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STELLAR ABLATION OF PLANETARY ATMOSPHERES
T. E. Moore1 and J. L. Horwitz2
Received 21 December 2005; revised 22 October 2006; accepted 3 January 2007; published 9 August 2007.
[1] We review observations and theories of the solar
ablation of planetary atmospheres, focusing on the
terrestrial case where a large magnetosphere holds off the
solar wind, so that there is little direct atmospheric impact,
but also couples the solar wind electromagnetically to the
auroral zones. We consider the photothermal escape flows
known as the polar wind or refilling flows, the enhanced
mass flux escape flows that result from localized solar wind
energy dissipation in the auroral zones, and the resultant
enhanced neutral atom escape flows. We term these latter
two escape flows the ‘‘auroral wind.’’ We review
observations and theories of the heating and acceleration
of auroral winds, including energy inputs from precipitating
particles, electromagnetic energy flux at magnetohydrodynamic
and plasma wave frequencies, and acceleration by parallel
electric fields and by convection pickup processes also known
as ‘‘centrifugal acceleration.’’ We consider also the global
circulation of ionospheric plasmas within the magnetosphere,
their participation in magnetospheric disturbances as absorbers
of momentum and energy, and their ultimate loss from the
magnetosphere into the downstream solar wind, loading
reconnection processes that occur at high altitudes near the
magnetospheric boundaries. We consider the role of planetary
magnetization and the accumulating evidence of stellar ablation
of extrasolar planetary atmospheres. Finally, we suggest and
discuss future needs for both the theory and observation of the
planetary ionospheres and their role in solar wind interactions, to
achieve the generality required for a predictive science of the
coupling of stellar and planetary atmospheres over the full range
of possible conditions.
Citation: Moore, T. E., and J. L. Horwitz (2007), Stellar ablation of planetary atmospheres, Rev. Geophys., 45, RG3002, doi:10.1029/
2005RG000194.
1.
INTRODUCTION
[2] This review has a focus upon the terrestrial ionosphere and upper atmosphere, and their expansion into
geospace and loss from the Earth, owing to energy inputs
from the Sun, both electromagnetic and mechanical. The
term ‘‘geospace’’ is used here in the sense suggested by the
NSF Geospace Environment Modeling (GEM) Program, to
encompass the coupled atmosphere, ionosphere, magnetosphere, and heliosphere. Over the past 40 years, the SunEarth system has been the subject of observations and
theoretical analyses of increasing sophistication, far beyond
what has been possible for the other planets of our solar
system, or what will be possible for planets of other astrospheres in the foreseeable future. Geospace studies are
directly applicable to other planets, solar and extrasolar,
and to the Earth during early solar system formation, or
geomagnetic reversals. These studies will contribute to our
preparedness for space exploration, as well as our understanding of our home planet in space. In addition, the results
of geospace studies are essential to advancing the frontiers
of space weather prediction.
1
NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.
Department of Physics, University of Texas at Arlington, Arlington,
Texas, USA.
2
[3] Some nomenclature explanations are in order. We
refer to the region dominated by terrestrial material
(‘‘geogenic’’) as the ‘‘geosphere,’’ and its outer interface,
beyond which lies mainly solar material, as the ‘‘geopause,’’
following Moore and Delcourt [1995]. Solar energy deposited in the geosphere, particularly the third (gas) and fourth
(plasma) geospheres, produces expansion of the gas and
plasma, initially producing what we term as ‘‘upflow’’
against gravitational confinement. With sufficient energy,
upflow becomes ‘‘outflow,’’ escaping gravity, but the plasma
is still trapped in the magnetosphere. When geospheric
plasma finds its way to the outer magnetospheric boundary
and escapes downstream in the solar wind, we call it
‘‘ablation’’ because material has then been completely
removed from the Earth and can never return, given the
supersonic expansion of the solar wind to fill the heliosphere.
[4] Planets and their early atmospheres are thought to
condense out of nebulae in parallel with their central stars
[Jeans, 1902]. By the time stellar ignition occurs, however,
condensation competes with the process of nebular and
atmospheric ablation, powered by an enhanced early stellar
wind thought to prevail during the T-Tauri phase for stars in
the size class of our Sun [Balick, 1987]. Volatiles may be
acquired by planets through impacts of long-period comets
[Chyba et al., 1990], but ablation continues and may
eventually dominate for planets sufficiently close to their
Copyright 2007 by the American Geophysical Union.
Reviews of Geophysics, 45, RG3002 / 2007
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8755-1209/07/2005RG000194$15.00
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Moore and Horwitz: ABLATION OF ATMOSPHERES
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Figure 1. Noon-midnight cross section of ionospherelower magnetosphere summarizing phenomena
involved in the heating and escape of ionospheric plasmas. After Moore et al. [1999a] with kind
permission of Springer Science and Business Media.
central star [Vidal-Madjar et al., 2004]. The terrestrial
planets of our solar system all bear evidence of long-term
atmospheric evolution to which solar photon or solar wind
ablation or erosion must have contributed. Mercury barely
has an exosphere [Hunten et al., 1988]. Venus’ atmosphere
has been desiccated of water [Kasting, 1988]. Mars has lost
most of an atmosphere that increasingly appears to have
contained substantial water vapor [Squyres and Kasting,
1994]. Earth is unique among these planets in the amount of
water that it acquired and retains. Intriguingly, of these
rocky inner planets, the Earth has by far the strongest
planetary magnetic field [Stevenson et al., 1983].
[5] The presence of a magnetic field has a strong influence on the interaction between a planet and the solar wind
[Stern and Ness, 1982], and thus the global distribution of
energy dissipated into atmospheric gases by the solar wind.
A planet without an intrinsic magnetic field might seem
more exposed to atmospheric ablation, with direct impact of
the solar wind on the entire upper atmosphere. However, as
we will discuss further below, the net loss may be comparable with or without a magnetic field, because its presence
concentrates electro-mechanical energy dissipation at the
planetary end of flux tubes that link with the boundary layer
between the solar wind and its magnetic field and the
planetary magnetic field and its enclosed plasma. This is
the region known as the auroral zone, where strong electrical currents link the solar wind to the ionosphere, and
charged particles are accelerated and in turn generate the
aurora [Ergun et al., 1998].
[6] This high-latitude ionosphere-magnetosphere region
is one of the richest and most interesting regions within the
space plasma universe that is accessible to direct measurements. The many energetic processes occurring there contribute to the expansion and escape of the ionosphere
beyond the outer regions of geospace. Figure 1 shows a
schematic illustration of the range of processes within the
ionosphere and lower magnetosphere that propel these outflows upward and outward.
[7 ] As depicted by Figure 1, ionospheric outflows
emerge from the high-latitude ionospherelower magnetosphere generation regions on magnetic field lines that guide
these flows into magnetospheric regions such as the tail
lobes and the plasma sheet, as shown in Figure 2. If these
ionospheric plasmas remained on polar lobe field lines,
which are ultimately connected to the interplanetary magnetic field, they would be lost from the magnetosphere.
However, these plasma outflows join the convective circulation of plasma in the magnetosphere as they expand out of
the ionosphere proper. Hence these emerging ionospheric
plasmas flow across the magnetic field into the closed field
line region of the plasma sheet, which carries the current
responsible for the stretched magnetotail. Ionospheric flows
from the nightside auroral regions are injected directly into
the plasma sheet, and since the plasma sheet itself is the
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Figure 2. Global circulation of plasmas in Earth’s magnetosphere, in the noon-midnight meridian. After
Hultqvist et al. [1999] with kind permission of Springer Science and Business Media.
primary source of plasmas for the quasi-trapped hot plasmas
of the inner magnetosphere, these ionospheric plasmas
ultimately populate the ring current in considerable quantities
as well. An ongoing controversy for the magnetospheric
community, particularly since the provocative article of
Chappell et al. [1987], has been the question of how much
of the plasma density and/or pressure within various regions
of the magnetosphere originates in the ionosphere, as
opposed to the other principal plasma source, the solar
wind.
[8] Figure 1 portrays several of the driving processes
and features of ionospheric upflow and outflows in a noonmidnight cross section of the high-latitude ionospherelower
magnetosphere region. Various types of ion conics and
beams [e.g., Øieroset et al., 1999], rings [Moore et al.,
1986], electron beams and conics [e.g., Menietti and
Weimer, 1998], counterstreaming and modulated fieldaligned electrons [e.g., Carlson et al., 1998; McFadden et
al., 1998], and other highly non-Maxwellian particle distribution functions have been consistently observed, often
in conjunction with a variety of wave and strong field
disturbances [e.g., Hirahara et al., 1998; Norqvist et al.,
1998; Vaivads et al., 1999]. The ion conics and rings are
often the results of perpendicular heating or acceleration of
ionospheric ions by various types of waves, including lower
hybrid, broadband, and ion cyclotron waves, whereas the
beam ion distributions are often caused by parallel electric
fields, either in quasi-DC or wave or impulsive forms. Also,
in addition to the light (H+, He+) ions and heavier, often
dominant O+, ions, at times, molecular ions (NO+, N+2 , O+2 )
are observed in the 1000- to 4000-km regions over the
auroral zones and the cusp, as well as over subauroral ion
drifts [e.g., Peterson et al., 1994; Wilson and Craven, 1998,
1999].
[9] The overall process of generating ionospheric outflows often occurs in a multistage sequence, particularly in
terms of the escape of normally gravitationally bound
heavier ions such as O+. One set of processes operates
within and just above the principal ionospheric production
region, at altitudes from 300 to 1000 km, to either produce
ionization or propel initial upflows, or both. Frictional
or Joule heating (FH in Figure 1), caused by the cross field
E B drift of ionospheric ions through the neutral
atmosphere at these altitudes, enhances the ion pressure
and produces an initial upflow [e.g., Wahlund et al., 1992;
Korosmezey et al., 1992; Ho et al., 1997]. Soft electron
precipitation produces both ionization and collisional heating of thermal electrons, which increases the local ambipolar electric field and lifts the ions upward [e.g., Wahlund et
al., 1992; Seo et al., 1997; Su et al., 1999]. The wave-
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particle driven ion heating, parallel electric field, and other
processes occurring at altitudes from 1000 km upward
[André et al., 1998] then supply the additional energy
needed, particularly for O+ to escape the Earth’s gravitational well (about 10 eV). Wu et al. [1999] has systematically investigated the effects of the different stage effects
on overall ion outflows, focusing on soft electron precipitation effects in the lower (<1000 km) ionospheric region,
and ion cyclotron wave – based ion heating above 1000 km
altitude.
[10] During the 1960s, ionospheric expansion into upper
geospace was viewed in terms of the so-called (classical)
polar wind [Dessler and Michel, 1966; Axford, 1968; Banks
and Holzer, 1968, 1969]. This classical polar wind outflow
was predicted as a light ion (H+ and He+) outflow along
open magnetic field lines threading the polar cap, in which
the principal upward driving forces are the plasma pressure
gradients and the ambipolar electric field sustained by
charge balance requirements involving the light electrons
and the heavy, gravitationally bound majority ion species,
O+. The first definitive measurements of this classical light
ion polar wind outflow were obtained by a polar orbiting
satellite above 1000 km altitude in the early 1970s [Hoffman
et al., 1974]. However, as a result of satellite measurements
of ions during the 1970s, the importance of outflows driven
by the stronger auroral processes, including the waveparticle interactions and parallel electric fields mentioned
above, became apparent. Furthermore, the term ‘‘polar
wind’’ has become somewhat imprecise in community
usage. Some authors include outflows driven by auroral
processes within the polar wind purview, while others tend
to confine use of the term mostly to the ‘‘classical,’’ thermal,
nonauroral processes and their natural extensions within the
paradigm arising from the analyses of Banks and Holzer as
suggested by Axford.
[11] In view of the above, we will thus organize the first
portion of this review into the physics considerations on two
relatively distinct classes of ion expansion into geospace.
We introduce the term ‘‘photothermal outflows’’ to describe
those outflows driven chiefly by photon energy from the
Sun. We introduce the term ‘‘auroral wind’’ for those
outflows driven by solar wind mechanical energy. Essentially, the ‘‘photothermal outflows’’ will be congruent with
what many refer to as the ‘‘classical polar wind,’’ while the
‘‘induced outflows’’ will be mostly consistent with ‘‘auroral
wind.’’ The remainder of the review will describe the
morphologies and larger-scale physics involved in highlatitude ‘‘fountain’’ and ‘‘plume’’ flows, the relationship of
the ionospheric expansion in terms of (lower) boundary
conditions set on the dynamics of the magnetosphere,
desirable directions for future theory and measurements
investigations, and discussion and conclusions.
[12] Progress in compiling and understanding the characteristics and significance of ionospheric expansion into
geospace has advanced in the past decade or so chiefly
through ongoing in situ measurements by sophisticated
particle instrumentation on such polar-orbiting spacecraft
as Akebono, Polar, Freja, FAST, and Cluster, augmented by
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promising remote energetic neutral atom imaging results
from the IMAGE spacecraft, in parallel with strides in
simulation techniques and applications utilizing fluid and
kinetic and combined fluid-kinetic computer codes over one
or more dimensions and different timescales. The aim of
this review is to present this progress within the organizational context described above. The last article within
Reviews of Geophysics that covered the topical purview of
the present review appears to be one by Ganguli [1996],
which had a focus primarily on the polar wind. Therefore
much of the detailed emphasis in the present review will be
on progress on understanding ionospheric expansion into
geospace since 1996, and with more emphasis on the
auroral wind, as defined above.
2.
GEOSPACE MODELING IMPLICATIONS
[13] The Heliophysics research community aspires to
incorporate its understanding of space plasma physics into
theoretical models and global simulations [Winglee, 1998;
Lyon et al., 2004; Siscoe et al., 2002b; Gombosi et al.,
2003; Raeder et al., 1997; Ashour-Abdalla et al., 1997].
The preference is clearly for simulations that run faster than
real time on available computer hardware, leading to
limitations that are gradually relaxed as hardware capabilities develop. Criticism of global models tends to focus on
the lack of kinetic or microphysical dissipative processes in
critical current carrying regions best exemplified by the
reconnection diffusion zone. Practical computer codes are
subject to numerical effects that give rise to diffusive
processes, independent of the true microphysics. A prominent objective of the code developers is to answer these
criticisms through reduction of the numerical effects to a
level that is negligible compared with the true physical
dissipative effects or their implementation in code. This is
an important goal and deserves all the attention it receives.
[14] However, there is another assumption that has traditionally been made to obtain practical global simulation
codes. This is an assumption that the dissipative medium of
terrestrial (ionospheric) plasmas is confined exclusively
within a thin layer of scale height less than 100 km, close
to the Earth. This thin layer is generally taken to lie
completely outside the global simulation space and is
lumped into the boundary condition. From the perspective
of electro-dynamic coupling, this is equivalent to assuming
that all solar wind-driven current systems close outside the
simulation space, or that the magnetosphere is exclusively
an electro-dynamic dynamo and transmission system with
substantial dissipative or inertial elements limited to the
ionospheric thin layer, beyond the system boundaries.
Exceptions may include the bow shock region where shock
heating accompanies compression, and the magnetosheath,
where the shocked solar wind is reaccelerated downstream.
Not surprisingly, in view of this assumption, an outstanding
problem of global simulations is their inability to produce
sufficient plasma pressure in the inner magnetosphere hot
plasma, or ring current region.
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[15] However, the ‘‘thin shell’’ assumption leads to a
number of problems. One fundamental problem is perhaps
that codes based on this assumption can never include the
plasmasphere because its ionospheric origin violates the
assumption of a thin shell ionosphere. The plasmasphere
is an important region of the inner magnetosphere that
was discovered early in the space age [Carpenter, 1963;
Carpenter et al., 1993]. Recent advances in imaging of
space plasmas have shown that the plasmasphere is a
dynamic participant in magnetospheric plasma circulation,
producing global-scale drainage plumes that supply plasma
to the dayside subsolar magnetopause region [Sandel et al.,
2001; Goldstein et al., 2002, 2003a], and to the polar cap
topside ionosphere [Foster et al., 2002]. It is clear from
these recent developments that the ionosphere, via the
plasmasphere, is at times an important source of plasma
mass density to the dayside magnetosphere. Given this new
validation of global ionospheric circulation at high altitudes,
a credible global simulation must evidently include these
plasmas to get the bulk characteristics of the magnetosphere
correct. The minimum requirement to include them is that
plasma be allowed to flow across the boundary between the
thin shell ionosphere into the magnetosphere at large.
[16] As important as the photothermal polar wind outflows may be, the auroral outflows at latitudes higher than
that of the plasmapause have greater potential impact upon
magnetospheric dynamics, given sufficient available energy,
owing to their much higher escape flux limits and more
massive ions. As solar wind mechanical energy is much
more variable than the solar photon flux, so is the auroral
wind much more strongly modulated than is the polar wind.
The area from which auroral wind is emitted is substantially
smaller than the area outside the plasmasphere that emits
light ion polar wind, but it is substantially more productive
of mass flux than the polar wind regions, when the solar
wind interaction is strong. Owing to recent observational
and theoretical advances, it is now becoming feasible to
relax the assumption of a thin shell ionosphere in global
magnetospheric simulations either by allowing flow of a
single plasma fluid into the system from the ionosphere, or
by introducing a separate ionospheric fluid (or fluids). We
can distinguish the outflows beyond the thin shell into
‘‘photothermal’’ and ‘‘auroral wind’’ categories. We further
consider and summarize the global outflow structures that
result from interactions between structured outflows and
magnetospheric circulation. Then we consider the role of
planetary magnetization, and the evidence that extrasolar
planets also experience atmospheric ablation by their stellar
photon flux and plasma winds. Finally, we summarize the
outstanding problems, new observations required, and theoretical developments that will allow us to incorporate our
new knowledge into our global simulations to further
improve their predictive capability and generality.
3.
PHOTOTHERMAL OUTFLOWS
[17] Our Sun is at a stage of its evolution during which
the electromagnetic energy flux of its photon emission
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(1 103 J/s-m2) greatly exceeds the mechanical energy flux
of its stellar wind (3 1017 eV/s-m2 or 5 102 J/s-m2).
Thus the predominant interaction between the Sun and Earth
is the deposition of the photon energy. However, the vast
majority of that is unabsorbed until it reaches the ground.
The UV component of the photon flux is absorbed in the
upper atmosphere and creates the ionosphere, maintaining its
temperature at a few thousand K in sunlight, by removing
electrons from atmospheric atoms to create a partially
ionized plasma. This plasma is substantially warmer than
the parent gases from which it is created and hence contains
a larger fraction of particles with enough energy to escape
from the Earth’s gravity. Photothermal escape of neutral gas
(that is, neutral escape which is driven by the photon flux
absorbed by the upper atmosphere) is for Earth limited
mainly to the lightest species, H and to some degree He.
The same is true of the plasma species H+ and He+, the
escape of which is called ‘‘polar wind,’’ following the
nomenclature of Axford [1968] and Banks and Holzer
[1968, 1969].
3.1. Jeans Escape
[18] ‘‘Jeans’’ escape [Jeans, 1902] refers to the escape of
an atmosphere from a gravitating body, owing simply to the
thermal motions of the gas atoms or molecules. Calculations
of the expected escape as a function of the temperature of
the gas have been made [Chamberlain and Hunten, 1987].
Loss of the more energetic particles into space constitutes a
heat loss from the atmosphere, which must clearly be
balanced by other energy sources, such as incoming radiation or upward heat conduction from below (normally
negligible or even negative).
[19] Analogous escape of ions is also expected from an
ionosphere or partially ionized atmosphere. The simplest
possible ionosphere is formed when ultraviolet radiation
from a nearby star partially ionizes an upper atmosphere,
giving rise to electron-ion pairs, and creating a plasma that
is far from thermodynamic equilibrium, reflecting the presence of a significant flux of photons with spectrum that
extends above the electron binding energy of the gaseous
atoms. Where the photoelectron energy is partly thermalized
by collisions with ions, the ion gas may be hotter than the
neutral gas, and hence have a larger than expected kinetic
escape rate, under this simple model. The enhancement of
mass flux is attributable to the energy flux of photons
absorbed by the gas in the processes of ionization and
thermalization.
3.2. Polar Wind and Plasmasphere
[20] In practice, ionization that occurs in the presence of a
magnetic field produces a more complex situation than that
described by Jeans escape. Moreover, the presence of the
solar wind in addition to the solar photon flux complicates
the situation substantially. Initial exploration of the Earth’s
magnetosphere revealed the presence of an extended ionosphere within the inner magnetosphere, and of plasma
circulation throughout the outer magnetosphere. Soon, the
implications for ionospheric escape flows were appreciated
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and a theory of polar outflows was outlined and subsequently developed [Dessler and Michel, 1966; Axford,
1968; Banks and Holzer, 1968, 1969]. This theory, as
further developed and reviewed by others [Raitt et al.,
1978; Schunk, 1988; Gombosi et al., 1985; Ganguli,
1996; Moore et al., 1999a], has been confirmed in most
respects [Abe et al., 1996; Su et al., 1998b; Drakou et al.,
1997; Huddleston et al., 2005], the main exception being
that the initial theory made no account of auroral zone
energetic phenomena that affect the nature of the ionospheric outflows. In recent studies of the polar wind, global
circulation has been considered [Demars and Schunk,
2002], and complex structures have been shown to result
that cannot be resolved well using statistical surveys of
spacecraft data.
[21] Because plasma electrons and ions are linked by
electrostatic forces, they are not free to separate and are
required to maintain very nearly equal densities by an
ambipolar electric field that swiftly forms if they begin to
separate. Since electrons are so much faster and more
mobile than ions, and are negligibly bound by gravity, the
topside ionosphere forms an approximately radially directed
ambipolar electric field that holds down the electrons,
exerting a balancing upward force on the ions. The strength
of the field depends on the mass of the dominant ion
species. When the field is strong enough to hold O+ in
equilibrium, it is strong enough to accelerate lighter ions
upward strongly, and this is an important correction to the
theory of the light ion polar wind. Such ambipolar electric
fields are important wherever differential transport of electrons and ions tend to make them separate because of either
gravity or inertia. Moreover, the ambipolar field links
electrons and ions such that heating of either species adds
energy to the system and enhances escape of both species as
an ambipolar plasma, though gravitational stratification by
mass will occur.
[22] Within high-latitude magnetospheric flows, as partially depicted in Figure 2, plasmas undergo a circulation
cycle whose timescale ranges from diurnal to much shorter
than diurnal, depending on the strength of the solar wind
interaction. As specified by Alfvén’s [1942] theorem, magnetized plasmas move on large scales as elastic ‘‘flux tubes’’
linked along their length by magnetic field lines of force.
During the course of this circulation cycle, a plasma flux
tube is first stretched from 10 RE to >100 RE in half-length
as it is convected antisunward, or it may be disconnected
from the conjugate hemisphere and reconnected into the
solar wind plasma during part of the cycle, permitting direct
plasma exchange between the two media. Later, after
reconnection in the tail (if disconnection has occurred),
the flux tube relaxes back to its starting point. During each
circulation cycle, the flux tube volume changes by a factor
on the order of 104.
[23] During the stretch part of the cycle, or when the flux
tube is connected to the downstream solar wind (which
evacuates rather than filling the flux tube), ionospheric
plasma expands freely into the flux tube as if there were a
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zero-pressure upper boundary condition, introducing light
ion plasma to the flux tube at a rate limited by the available
source of plasma. The net result is a very low density,
supersonic flux of cold light ions through the polar caps and
into the magnetospheric lobes. This outflow corresponds
closely to the polar wind and has been observed from
ionospheric topside heights [Brinton et al., 1971; Hoffman
et al., 1974; Hoffman and Dodson, 1980] up through
altitudes around 1 RE [Abe et al., 1993a, 1993b, 1996],
around 3 – 4 RE [Nagai et al., 1984; Olsen et al., 1986; Su et
al., 1998b], and at altitudes up to 9.5 RE [Moore et al.,
1997; Su et al., 1998b; Liemohn et al., 2005], extending into
the lobes of the magnetosphere. However, as has been noted
by many of these authors, the cold polar wind is often
accompanied by comparable or larger fluxes of O+ ions,
even under conditions of low solar activity. The highaltitude, cold supersonic outflows into the magnetotail,
including O+ as well as light ions, have recently been
studied as the ‘‘lobal wind’’ [Liemohn et al., 2005].
[24] The polar wind has recently been considered as a
three-dimensional system, in the context of global ionospheric convection, with Joule heating and auroral electron
precipitation effects on the ambipolar electric field [Demars
and Schunk, 2002; Coley et al., 2003]. A complex threedimensional picture emerges of light ion outflow variations,
especially in velocity. Moreover, heavy ion ‘‘breathing’’
motions inflate and deflate the topside ionosphere with
mixed upflows and downflows, in response to these processes. Particularly in the case of the heavy O+, what goes
up in regions of Joule heating or precipitation for the most
part falls back into the ionosphere in the polar cap, or
subauroral regions that are downstream of the aurora in the
ionospheric circulation pattern. This leads to complex
patterns of counterstreaming in which the light ions are
flowing upward in the same region where O+ downflows are
occurring. The authors cite good agreement with observational studies of O+ downflow [Chandler et al., 1991;
Chandler, 1995].
[25] During the relaxation part of the flux tube convection cycle, stretched flux tubes contract through the magnetotail toward the Earth. Disconnected flux tubes also
reconnect there and circulate back toward the dayside
subsolar region. However, these flux tubes do not pass
through the inner magnetosphere, except during conditions
of strong magnetospheric circulation. When the interplanetary magnetic field (IMF) tends northward, convection
stagnates in the inner magnetosphere, while the polar wind
escapes downstream as flux tubes are peeled off the
magnetospheric lobes by high-latitude reconnection. This
is illustrated in Figure 3 (left), from a global simulation that
tracks ionospheric fluids in addition to the solar wind fluid
[Winglee, 1998]. The surface plotted in Figure 3 bounds the
region dominated by polar and auroral wind plasmas
and excludes the region dominated by solar plasmas.
Polar wind outflow at lower, stagnant latitudes, closer to
the Earth, continues until large densities are accumulated
(up to >1000 cm3), forming the region of extended iono-
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Figure 3. A simulated configuration of ionospheric plasmas in the magnetosphere [Winglee, 1998]. The
highlighted surface represents the limits of the region within which the plasma is predominantly of
ionosphere origin, regardless of its absolute density, that is, the geopause of Moore and Delcourt [1995].
sphere known as the plasmasphere. In Figure 3 (left),
intermediate latitudes are dominated by solar wind plasmas
that circulate inward from the low-latitude boundary layers in
this case.
[26] For normal (BZ 0, where Z points northward
normal to the ecliptic plane) or southward IMF, as shown
in Figure 3 (middle and right), magnetospheric circulation is
excited by a combination of dayside and nightside reconnection that grows increasingly stronger with more southward IMF, penetrating deeper into the inner magnetosphere
and eroding the outer parts of the dense plasmasphere (see
also section 3.3). Polar wind outflows are convected into
the plasma sheet and then sunward, replacing the denser
plasmaspheric material being removed toward the subsolar
magnetosphere. The surface separating solar and geogenic
plasmas, the geopause [Moore and Delcourt, 1995],
changes in shape and grows in size with increasing magnetospheric convection. In contrast, the classically defined
plasmapause, which bounds the high-density light ion
region, is eroded to smaller size during the strong circulation driven by southward IMF.
3.3. Plasmasphere Filling, Erosion, and Refilling
[27] The basic paradigm is that the outer portion of the
dense plasmasphere is eroded away by enhanced magnetospheric circulation and then refills via upward ionospheric
plasma flow during the recovery phase of magnetospheric
storms. This conceptual framework dates from the seminal
papers by Nishida [1966] and Brice [1967]. In this picture, a
period of enhanced magnetospheric convection during a
storm’s main phase causes the boundary between closed
(Earth-encircling) flux tube trajectories and circulating flux
tube trajectories to shrink. The plasma on the flux tubes that
are on open trajectories (in which the flux tubes link with
the interplanetary magnetic field at some point during the
cycle) may then vent to the interplanetary medium, resulting
in low densities. In steady state conditions, this boundary
Figure 4. Images of the plasmasphere in scattered He+ resonant sunlight [Goldstein et al., 2003a],
illustrating the observed and modeled plasmasphere as it is eroded by a period of enhanced
magnetospheric convection, from left to right. Sun is to the right.
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coincides with the density gradient known as the plasmapause [Carpenter, 1963]. At the onset of the storm recovery
phase, when the intensity of magnetospheric convection is
reduced, there is an outer region of initially low density
between the quiet and active closed/open flux tube trajectory boundaries. This outer region of newly closed or Earthencircling trajectories is subject to plasma upflow from the
ionosphere below on the same field lines. The reality of this
description was confirmed when the IMAGE mission made
possible global imaging of the plasmasphere, showing that
it expands during quiet times and is then eroded by
convection, producing pronounced sunward extending
plumes, as well as other dynamic features, as illustrated in
Figure 4.
[28] A particularly influential theoretical paper on how
the refilling could proceed was that of Banks et al. [1971].
They considered a single-stream hydrodynamic model treatment in which equal supersonic polar wind streams from
conjugate ionospheres flowed onto initially empty flux
tubes and interacted at the magnetic equator. The interaction
in that treatment led to a ‘‘slab’’ of thermalized denser
plasma at the equator, whose edges were shock fronts which
propagated back down the field lines toward the ionosphere.
When these shock front slab edges reach the flow critical
points near the topside ionosphere, the overall flow character
changes to subsonic flow throughout. The flow proceeds
through this second, subsonic flow stage until the flux tube
density distribution attains diffusive equilibrium or is interrupted by a new storm or gust of enhanced magnetospheric
convection.
[29] One of the principal issues with this scenario concerns
the single-stream restriction and the interaction between the
opposing ionospheric flows. In the above picture, the
modeling requirement that the ion plasma be described as
a single stream everywhere, with a single bulk velocity (as
well as temperature and density), necessarily forces a
stationary slab around the equator as a consequence of the
interaction. However, as Banks et al. [1971] acknowledged,
the densities of the interacting streams would be sufficiently
low that such a strong interaction should not occur purely on
the basis of Coulomb collisions. Other processes, such as
wave-particle interactions, would be required in order to
sustain such an interaction. This issue led to treatment by
two-stream models [e.g., Rasmussen and Schunk, 1988].
Here the streams originating from the Northern and Southern
Hemispheres are treated as identifiably separate populations.
Owing to the low collision frequencies involved, the
streams typically would pass through each other largely
undisturbed as interpenetrating streams, until each would
interact with the conjugate ionosphere. Each stream would
then reflect from that conjugate ionosphere, but would now
create a shock front that would propagate back up the field
line toward the equator. The reverse shock in this framework was presumably a consequence of the fact that above
the topside ionospheres of each hemisphere, the incident
stream and its reflection were required by the treatment to
be considered as a single, largely stationary stream. The
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low-altitude shocks stem from the same physics as the
equatorial thermalized plasma slab formation in the equatorial region for the single-stream formulation.
[30] An alternative to these single-stream and two-stream
hydrodynamic approaches is to use a kinetic treatment, in
which the ions are treated as particles. Wilson et al. [1992]
considered a refilling flux tube simulation in which ions
were injected above the conjugate ionospheres onto an
initially empty flux tube. The electrons were treated as a
Boltzmann neutralizing fluid. No shocks were formed in the
ensuing development, but rather a more gradual isotropization of the counterstreaming polar wind flows, caused
primarily by small-angle Coulomb collisions, until eventually a diffusive equilibrium condition was achieved. Under
the same kinetic framework, the effect of equatorially
concentrated ion heating perpendicular to the magnetic field
by ion cyclotron waves was considered by Lin et al. [1992,
1994]. These processes, in combination with a presumption
of a heated, field-aligned or isotropic electron component
around the equator, produced an electric potential peak at
the magnetic equator of order 1 V, an electric field directed
away from the magnetic equator. Since the refilling flows
from each conjugate ionospheric source region had (polar
wind) energies below this potential level, this led to a
‘‘hemispheric decoupling’’ in the refilling. That is, the ion
streams in each hemisphere reflected off this potential layer
back toward the ionospheric end, to be either isotropized via
collisions, mirrored, or lost to the atmosphere below. These
simulation results were in accord with observations about
the magnetic equator on L 4.5 flux tubes by Dynamics
Explorer 1 (DE-1) [Olsen et al., 1987].
[31] Other kinetic aspects of refilling that have been
studied with similar models include velocity dispersion
[Miller et al., 1993], other ion heating by waves [Singh,
1996], and further examination of reflection of incoming
ionospheric polar wind streams by potentials created
through resulting differential ion-electron anisotropies
[Liemohn et al., 1999]. However, a serious limitation with
these kinetic-focused studies is that the altitudinal boundary
conditions typically involved presumed plasma injection
into the simulation regions at topside altitudes (generally
10002000 km). Thus there has been no true self-consistent
coupling of the ionospheric plasma and energy sources and
losses and chemistry to be synergistic with the intriguing
kinetic aspects of these simulations.
[32] There are a number of intriguing features of the outer
plasmasphere and trough regions that have been observed
by spacecraft but are not presently understood, and probably
require incorporation of high-altitude kinetic processes and
self-consistent coupling of ionospheric processes to model
and understand quantitatively. For example, in the Horwitz
et al. [1984] DE-1 examination of plasmaspheric refilling
during the recovery of a magnetospheric storm, it was found
that O + densities were greatly elevated and actually
appeared to be temporarily the dominant ion species at
mid magnetic latitude refilling plasmasphere. Horwitz et al.
[1984] also showed that the O+ in this event was flowing
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along the field lines out of the northern ionosphere. It is
possible that such brief strong impulsive O+ upward flows
may represent the incipient formation of the heavy ion
density enhancements characteristically observed in the
outer plasmasphere in the vicinity of steep plasmapause
density gradients and associated with enhanced ion and
electron temperatures at both ionospheric and plasmaspheric
altitudes [e.g., Horwitz et al., 1986; Roberts et al., 1987;
Horwitz et al., 1990].
[33] Another intriguing feature is the apparent distribution
of trapped light ions and heavy ion density enhancements on
outer plasmaspheric field lines. Olsen et al. [1987] showed
that perpendicularly peaked fluxes of light ions (H+ and He+)
were predominantly observed by DE-1 within about 10° of
the magnetic equator on L = 3 4.6 flux tubes. Craven
[1993], on the other hand, showed that DE-1 measured heavy
N+ ion fluxes were detectable only at midlatitudes. Although
O+ fluxes were not specifically represented, N+ is typically a
‘‘proxy’’ for O+ with N+/O+ density ratios typically around
0.10 in the plasmasphere [Craven et al., 1995]. The combination of these and other observations may suggest that
the heavy ion density enhancements occur at midlatitudes
and the light ion ‘‘pancake’’ or trapped distributions occur
around the equator, on field lines threading the ring
current-plasmasphere overlap region near the plasmapause,
possibly also related to SAR arc types of phenomena [e.g.,
Horwitz et al., 1986, 1990].
[34] Other species-dependent behavior on outer plasmaspheretrough flux tubes has been observed, particularly
for He+, which is especially significant in view of the
remote large-scale plasmasphere He+ observations by the
magnetospheric IMAGE mission via the 30.4-nm EUV
scattering line of He+, and others bearing on this issue,
including the refilling process, for example by Yoshikawa et
al. [2003]. Chandler and Chappell [1986] examined the
field-aligned flow velocities of He+ and H+ from DE-1
observations in the plasmasphere, and found instances when
the H+ and He+ were counterstreaming toward opposite
hemispheres. Anderson and Fuselier [1994] examined lowenergy H+ and He+ in the presence of electromagnetic ion
cyclotron waves, in the dayside morning trough region with
AMPTE observations. They found that ‘‘X’’ distributions
(or bidirectional conics) of He+ were observed near the
magnetic equator with energies around 35 eV on average,
while colocated H+ perpendicularly peaked had temperatures of 5 eV. Anderson and Fuselier [1994] argued that
these He+ ‘‘X’’ distributions were gyroresonantly heated by
the EMIC waves at off-equatorial latitudes.
[35] Still further intriguing features reported by Olsen
and Boardsen [1992] were not only interesting density
minima near the magnetic equator, but also latitudinally
asymmetrical densities around the magnetic equator, on outer
plasmasphere field lines. If such latitudinal asymmetries in
the densities do exist, it is possible they could involve the
above-noted electrostatic hemispheric decoupling [Lin et al.,
1994] of asymmetric Northern and Southern Hemisphere
ionospheric flows.
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[36] The IMAGE spacecraft, launched in 2000, opened a
new window into understanding the large-scale dynamic
response of the plasmasphere to magnetospheric electric
field changes. For example, Goldstein et al. [2003b] examined observations by the IMAGE EUV imager that showed
erosion of the nightside plasmasphere occurring in two
bursts during one interval in July 2000, and indicated that
these erosion bursts were correlated with solar wind southward turnings of about 30 min prior (after correcting for
transit time from observing satellite). Foster et al. [2002]
examined total electron content (TEC) measurements from
GPS satellites in conjunction with Millstone Hill incoherent
scatter radar observations of storm-associated ionospheric
electron densities during IMAGE EUV observations of
plasmasphere erosion and formation of elongated plasmaspheric tails formed by convection of the main plasmasphere, including the dusk bulge region. Foster et al. [2002]
found that enhanced electron densities in the incoherent
scatter radar and TEC measurements corresponded to
regions of these plasmaspheric plumes mapped down along
magnetic field lines into the ionosphere.
4.
AURORAL WIND
[37] The photon flux of the sun is uniformly distributed
over the dayside of the Earth, and the mechanical energy
flux of the solar wind is similarly distributed over the
boundary of the magnetosphere. However, that mechanical
energy is by no means uniformly transmitted to the upper
atmosphere. Ring-like concentrations of energy input to the
atmosphere exist in the regions of bright auroras, known as
the auroral ovals. Indeed, the auroral ovals can usefully be
thought of as the footprint of the magnetospheric boundary
layer in the ionosphere, defined by the conduction of
electrical currents connecting the boundary layer with the
ionosphere. The largest energy inputs to the atmosphere
occur in these twin halos roughly centered on each magnetic
pole. These mark the cleft between magnetic field lines that
connect directly between hemispheres and those which have
been recently reconnected to the solar wind or have been
swept back into the magnetotail by earlier connection to the
solar wind, and may still be so connected. Plasmas
connected to different parts of the auroral ovals are substantially separated in space and time and may have quite
different histories, so the ovals are not monolithic. They
may be characterized by quite different behavior at any
given instant, their dynamics revealing to some degree the
different dynamics going on upstream, downstream, at the
north or south polar regions, or on the dawn or dusk side of
the Earth, in the solar wind and boundary layers. Nevertheless, the energy fluxes delivered to the ionosphere are
substantial. A small fraction of those energy fluxes, in the
form of thermal energy of the electrons and ions, is
sufficient to greatly enhance the escape flux of plasma into
space, even for the heavier ions O+, N+, and at times also
NO+, O+2 , and N+2 [Wilson and Craven, 1999]. As noted
above, this heavy ion outflow (with accelerated light on ion
outflows) will herein be referred to as the ‘‘auroral wind.’’
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In this section we consider the detailed causes and consequences of the auroral wind.
4.1. Overview
[38] While photothermal outflows originate from the
entire sunlit globe, only the lighter ion species have significant escape fluxes at Earth, since only they have thermal
energies exceeding their escape energy. The gravitational
escape energy for the C-N-O group is of order several eV,
and a negligible fraction of these ions can escape for typical
ionospheric temperatures. Thus the photothermal polar wind
contains relatively small amounts of these heavy ions,
owing to gravitational confinement.
[39] The auroral zones are defined as regions in which
solar wind energy dissipation is frequent and substantial,
often leading to auroral light emissions. This is equivalent
to saying that the auroral zones are magnetically conjugate
with the boundary layers and associated region-1 fieldaligned current system, along and inside the magnetopause,
or with the hot plasmas created inside the magnetosphere,
and their associated region-2 field-aligned current system.
For our purposes, the auroral zones represent a localized
source energy deposited in the topside ionosphere. This
energy comes in various forms, with the net effect of
enhancing ionospheric expansion, outflow and escape, or
in short, ablation.
[40] The altitude distribution of solar wind energy dissipation is also important and relevant. The theory of astrophysical winds [Leer and Holzer, 1980] states that energy
added below the supersonic outflow transition (critical
height) increases the escaping mass flux, while energy
added above the critical height increases the velocity of
outflow without increasing the mass flux. We thus anticipate
this same effect in the auroral wind, with energy added low
down producing enhanced mass flux, while energy added
higher up will mainly accelerate the flux determined lower
down. However, some level of high-altitude acceleration
may be required to sustain escape, turning upflow into
outflow.
[41] Two types of low-altitude heating have been identified as relevant to ionospheric upflows [Banks and Holzer,
1968, 1969; Wahlund et al., 1992]. First, ion heating by
frictional collisions with neutral gas is an important heating
mechanism in the topside F layer. It is often referred to as
‘‘Joule heating’’ even though that term is usually reserved
for the heating of current conduction electrons by electric
fields. In the ionosphere, horizontal currents are carried
largely by ions, owing to their demagnetization by collisions with neutrals, so this may be appropriate for that
medium. Second, electron heating by the impact of precipitating energetic auroral electrons is a substantial source of
energy for the ionospheric electron gas. Owing to electrostatic forces, heating of the electron gas is just as effective in
producing expansion and upflow as heating of the ion gas,
but clearly has a different signature when observed, for
example, by upward looking ionospheric radars.
[42] Another important concept for ionospheric outflows
is that of limiting flux [Hanson and Patterson, 1963].
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Though originally based on the subsonic flow regime of
plasmaspheric filling, in steady state, this concept describes
a real limitation on the average rate at which ions of a
specific species can flow upward out of the ionosphere.
Higher transient fluxes are possible but cannot be sustained,
so a longer term limit is ultimately enforced. The limit
results from a limit either on how rapidly the species in
question is created through ion-neutral chemistry or a limit
on the rate at which a species can diffuse through the
frictional medium of other species or neutral gas atoms
[Barakat et al., 1987]. For current purposes, it is sufficient
to note that H+ in the polar wind is limited by typical
ionospheric conditions to a flux on the order of 2 108 cm2 s1 (at 1000 km altitude). On the other hand, the
flux of O+ has a substantially higher limit on the order of
3 109 cm2 s1, depending somewhat on local conditions.
[43] In summary, depending on the amount of energy
available in either imposed magnetospheric convection
(Poynting or electromagnetic energy flux), or imposed
electron precipitation (heat flux), auroral wind fluxes of
heavy ions will range from negligible to more than an order
of magnitude greater than polar wind fluxes. In this section,
we consider the various types of energy and their contributions to localized heavy ion escape from the ionosphere.
4.2. Polar Rain Precipitation
[44] One of the most basic sources of energy to the
ionosphere, erroneously cited by most encyclopedia
accounts as the cause of the aurora, is the direct ‘‘polar
rain’’ of solar particles entering along reconnected magnetic
field lines linking the Earth and the solar wind [Gosling et
al., 2004]. The ion polar rain is confined largely to the
cusp proper, where freshly injected magnetosheath plasma
penetrates quite low and precipitates into the conjugate
ionosphere, where it can be imaged as proton aurora, and
has been by the IMAGE mission [Frey et al., 2002]. Most
such solar wind ions are mirrored and reflected back into the
boundary layers, forming the plasma mantle flow along the
flanks of the magnetosphere, following the convective flow
of plasma along the magnetopause.
[45] Solar wind thermal electrons, though much faster
than the thermal ions, are required to stay with those ions
unless replaced by addition of electrons from the Earth. A
subset of them is able to move along reconnected field lines
all the way to the ionosphere, barring adverse electrostatic
potentials. The region of polar rain can include much of the
polar cap when linked to the upstream solar wind, because
the solar wind electron gas often includes a highly field
aligned, superthermal component known as the ‘‘strahl’’
[Fitzenreiter et al., 1998], the presence of which will
enhance the energy flux of electrons precipitating into the
atmosphere in the polar regions.
[46] Polar rain precipitation has been noted and studied
[Fairfield and Scudder, 1985], but it does not appear to be
associated with geophysically significant ionospheric outflows, at least in the observations obtained to date. It is
possible that polar rain precipitation is responsible for
certain O+ outflows that have been suggested to originate
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Figure 5. Electron precipitation as a driver of outflows via electron heating. (left) Ion outflow velocities
in the auroral topside ionosphere (850950 km altitude) from DE-2 versus electron temperature. Straight
line fits and correlation coefficients are based on binned data. (middle) Ion upflow velocities versus
average energy of soft electron precipitation. (right) Ion upflow fluxes versus average energy of soft
electron precipitation. From Seo et al. [1997].
from the polar cap region rather than from the auroral zones
[Tam et al., 1995, 1998], but this needs more study.
4.3. Auroral Precipitation
[47] A major driver for F region/topside upward flows in
the topside ionosphere appears to be the effect of soft
auroral electron precipitation. Seo et al. [1997] examined
field-aligned flows at topside ionospheric altitudes
(850950 km) from Dynamics Explorer 2 (DE-2) and found
strong correlations between the upward flows observed in the
auroral zones and elevated electron temperatures, as indicated
in Figure 5. They also found significant correlations of the
electron temperatures themselves with the soft electron
precipitation energy fluxes, and anticorrelation with the
characteristic or average energy of the precipitating electrons
as indicated in Figure 5. Moreover, they found that unusually
large upward ion fluxes exceeding 1010 ions/cm2-s were only
observed during periods when the average soft electron
precipitation energy was less than 80 eV, or ‘‘ultrasoft’’
precipitation cases (Figure 5).
[48] These results were interpreted by Seo et al. [1997] as
indicative that soft electron precipitation played a significant role in driving upward F-region/topside ionospheric
plasma flows. The principal ionospheric effects of the
soft electrons in this regard were to directly enhance the
F-region ionization and to heat the thermal electrons, either
via Coulomb collisions or more indirectly through waveparticle processes. Elevating the ionospheric electron
temperatures increases the local ambipolar electric field to
lift the ionospheric ions. The fact that unusually high
ionospheric ion upward fluxes were observed only during
periods when the average electron precipitation energy was
very low is consistent with the concept that ultrasoft
precipitation is most efficient at both ionization and heating
at upper F-region and topside altitudes. Higher-energy
precipitation penetrates to lower ionospheric altitudes, in
which ionization tends to remain in local equilibrium and
not diffuse upward.
[49] Modeling of specific auroral ionospheric upflow/
outflow observations in which effects of soft electron
precipitation were incorporated was performed by Liu et
al. [1995] and Caton et al. [1996], while Su et al. [1998a]
performed systematic fluid-based ionospheric modeling of
the effects of electron precipitation on upflows by examining
the effects of a range of electron precipitation parameters on
ionospheric parameters, such as the upflows versus time
and altitude, the electron and ion temperatures. Wu et al.
[1999, 2002] incorporated the effects of soft electron
precipitation into a fluid-kinetic transport model that examined the synergistic effects of soft electron precipitation and
transverse ion heating on the transport of ionospheric plasma
through the auroral ionosphere-magnetosphere region.
4.4. Auroral Heating
[50] Transverse ion heating in the auroral zone was
observed well before high-altitude observations of the polar
wind were made [Sharp et al., 1977; Whalen et al., 1978;
Klumpar, 1979]. This is owed at least in part to the fact that
it is relatively easier to observe, as directional hot tails of the
ionospheric energy distribution, extending readily up to tens
of eV and often up to hundreds of eV or even several keV,
depending on event intensity and instrument sensitivity
[Hultqvist et al., 1991; Yau and André, 1997; André et al.,
1998; Norqvist et al., 1998].
[51] The basic observation of transversely accelerated
ions (TAI) clearly implies energy gain principally transverse
to the local magnetic field. This was often envisioned as
taking place in a narrow altitude range, and it was assumed
that the subsequent behavior of the ions would be essentially that of a free particle gyrating and mirroring up out
of the divergent magnetic field of the auroral zone, leading
to a ‘‘conical’’ distribution whose cone angle would vary
with altitude according to the conservation of the first
adiabatic invariant and total energy. However, it soon
became apparent that transverse heating is in general
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widely distributed along auroral flux tubes in altitude [e.g.,
Moore et al., 1999a].
[52] Transverse heating also occurs through multiple
different physical processes. At the lowest altitudes, in the
F region, the dominant mechanism is ion-neutral friction as
magnetospheric motions drag the ionized plasma through
the embedded neutral gas. The collisions become infrequent
relative to the gyro frequency at some altitude, with charge
exchange interactions persisting to the greatest heights.
Subsequent to a collision, the scattered or newly formed
ion is picked up by the convection electric field, imparting
a perpendicular energy that exceeds the ion thermal
energy for strong convection. Transverse ion heating is
the relatively straightforward result, but it is limited to
energies corresponding to a few km/s or a few eV for O+
ions, and much less for H+ [e.g., Moore et al., 1996].
[53] Plasma waves with substantial power in the electromagnetic ion cyclotron range (EMIC waves) are frequently
observed in the auroral ionosphere, with frequencies well
above those associated with MHD instabilities. These are
thought to result from turbulent cascades of MHD wave
energy, or to be associated with free energy of auroral
current systems, or possibly with the occurrence of certain
types of ion anisotropy, for example ion shell distributions.
EMIC waves or broadband extreme low frequency (BBELF)
waves have sufficient power in the cyclotron range to raise
transverse tails on the ion velocity distributions and are
thought to do so frequently. Some of these are Alfvén waves
propagating into the ionosphere along flux tubes, which are
thought to be effective in heating ionospheric ions, and have
been associated with the most productive region of nightside
outflows at the poleward boundary of the midnight auroral
oval [Tung et al., 2001; Keiling et al., 2001]. These lowfrequency waves are thought to be generated by boundary
layer instabilities or other sources of turbulent motions, such
as reconnection.
[54] At higher plasma wave frequencies, ion transverse
motions are though to be coupled with parallel electron
motions by means of lower hybrid waves [Retterer et al.,
1994; Kintner et al., 1992]. These waves are not thought
capable of influencing the core of the ion velocity distributions, but are thought to create or extend transverse tails of
the distribution to energies well above thermal. The waves
are thought to be current-driven, deriving energy from the
parallel motion of current-carrying electrons.
[55] Recent observations from the FAST Mission have
added many details to our knowledge of auroral transverse
ion heating. These include events in which He+ ions are
resonantly accelerated perpendicular to the magnetic field to
energies of several keV [Lund et al., 1998]. These He+
energizations occur during periods of EMIC waves and
conic angular distributions of up to a few hundred eV in H+
and a few keV in O+. This was identified by Lund et al.
[1998] as a process similar to that which may occur in solar
flares. Lund et al. [1999] also examined FAST observations
indicating that for ion conics produced by interactions with
BBELF waves, the energies of different ion species are
often nearly equal. They suggested that this discrepancy
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with theoretical predictions, which predict preferential
heavy ion acceleration, can be understood through a version
of the ‘‘pressure cooker’’ mechanism. With simple single
particle computations, Lund et al. [1998] showed that for a
significant downward electric field within the transverse
heating region, ions of different masses are energized to
approximately the same energy as required to escape the
downward electric field ion trapping region.
[56] It seems likely that all these processes contribute to
the inferred wide altitude range of auroral ion transverse
heating, beginning in the F region with frictional heating,
extending through the topside with EMIC and LHR wave
heating of ions, with the latter extending to altitudes of
several RE. All of the processes derive energy from the
dissipation of MHD energy in the form of medium frequency
(approximately cyclotron) waves or turbulence, including
the auroral current systems and wave driven by them.
4.5. E// Acceleration
[57] Parallel electric fields occur in the auroral region in
both upward and downward current regions. Their presence
and significance for auroral acceleration had been discussed
for many years, certainly in Scandinavian perspectives, and
evidence for their existence had been found in rocket [e.g.,
Evans, 1974] and satellite [e.g., Frank and Gurnett, 1971]
electron measurements. However, the first systematic and
relatively conclusive measurements of such parallel electric
fields probably came from the measurements in the mid1970s by the S3-3 spacecraft near 1 RE altitude, an altitude
region hitherto largely unsampled. Direct measurements of
parallel electric fields [Mozer et al., 1977] and upgoing ion
beams [Shelley et al., 1976] confirmed the existence of
strong parallel electric fields somewhere below 6000 km
altitude in the auroral zones. Moreover, in addition to the
upward parallel electric field observations, associated with
upward current regions and generally linked with discrete
auroral arcs, S3-3 observations also indicated evidence of
downward electric fields having important effects ion outflows by trapping ions in transverse heating regions for
extended periods of time and thus leading to higher transverse energization [e.g., Gorney et al., 1985].
[58] An example of a recent simulation investigation
which sought to elicit the effects of parallel electric fields
on ionospheric ion outflows is the work of Wu et al. [2002],
mentioned earlier. As noted above, Wu et al. [1999] used a
Dynamic Fluid Kinetic (DyFK) code to simulate the effects
of soft electron precipitation and transverse ion heating on
ionospheric outflows. Wu et al. [2002] added to these
influences a distributed field-aligned potential drop produced self-consistently by a high-altitude (magnetospheric)
population of hot ion and electron populations. When
the presumed upper boundary magnetospheric population
had significant differential anisotropy between the ion
distribution and the electron distribution, significant parallel
potential drops resulted. When the upper boundary ion
population was more perpendicularly peaked than the
electron distribution, such as a trapped type of ion distribution with a more isotropic electron distribution, this led to
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Figure 6. Observed relationship between ionospheric outflow flux in the dayside auroral zone and the
hourly standard deviation of the solar wind dynamic pressure [Fok et al., 2005].
a positive potential peak and downward directed parallel
electric fields. When the upper distribution of magnetospheric electrons was more perpendicularly peaked than that
of the magnetospheric ions, an upward electric field distribution resulted. In both cases, the upward flow of ionospheric plasma into these hot plasma regions gradually
neutralized and reduced the potentials over timescales of
the order 30 min.
[59] In the case of the hot plasma differential anisotropy
circumstances associated with upward electric fields, these
fields of course accelerated the ionospheric ions, which
were first transversely heated above the topside by transverse waves, to parallel beam energies of the order of
100200 eV of energy. In the case of the hot plasma
differential anisotropies leading to downward electric fields,
various types of ionospheric distributions were produced,
including classical conics as well as ‘‘butterfly’’ distributions, in which occurred simultaneous upward and downward conical distributions, at times with ‘‘holes’’ or
absences of particles at the low-energy part of phase space.
These holes presumably were the result of exclusion of
ionospheric ion access by the positive potential barrier.
Some observations of toroidal or quasibutterfly distributions
which were observed by POLAR [e.g., Huddleston et al.,
2000] could possibly be interpreted along such lines,
although other alternative explanations might be involved
as well. While anisotropies clearly require parallel potential
drops, it is field-aligned currents that are thought to
ultimately determine their necessity [e.g., Lyons, 1980].
[60] More recently, instruments aboard the FAST spacecraft have been used to observe effects of parallel electric
fields on ion beams and other ion outflows. For example,
McFadden et al. [1998] exploited very high time resolution
measurements of ion distributions to show kilometer-scale
spatial structures, which involve ‘‘fingers’’ of parallel
electric potential structures which extend hundreds of kilometers along the magnetic field line but are only a few to
tens of kilometers wide. McFadden et al. [1998] were able
to show that integrations of the electric field along the
spacecraft track to calculate the parallel potential below the
spacecraft were typically in reasonable agreement with
the observed ion energy. In another paper based on FAST
measurements in the cusp region, Pfaff et al. [1998] found a
complex mixture of DC electric fields and plasma waves in
association with observed particle accelerations as manifested by upgoing ion beams, ion conics, and related electron
distributions. In a particular case, injections of magnetosheath ions were associated with upgoing ion beams and
electrons going earthward, indicative of potential structures
set up by the magnetosheath injections.
[61] In another result from FAST, Ergun et al. [2000]
analyzed data from the upward current region of the auroral
zone, which inspired Vlasov simulations of the static
particle distributions and potential structures. From
the observed electron distributions, they inferred density
cavities, and regions in which the densities were dominated
by either plasma sheet electrons or cold ionospheric electrons. Ergun et al. [2000] conducted Vlasov calculations
with one-spatial and two-velocity dimensions to obtain
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Figure 7. A flowchart depicting the relationships between energy inputs to the ionosphere and
ionospheric ion outflows detected by FAST near 4000 km altitude. From Strangeway et al. [2005].
large-scale self-consistent solutions of the parallel electric
field. Depending on the ionospheric H+ density level and
other parameters, the electric potential along the magnetic
field lines shows three different potential and plasma
population regions, and includes distinct electron and ion
‘‘transition’’ layers. Such potential distributions could then
be responsible for the upward ion beams emerging from
these types of regions.
4.6. Specifying Global Outflows
[62] To compute and appreciate their consequences for
the solar wind interaction with Earth, as well as their
ultimate loss into the downstream solar wind, it is essential
to specify how these outflows are driven by that interaction.
Prior to the advent of upstream solar wind observations,
efforts focused on the dependence of outflows upon
internal magnetic indices such as Kp (planetary activity),
AE (auroral electrojet), or Dst (storm time disturbance)
[Yau and Lockwood, 1988]. The results were usually stated
in statistical terms of global aggregate outflow (ions/s),
sometimes treating the Earth as a point source, but usually
sorted by longitude and latitude [e.g., Giles et al., 1994].
More recently, outflows have been regarded as directly
driven by solar wind parameters [Pollock et al., 1990;
Moore et al., 1999b; Cully et al., 2003a; Lennartsson et
al., 2004]. It has become clear from these studies, as shown
in Figure 6, that the dynamic pressure of the solar wind is a
very strong driver of outflows, particularly from the dayside
auroral zone where the solar wind interaction is quite direct.
Somewhat surprisingly, the IMF Bz appears to be more
weakly correlated with auroral wind outflows [Pollock et
al., 1990]. While illuminating, these observed relationships
clearly suffer from a large amount of scatter indicating that
multiple processes mediate the interaction with the solar
wind. Any attempt to relate global outflow with typical
upstream solar wind conditions begs the question of how
auroral wind outflows respond to the detailed local distribution of energy inputs into the topside ionosphere gas, as
determined by magnetospheric global dynamics [Andersson
et al., 2004]. Only when such detailed outflows can be
specified as functions of global dynamics can a realistic
global simulation be constructed. Even then, the solar wind
is far from uniform across the magnetopause and the system
responds to prior history as well as instantaneous conditions, and no solar wind sequence is ever repeated exactly.
[63] A significant advance in our ability to observe global
patterns and time variability of ionospheric outflows and
atmospheric escape came with the IMAGE mission and its
neutral atom imagers, including especially the Low Energy
Neutral Atom (LENA) Imager [Moore et al., 2000]. LENA
provides a new capability to both remotely sense ion plasma
heating and acceleration, and to observe the escape of fast
neutral atoms from the atmosphere [Moore et al., 2003].
Using LENA it was confirmed that heavy ion outflows are a
prompt response to solar wind dynamic pressure enhancements [Fuselier et al., 2001, 2002; Khan et al., 2003]. It was
also confirmed that the creation of outflows is strongly
related to internal geomagnetic activity of the magnetosphere [Wilson and Moore, 2005]. A hot neutral oxygen
exosphere was shown to be created by auroral activity
[Wilson et al., 2003]. The related outflows of fast neutral
atoms were initially surprisingly intense [Moore et al.,
2003], but recent theoretical work suggests that this is
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Figure 8. (top) Hemispheric distribution of NeHot, Poynting energy flux, and parallel current density,
mapped to ionospheric heights in the Northern Hemisphere, for a snapshot of the LFM MHD simulation
of the magnetosphere during SBZ. (bottom) The hemispheric distribution of ion Tperp, V//, and escape
flux corresponding to the conditions in the top row (simulation results courtesy of S. Slinker (personal
communication, 2005); ionospheric outflow conditions computed per Strangeway et al. [2005]; R. J.
Strangeway (private communication, 2005) for ETH).
expected when ion acceleration begins sufficiently low in
the ionosphere [Gardner and Schunk, 2004].
[64] Recently, Strangeway et al. [2000, 2005] examined
ionospheric outflows near 4000 km altitude using the FAST
spacecraft in order to understand the factors that drive the
outflows. Zheng et al. [2005] performed a similar study for
the Polar data set. Both found high correlations between the
logarithms of the observed outflow ion fluxes and both
the density of the soft electron precipitation as well as
the electromagnetic energy flux. Both concluded that the
principal effects were Poynting flux and electron precipitation inputs in the low ionosphere, which would be broadly
consistent with incoherent scatter radar observations by
Wahlund et al. [1992] and others, and the DyFK simulations
of Wu et al. [1999]. A summary ‘‘flowchart’’ of their
findings is indicated in Figure 7.
[65] There is a strong internal correlation between the
precipitation and Poynting energy fluxes themselves, and
therefore it is somewhat difficult to separate their effects on
the outflows purely from such statistical observational
methods. Recently, Moore et al. [2007] and Zeng and
Horwitz [2007] concluded that both are necessary at some
level for outflow to occur. Owing to the transit times of ions
from F-region/topside ionosphere to the observation altitude, as the observation altitude increases, spatiotemporal
discrepancies between responsible energy inputs and outflow plumes are likely to be more significant. Such transit
time – produced uncertainties may well have reduced possible correlations, especially at Polar altitudes of 5000 km.
However, the soft electron precipitation and the Poynting
flux appear to have somewhat separable effects on the
outflows. If so, this relationship is consistent with the
concept of ‘‘two-stage’’ processes driving auroral ionospheric outflows, as modeled for instance by Wu et al.
[1999, 2002]. In this paradigm, auroral outflows consist
typically first of ‘‘upwelling’’ in the F-region and topside
ionosphere (driving the mass density of outflows), followed
by stronger energization at higher altitudes (driving the
velocity and therefore escape of outflows). The processes
that are thought to drive ‘‘upwelling’’ at low altitudes, such
as soft electron precipitation-driven ionospheric electron
heating and Joule/frictional ion heating, may propel significant fluxes to higher altitudes but only to relatively small
energies (typically below 1 eV), which are too small alone
to cause O+ escape. Thus a second energization stage, for
instance involving wave-driven transverse ion heating processes, at altitudes down extending toward 1000 km or even
lower, is then required to produce significant fluxes of
escaping O+ ions that could overcome the gravitational
barrier, approximately 10 eV from the Earth’s surface, and
attain magnetospheric altitudes.
[66] Another important factor in the acceleration of ions
to magnetospheric energies and altitudes is the parallel
electric field, generally considered to be significant in the
altitude range of 1 RE, as discussed above. From a global
magnetospheric perspective, E// is a relatively low altitude
influence on the properties of outflowing auroral ions. E// is
thought to be determined largely by the sense and magni-
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Figure 9. Velocities of H+, He+, and O+ during a substorm
dipolarization event observed by DE-1 in the middle
magnetosphere [Liu et al., 1994].
tude of the local parallel or Birkeland current density [e.g.,
Lyons, 1980; Carlson et al., 1998]. High-altitude flow shear
(or vorticity) producing J// away from the ionosphere, and
exceeding a threshold current density that is easily supplied
by low-density magnetospheric electrons, implies parallel
potential drop that increases with the current density. In
contrast, when the current J// flows toward the ionosphere, it
is relatively easily carried by ionospheric electrons, and
implies quite small potential drops.
[67] In Figure 8, we exhibit the inner boundary conditions relevant to auroral wind outflows, mapped to ionospheric heights, from the LFM simulation as a snapshot
during an interval of southward IMF. In Figure 8 (bottom),
we have translated these boundary conditions into ion
outflow conditions using relations based on those of
Strangeway et al. [2005] and Zheng et al. [2005]. Here
we have assumed that the total ion flux has superposed
contributions from the Poynting energy flux and from the
hot electron precipitation density, but a geometric mean has
proven more realistic [Moore et al., 2007; Zeng and Horwitz
2007]. We have taken the Poynting flux to drive the ion
temperature, and we have taken the parallel current density
to drive the parallel ion velocity according to the modified
Knight relationship of Lyons [1981]. It can be seen that the
result is highly structured outflow distribution, in both flux
and velocity, and it can readily be appreciated that this
pattern is highly variable in response to system dynamics as
simulated by a global MHD simulation.
5.
GLOBAL CIRCULATION
5.1. Centrifugal Acceleration
[68] The terminology ‘‘centrifugal acceleration’’ for
convection-driven acceleration of high-latitude ionospheric
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outflows was introduced by Horwitz [1987a], while the
basic effect itself was identified by Cladis [1986], Mauk
[1986], and Delcourt et al. [1988] as a curvature drift. The
effect may be described [e.g., Horwitz, 1987b] as similar to
the acceleration of a bead moving easily on a rotating stiff
wire, in which the bead is flung radially outward along the
wire as a result of the rotation. In the case of diverging
magnetic field lines of the Earth, convection can be viewed
as a quasirotation of charged particles along these field
lines, again effectively accelerating them outward toward
higher altitudes in the magnetosphere.
[69] Several authors have included and investigated the
significance of this effect on high-latitude and magnetospheric particle motion. For example, Horwitz et al. [1994]
examined the possible effect of centrifugal acceleration on
the polar wind in a kinetic ionospheric transport simulation,
and found that such acceleration can in principle explain
observed polar electron density profiles and elevated O+
densities for moderate convection speeds. However,
Demars et al. [1996] calculated that the effect of centrifugal
acceleration on the polar wind H+ may be slight for normal
convection values. Centrifugal acceleration has been incorporated in numerous single particle trajectory studies of
ionospheric and other particle flow into and through the
magnetosphere [e.g., Delcourt et al., 1990, 1994]. Also, the
one current global MHD model of magnetospheric dynamics
[Winglee, 1998; Winglee et al., 2002] that incorporates the
effects of ionospheric plasma outflow into the magnetospheric dynamics at present relies chiefly on centrifugal
acceleration as the agent for energization of ionospheric
plasma to be ejected into the magnetosphere.
[70] Arguably the most compelling discrete observational
evidence for centrifugal acceleration of ionospheric
ions comes from the observations of energized H+, He+,
and O+ ions in the middle magnetosphere during substorm
‘‘dipolarization’’ events [Liu et al., 1994], as illustrated in
Figure 9. Liu et al. [1994] observed energized ionospheric
ion species having similar parallel (and convection) velocities, and thus characteristic energies roughly in the proportion
H+:He+:O+ = 1:4:16. This is a telltale characteristic of
centrifugal acceleration and should be observed when it
dominates other processes.
5.2. Fountain Flows
[71] The concept of a localized fountain of outflowing
ionospheric ions ejected into a horizontal wind of convection was introduced in the form of the ‘‘cleft ion fountain’’
(CIF) during the mid-1980s [Lockwood et al., 1985;
Horwitz and Lockwood, 1985]. These formulations were
based on observations of low-energy ionospheric ions from
DE-1 in the region of the cleft and on into the polar cap
lower-intermediate altitude magnetosphere. Observations
and corresponding simulations depicted a cleft-associated
source of upward moving ions that spread into the polar
cap magnetosphere under the influence of antisunward
(or sunward) convection.
[72] Among the interesting phenomena observed and
simulated were the effects of separation of ion species,
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Figure 10. Simulated variations of the O+ densities and velocities along a noon-midnight oriented
convection trajectory with the horizontal distance and altitude. Vertical bars on the bottom x axis mark the
locations of the equatorward and poleward boundaries of the cusp/cleft region. (Reprinted/modified from
Tu et al. [2005], with permission from Elsevier.)
termed the ‘‘geophysical mass spectrometer’’ [Moore et al.,
1985], or ‘‘geomagnetic mass spectrometer’’ [Lockwood et
al., 1985] and ‘‘parabolic’’ heavy ion flows [e.g., Horwitz,
1984, 1986; Horwitz and Lockwood, 1985]. The picture of a
‘‘fountain’’ followed from the discovery of the geophysical
mass spectrometer effect, in which midaltitude spacecraft
observe, in sequence ‘‘downstream’’ from the cleft source,
first H+, then He+, and then O+ as the spacecraft moves
antisunward from near cleft threading magnetic field lines
into the polar cap region. This results because, for similar
thermal energies, light H+ ions have larger field-aligned
velocities than do heavier He+ and O+, while for all energies
and all species the horizontal convection velocity is the
same. Thus the faster H+ ions tend to remain closer to the
source field lines, while the slower O+ ions are transported
further into the polar cap by convection during the transit
time required to attain similar spacecraft altitudes. ‘‘Parabolic’’ flows, involving upward and then downward motion
of O+ ions with energies below the gravitational escape
barrier, were indicated in the trajectory modeling of Horwitz
[1984], and the downward O+ flows in the polar cap were
observed near 1 RE altitude by Lockwood et al. [1985].
[73] Soon thereafter, Chappell et al. [1987] incorporated
the cleft ion fountain concept, together with polar wind and
‘‘auroral ion fountain’’ sources, to consider whether circulation of ionospheric origin plasma into the magnetosphere
could account for the observed magnetospheric plasmas.
They concluded that indeed this was the case and in
particular that the ionosphere was a ‘‘fully adequate source’’
for the plasma sheet. Several other efforts have employed
large-scale trajectory models to track ionospheric particles
through the magnetosphere, including Delcourt et al. [1989,
1994] Ashour-Abdalla et al. [1992], Peroomian [2003], and
Fok et al. [1999], seeking to account for observed beams
and other specific ionospheric ion distributions in the larger
magnetosphere and also under the influence of centrifugal
acceleration and related effects. An example from a recent
effort [Tu et al., 2005] is used to visualize the auroral
plasma fountain in Figure 10.
[74] One of the ongoing issues concerning ‘‘fountain
flows’’ is the question of whether or not the cleft ion
fountain, or more generally the auroral fountain, is the
dominant source of observed O+ in the polar cap magnetosphere. Additional observations of frequent downward
flows of O+ in the main polar cap under southward IMF
conditions [e.g., Chandler, 1995] have led others [e.g.,
Horwitz and Moore, 1997] to at least tentatively conclude
that indeed the cleft ion fountain, and not the polar cap
ionosphere, must be the source of the O+ in the polar cap
magnetosphere proper, at least during southward IMF when
the polar cap is not normally threaded by transpolar arcs that
might be sources of energized upflowing O+. More recently,
papers by Tu et al. [2004, 2005] have demonstrated that
sophisticated ionospheric plasma transport modeling based
on the CIF paradigm can produce close agreement
with observed polar cap density profiles and from observed
O+ field-aligned velocity and density profiles seen near
5000 altitude from cross-polar spacecraft passes. However,
other authors [e.g., Abe et al., 2004] maintain that the solarilluminated polar cap ionosphere can directly supply the O+
observed in the midaltitude polar cap magnetosphere,
through effects such as photo-electron acceleration of the
polar wind and/or elevated effective electron temperatures
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(such as through effects of the polar rain [e.g., Barakat and
Schunk, 1983]).
[75] As indicated previously, the idea that photo-electron
effects can accelerate relatively low altitude heavy ionospheric ions to escape energies comes in large part from
papers by Tam et al. [1995, 1998], whose modeling calculations suggested that O+ could attain supersonic speeds
at altitudes as low as 500 km. However, as also noted
earlier, these predicted low-altitude photo-electron acceleration effects have never been identified in observations, and
other efforts on modeling the photo-electric polar wind by
Khazanov et al. [1997], Wilson et al. [1997], and Su et al.
[1998a] have instead indicated that photo-electron acceleration occurs at relatively high altitudes of 2– 3 RE, in conjunction with a thin double layer of tens of volts. Hence we
maintain that the cleft, or more broadly the auroral, ion
fountain, is the principal source of O+ in the polar cap
magnetosphere ‘‘proper,’’ that is, on magnetic field lines
not threaded by polar cap/transpolar arcs such as occur during
northward IMF conditions. However, the most powerful
evidence that photothermal outflow cannot in itself supply
significant heavy ion plasmas at high altitudes is the wellknown light ion dominance of the plasmasphere, a region
that is supplied almost exclusively by the sunlit low latitude
ionosphere.
5.3. Lobal Wind
[76] The polar and auroral winds are easily distinguished
in terms of their causative agents, but they are not so easily
distinguished when observed from a given point in geospace. Owing to the fountain flow effects discussed in
section 5.2, ions are dispersed according to their differing
parallel velocities, and are thus selected for velocity by
the geophysical mass spectrometer effect. The result is a
mixture of polar wind and auroral wind that varies substantially with location within the high-latitude regions of
magnetospheric circulation, but tends to have a restricted
range of parallel velocities in any particular spot [Horwitz et
al., 1985]. This effect is accentuated for auroral wind ions
that are emitted from a region of relatively narrow extent
embedded within magnetospheric convective flow, as
described above, creating the geophysical mass spectrometer
effect.
[77] The resultant mixture flows outward through the
magnetospheric lobes, in part supplying the plasma sheet
with relatively cold source plasmas, and in part escaping
downstream beyond the region convecting earthward. To
preserve the distinction between polar and auroral winds,
we have adopted the term ‘‘lobal wind’’ for the resultant
mixed plasmas that flow out of the polar cap into the lobes
of the magnetosphere, and then convect into the plasma
sheet or escape beyond.
[78] Recently, Liemohn et al. [2005] have surveyed the
properties of the lobal wind observed from the Polar
spacecraft and determined that it is a pervasive feature of
the lobes, providing a continuous supply of cool plasmas to
the central plasma sheet inside the location of any persistent
reconnection X line. The unidirectional outward flows are
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transformed into bidirectional interpenetrating streams from
the two lobes as they convect onto closed field lines. These
bidirectional streams are isotropized and strongly heated
when the neutral sheet thins during periods of higher
activity. This behavior is associated with the onset of
spatially nonadiabatic trajectories as the plasma sheet thins.
Temporal nonadiabatic behavior is also triggered when
reconnection or other effects change the neutral sheet on
gyroperiod timescales for specific ion species.
5.4. Geospace Storms
[79] Downstream of the midtail plasma sheet, during
normal slow magnetospheric convection, lies the magnetopause along the dawn and dusk flanks of the magnetosphere. When dayside reconnection complements nightside
reconnection, the plasma circulation is drawn deeply
through the inner magnetosphere toward the subsolar
magnetopause region. Then heated ionospheric and solar
plasmas supply the inner magnetosphere and ring current
region, and a lesser or greater geospace storm results,
depending upon the strength and persistence of these
effects, combined with the number and frequency of substorm dipolarizations that heat and compress plasma sheet
plasmas into the inner magnetosphere.
[80] Mitchell et al. [2003] have shown that substorms, in
particular, modulate primarily the heavy ion component of
ring current hot plasmas, while the proton component is
relatively unresponsive to them. Moore et al. [2005a] have
argued that this is a natural consequence of the different
circulation paths of solar wind and auroral wind contributions to the ring current. In the global test particle simulations they performed in MHD fields, solar wind proton
entry occurred principally through the low-latitude flank
magnetopause boundary layers, with direct transport into
the ring current region. In contrast, auroral wind O+ was
transported via the traditional route through the polar caps
and lobes, into the plasma sheet, and then back to the inner
magnetosphere via the midnight plasma sheet. They argued
that this implies a greater substorm dipolarization effect on
plasmas of ionospheric origin. Recent work with simulated
substorms has confirmed this conclusion [Fok et al., 2006].
[81] The amount of auroral wind O+ in the energetic
plasmas of the ring current is a direct function of the
magnitude of that current [Sharp et al., 1985, 1999, 1994;
Daglis and Axford, 1996; Daglis, 1997; Moore et al., 2001,
2005b]. Recently, Nosé et al. [2005] have studied the
October 2003 superstorm and reviewed its place in the
existing observations of storm plasma composition, as
shown in Figure 11. They show that a trend line can be
identified in which the ratio of O+ to H+ pressure increases
exponentially with the magnitude of storms as measured by
Dst. The pressure ratio rises from near zero for very small
Dst and passes through unity at Dst 200, making the
origin of the largest geospace storm plasmas primarily
ionospheric.
[82] Energetic neutral atoms (ENA) imaging from the
IMAGE mission has confirmed that the principal mechanism of ring current decay is charge exchange production
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Figure 11. Relationship between ring current composition and magnitude of the ring current as
measured by Dst or Sym-H (limited Dst). The ‘‘Halloween 2003’’ event appears at the top right of the
plot [Nosé et al., 2005].
of fast neutral atoms that immediately escape from the
magnetosphere [Burch et al., 2001]. Thus even that part
of the auroral wind outflow that is trapped within the
magnetosphere and fails to escape to the downstream solar
wind eventually manages to escape through charge exchange
(on cold geocoronal hydrogen) back to a neutral state. Since
the Earth is such a small object within the realm of the ring
current, a small fraction of such fast atoms will precipitate
back into the atmosphere, and most of them will escape into
the solar wind.
5.5. Plume Flows
[83] The polar wind fills the plasmasphere to pressure
equilibrium with the ionosphere and then shuts off in the
stagnant, corotating, inner magnetosphere. Beyond the
plasmasphere, magnetospheric circulation carries flux tubes
away to the dayside magnetopause and boundary layers.
Some of these flux tubes are opened up to the solar wind by
dayside reconnection and then dragged downstream through
the polar caps or low-latitude boundary layers. Viscosity in
the ionosphere spreads this motion to neighboring flux
tubes even if they are not actually opened up by reconnection, and these flux tubes are also stretched dramatically as
they flow down the low-latitude boundary layers.
[84] Magnetospheric circulation varies greatly in its depth
of penetration into the inner magnetosphere, depending on
the synchronization, or lack thereof, between dayside and
nightside reconnection. When reconnection is weak, and
circulation is restricted to the outer magnetosphere, the
plasmasphere grows larger as flux tubes fill with plasma,
with longer timescales for the outer, longer flux tubes at
greater radius. When reconnection increases simultaneously
on the dayside and on the nightside, the night-day plasma
pressure gradient increases and deep convection occurs
through the innermost magnetosphere. Then plasma built
up on outer flux tubes of the plasmasphere is entrained in
the flow and transported to the dayside magnetopause where
it enters the reconnection regions there, or becomes part of
the low-latitude boundary layer flows.
[85] This process was hypothesized in the late 1960s and
early 1970s [Grebowsky, 1970], and shown then to create
a plume of plasma leaving the outer plasmasphere and
moving toward the postnoon sector of the dayside magnetopause. Borovsky et al. [1997] and Elphic et al. [1997]
further hypothesized that such transient release of plasma
from the plasmasphere could create superdense polar wind,
and possibly a superdense plasma sheet. Evidence of this
was observed during events when the magnetopause was
compressed to geosynchronous orbit [Su et al., 2000, 2001],
but our understanding of this process was greatly expanded
when the IMAGE spacecraft was able to image the formation of such plumes [Sandel et al., 2001], permitting
detailed studies of their formation and variability [Goldstein
et al., 2002].
[86] More recently, direct observations of cold convecting
plasmas have been obtained at the normal equatorial magnetopause [Chandler and Moore, 2003]. Chen and Moore
[2004] found further that sunward flow bursts approaching
the local Alfvén speed were observed in these flows,
correlated with southward IMF in the upstream solar wind.
More recently, statistical studies [Chen and Moore, 2006]
have shown that the occurrence and sense of convection
of these flows is fully consistent with global convection
as driven by dayside reconnection, and its variation
with interplanetary magnetic field, Bz. Figure 12 gives an
overview of these results in terms of the distribution of cold
convecting plasmas observed in the dayside magnetopause
region.
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Figure 12. Distribution of cold convecting ions at the dayside magnetosphere for (a) northward and
(b) southward IMF. After Chen and Moore [2006].
[87] The significance of these plasmas is that they have
not been observed systematically before, yet they substantially reduce the Alfvén speed of the inflow to the lowlatitude reconnection region at the dayside magnetosphere.
Freeman et al. [1977] have previously noted that this
could create a negative feedback on deep magnetospheric
convection, whereby reconnection would be slowed if a
sufficient amount of ionospheric or plasmaspheric plasma
was convected to the dayside subsolar region. This may be
related to recent interest in the saturation of the transpolar
potential [Siscoe et al., 2002a]. More recently, it has
been shown that enhanced densities of plasma, especially
cold protons, will produce essentially a ‘‘cessation of reconnection’’ [Hesse and Birn, 2004]. O+ ions are observed to be
part of the hot magnetospheric plasma when arriving at the
dayside magnetosphere [McFadden et al., 2003]. This is
consistent with most O+ outflow being driven by auroral
processes, as part of what we have called the auroral wind.
Such hot plasmas may also load the dayside reconnection
processes, especially for heavy ions.
5.6. Global Simulations
[88] We have learned from observations that ionospheric
plasma permeate the magnetosphere and come to dominate
even the energetic particle populations during great geospace storms. Many of the groups performing global simulations have begun to modify their codes to take account of
ionospheric plasmas as circulating dynamical elements of
the system, literally above and beyond their role as a
resistive medium in the thin F layer. The most needed
improvements are, first, the specification of ionospheric
outflows in terms of energy inputs and, second, the simple
inclusion of multiple fluids in global simulations [Winglee,
1998, 2000]. This will slow the calculations of such
simulation codes proportionately to the number of fluids.
However, we have made the case earlier that it is a serious
oversimplification to assume that the ionospheric load is
well described by friction in the topside ionosphere, that is,
that no ionospheric energy appears in the magnetosphere
proper, when the observed fact is that ionospheric pressure
dominates the largest storms.
[89] Nevertheless, the full task is more difficult than it
appears because heavy ions are nonadiabatic in many
magnetospheric regions. Even when they are reasonably
adiabatic, their transport departs substantially from the
convective paths for cold particles. We must move in the
direction of accounting for ion drifts and finite gyroradius
effects, which will ultimately require hybrid simulations,
with fluid electrons and particle ions. In the meantime,
progress is being made in understanding and assessing the
character of ionospheric circulation in the magnetosphere
using global simulation fields to compute the motions of test
particles [Winglee, 2003; Peroomian, 2003; Cully et al.,
2003b; Moore et al., 2005a, 2005b].
[90] In Figure 13, we illustrate this work with an example
result from Cully et al. [2003b]. Here the supply of ionospheric plasmas to the plasma sheet has been assessed in
aggregate as a function of time during two different step
changes in the IMF, one a northward turning and the second
a southward turning. It can readily be seen that an abrupt
northward turning results in a sharp and relatively prompt
cutoff in the supply of O+ ions to the plasma sheet. This is
because convection in the lobes stops and reverses direction
in response to this change. In contrast, when the IMF turns
southward, convection through the lobes is enhanced, but
enhanced ion outflow delivery to the plasma sheet increases
gradually and with a delay owing to the transport time of
relatively slow O+ ions from their source regions.
[91] An important point made by Cully et al. [2003b] is
that auroral wind outflow through the lobes is constant and
ongoing, though the magnitude of the flux can vary greatly
in response to ionospheric energy inputs. Even though the
transit time from the ionosphere to the plasma sheet is long,
the outflow stream responds instantaneously to changes in
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Figure 13. Simulated supply of O+ to the CPS in the region 30 RE < x < 5 RE, jyj < 10 RE during a step
change in the convection field. The electric field changes at time t = 0. Supply is cut off by a northward
turning, and enhanced with a delay for southward turning [Cully et al., 2003b].
magnetospheric convection or configuration. Cully et al.
suggest further that a cutoff in the supply of O+ to the nearEarth magnetotail may deprive the region of current carriers
and cause a reconfiguration of the magnetic field in that
region as a result, with possible implications for substorm
dynamics [Lyons et al., 1998]. Cully et al. did not consider
an abrupt compression of the magnetosphere, as would be
produced by a sudden increase in solar wind dynamic
pressure. Such compression would likely have an energizing
and intensifying effect on the plasma sheet and the auroral
winds feeding it, with no significant transport delay. Simulation of this is best done with MHD fields for which the
global response can be calculated self-consistently, as
reported by Moore et al. [2007].
[92] In addition to supplying current carriers, the presence
of polar and auroral winds in the magnetosphere also
represents a moderating load on the system, and this appears
when they are treated as true dynamical elements of the
global simulation, as noted by Winglee et al. [2002]. While
illustrating the processes of transport and energization that
operate on polar and auroral wind plasmas, current single
particle simulations must in the future be adapted to run
within self-consistently calculated global fields so that their
responses to dynamical solar wind events can be properly
assessed.
6.
COMPARATIVE PLANETARY ABLATION
[93] It is interesting to compare the situation at Earth with
our best knowledge of other planets. Within the solar
system, the gas giant planets with large magnetospheres
are so massive and cold that the loss rates of their atmospheres are negligible, even though hydrogen dominates
them, accounting for the tremendous thickness of their
atmospheres compared with those of the inner terrestrial
planets. Replete with moons of varying sizes and compositions, the outer planet magnetospheres tend to be dominated
by gas and heavy ion plasmas escaping from those satellites
rather than hydrogen escaping from the planets themselves
[Belcher, 1983; Belcher et al., 1990]. The focus of interest
in ablation is then upon the atmospheres of the moons.
These are generally well shielded from direct solar wind
impact by the large magnetospheres of these planets. Still,
substantial amounts of energy derived either from the solar
wind or the rotation of these large magnetospheric systems
is clearly present, and it generates robust populations of
energetic particles comparable to or far exceeding the
intensity of the Van Allen radiation belts at Earth [Bolton
et al., 2004; Jun and Garrett, 2005]. The moons are
themselves highly diverse, with or without their own
magnetospheres. As this is written, a handful of flyby
missions have briefly explored the gas giant magnetospheric
systems in the past, and the Saturnian system is now being
explored by the orbiting Cassini Mission. New results are
appearing rapidly, so the detailed exploration of gas giants
and their satellite systems is therefore beyond the scope of
this review.
[94] Among the terrestrial planets, only Earth has a
planetary magnetic field strong enough to hold the solar
wind off at about 10 times the planetary radius for typical
solar wind conditions. Earth is also unique in having
acquired and retained substantial amounts water in all
forms. Mercury has a relatively weak magnetic field that
holds the solar wind off the planet for average conditions,
but stronger solar winds are directly incident on the planet,
beginning in the magnetic cusps and spreading with
increasing solar wind dynamic pressure [Delcourt et al.,
2002]. Mars has scattered regions of remnant crustal magnetization [Connerney et al., 1999], which will produce
features in the solar wind interaction, but do not hold the
solar wind off Mars’ ionosphere. Venus has no measurable
magnetodynamo so the solar wind is continuously incident
upon the ionosphere [Luhmann, 1991].
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[95] Mars and Venus are both are believed to have
become deficient in water relative to their earlier states.
Their solar wind interactions have been observed extensively
[Lundin and Dubinen, 1992; Brace and Kliore, 1991] and
are known to involve considerable rates of loss of water
products into space. The Hermean solar wind interaction
has not been observed since Mariner 10 [Glassmeier, 1997],
but is about to be revisited by the NASA Messenger
Mission. Mercury is believed to lack any significant water
or water product escape and has an exosphere in which
sodium is prominent [Killen and Ip, 1999].
[96] It is tempting to hypothesize causality in the magnetized Earth’s retention of water, but we should first
consider what we have learned about the expansion of
Earth’s ionosphere into space. Water loss begins with
photodissociation of water in the upper atmosphere. Photothermal expansion of water products into space is oxidizing
for larger planets that trap heavier species. Smaller planets
like Mars may be an exception. Solar wind ablation tends to
balance the loss of light species, or to be reducing (when
oxygen loss exceeds light species loss). Jeans escape and
photothermal escape via the creation of an ionosphere are
common to the inner planets. Mars and Mercury are smaller
and can lose heavy species such as O+ photothermally.
Venus (and Earth) loses only lighter species to any significant degree owing to photothermal effects, but solar wind
ablation may be active at Venus, though different in
character from Earth [Fok et al., 2004].
[97] The solar wind is directly incident upon the ionospheres of the unmagnetized planets, Mars and Venus,
exposing dayside ionospheres to an energy flux on the
order of 0.3 mW m2. A number of processes are active
in these cases, including charge exchange between atmospheric atoms and solar wind ions, impact ionization of
atmospheric atoms by solar wind electrons, and some highaltitude photoionization [Breus et al., 1991; Brecht et al.,
1993; Kallio et al., 1997, 2006]. Some atoms and ions are
sputtered to escape energy by direct collisions with solar
wind ions, and ions are picked up by the interplanetary
electric field. All of these processes contribute to the
contamination of the planetary magnetosheath by planetary
material that is lost from the planet.
[98] Similar fluxes (scaled for distance from the Sun) are
incident upon the regions of the ionosphere conjugate with
the magnetospheric cusps for the magnetized Mercury and
Earth. On the other hand, an appreciable fraction of the solar
wind energy incident upon the entire magnetospheric cross
section (15 – 20 RE radius) is dissipated in the terrestrial
auroral zones, with energy fluxes at the ionosphere ranging
from 3 to 100 mW m2. The lower end represents a typical
value for average conditions, while the upper end represents
very active solar wind and aurora. Thus the energy flux
available for heating and ablation of ionospheric plasmas is
10 to 300 times larger in the localized auroral zones than it
would be if the solar wind were simply incident upon an
unmagnetized Earth, as it might possibly be during a
geomagnetic field reversal.
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[99] Given that heavy ion expansion out of the gravitational well of the planet is dependent upon auroral levels of
energy flux, it might seem that an unmagnetized Earth
would lose only lighter species such as H and He, while
the magnetized Earth loses substantial quantities of N, O,
and the diatomic molecules of those species. However,
studies of Mars and Venus [Cravens, 1991; Luhmann and
Kozyra, 1991; Fok et al., 2004] (the latter based on the
MHD simulation of Tanaka and Murawski [1997] with
embedded aeronomy models) indicate that they suffer
nearly as much total heavy ion and heavy atom loss as
the Earth does, and more steadily over time. The interplanetary magnetic field diffuses into the dayside ionosphere of
Venus and couples energy from the solar wind, even though
there is no planetary magnetic field. Thus it appears that the
solar wind finds ways to carry off as much of a planetary
atmosphere as is possible (given its momentum flux),
independent of planetary magnetization, though this needs
much more investigation.
7.
EXTRASOLAR PLANETS
[100] So far, more than 160 extrasolar planets have been
identified (J. Schneider, Interactive Extra-Solar Planets
Catalog, http://www.obspm.fr/planets, retrieved on 4 November
2005), and almost all of them have masses comparable to
(within a factor of 5) or larger than Jupiter. About 20% of
the planets are classified as close-in extrasolar giant planets
(CEGP) (or ‘‘Hot Jupiters’’), as they orbit their center stars
within a distance of 0.10 AU or less, noting that the smallest
star-planet separation distance identified to date is 0.021 AU
[Rivera et al., 2005], equivalent to 4.5 solar radii. Magnetic
interaction between CEGPs and their host stars was first
postulated by Cuntz et al. [2000] and later confirmed by
Shkolnik et al. [2003] for HD 179949 through the detection
of temporal variations of Ca II H, K line emission in the
stellar plasma, which were found to be synchronized with
the planetary orbit rather than the stellar rotation.
[101] Direct observations of planetary atmospheric plasma
flows, revealed through hydrogen Lyman-a [Vidal-Madjar
et al., 2003], oxygen and carbon [Vidal-Madjar et al.,
2004], and sodium detections [Charbonneau et al., 2002]
have been obtained for the planet of HD 209458, which
constitutes the ‘‘best studied to date’’ extrasolar star-planet
system. Vidal-Madjar et al. [2003] concluded that the
Lyman-a absorption feature is formed outside the planetary
Roche limit, thus constituting evidence of escaping planetary
hydrogen plasma. As a way of visualizing the planet in
question, these authors developed an artist’s conception,
which is shown in Figure 14, accompanied by their principle
result plot.
[102] A further possible consequence of planetary plasma
flows and evaporation is the existence of ‘‘close-in hotNeptunes,’’ which have been found around GJ 436, r Cancri,
and m Ara and are believed to be caused by extreme stellarinduced mass loss [Baraffe et al., 2004, 2005] of formerly
intact Jupiter-mass planets. This interpretation is strongly
supported by planetary evolution studies, which in particu-
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Figure 14. (left) Total intensity of lines as function of the orbital phase for extrasolar planet HD
209458. Circles, squares, triangles, and diamonds are for first to fourth transits, respectively. Vertical
dotted lines correspond to the position of the first to fourth contacts of the planetary disk [Vidal-Madjar et
al., 2004] (reproduced by permission of the AAS). (right) Artist’s conception of the extrasolar planet of
HD 209458.
lar take into account effects of stellar irradiation. An even
more extreme outcome of this type of process is suggested
by observational evidence that CEGPs can even be engulfed
by their host stars. Israelian et al. [2001] reported the
presence of the rare 6Li isotope in the stellar atmosphere
of HD 82943. This isotope is normally destroyed in the
early evolution of solar-type stars, but is preserved intact in
the atmospheres of giant planets. In fact, the 6Li isotope has
hitherto not reliably been observed in any metal-rich star
besides HD 82943. The authors interpret this finding as
evidence for a planet (or planets) previously engulfed by the
parent star.
[103] While most of the extrasolar planets discovered to
date are in circumstances that are far more extreme than
those of the Earth in our own solar system, they serve as
reminders of the range of possible conditions over which we
would like our theories to be applicable. As observing
capabilities are refined, future discoveries are likely to
reveal planets that are much more similar to the terrestrial
planets.
8.
FUTURE MEASUREMENT REQUIREMENTS
[104] This section deals with the requirements for future
progress in the observation of ionospheric outflows as
driven by stellar wind energy dissipation. Some of the areas
of theory and simulation of ionospheric outflows that appear
to be ready for near-term rapid advances include the
following.
8.1. Removing Low-Energy Plasma Obstacles
[105] Recent observations have largely overcome the
problems of observing cold plasmas from spacecraft in
sunlight, owing to photoelectric charging. Another problem
has been the prevalent assumption that cold plasma densities were so low that they were negligible, which made it
difficult to justify the expenditure of resources to measure
them. These problems delayed a beginning survey of
the polar wind circulation throughout the magnetosphere
from the first theoretical predictions in the 1960s until
the Dynamics Explorer-1 (DE-1) mission in the 1980s
[Chappell, 1988]. Akebono, Polar, and Cluster have established the technology [Moore and Delcourt, 1995; Comfort
et al., 1998; Singh et al., 2001; Torkar et al., 2001] to
actively control spacecraft floating potential, leading to
successful tracing of polar and auroral winds from the
ionosphere to the magnetotail [Moore et al., 1997; Su et
al., 1998b; Liemohn et al., 2005; Huddleston et al., 2005].
On the Cluster mission, Active Spacecraft Potential Control
(ASPOC) is an ion emitter that effectively neutralizes positive spacecraft charging in sunlight. By avoiding emission
of electrons, it creates a benign amount of plasma wave
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noise, and appears to improve the accuracy of double probe
electric field measurements [Eriksson et al., 2006] in lowdensity polar wind regions. This technology is included in
plans for the Magnetospheric MultiScale Mission currently
entering development.
8.2. Observing the Heating Mechanisms
[106] Sounding rockets offer direct low velocity probing
of topside ionospheric heating processes, albeit with limited
scope and duration. They have contributed a great deal to
our knowledge and should certainly continue with new
strategies and observing technologies. The Polar Outflow
Probe (POP) [Yau et al., 2002] spacecraft is being developed for the Canadian Space Agency by the University of
Calgary Institute for Space Research, and will explore
ionospheric outflow origins in the altitude range from
300 to 1500 km, where energy goes into creating mass flux
(and where photoelectric charging is tolerable). The science
objectives of POP are to quantify the microscale characteristics of plasma outflow in the polar ionosphere and proberelated microscale and mesoscale plasma processes at
unprecedented resolution, and explore the occurrence
morphology of neutral escape in the upper atmosphere.
This type of mission is much needed to better understand
the fundamental causes of ionospheric outflows so they can
be predicted on the basis of their causal physics rather than
through empirical scaling.
8.3. Determining Exospheric Structure and Variability
[107] Recently, owing in large part to the IMAGE mission
[Burch, 2005], it has been possible to better observe and
understand the ionospheric topside, geocoronal, and plasmaspheric media that supply gas and plasma to magnetospheric reservoirs encompassing the plasmaspheric plumes,
polar lobes, plasma sheet, and ring current regions. Both the
gas and plasma of the ionosphere expand into the magnetosphere [Gardner and Schunk, 2004, 2005], impeded
mainly by gravity. The gas creates a relatively steady, nearly
spherical geocorona of light hydrogen gas with a scale of a
few Earth radii. The denser gas species form a smaller
geocorona or exosphere with a height scale that about
100 km at normal F-layer heights but increasing with
altitude above that in a highly variable way that depends
on the amount of energy deposition into the auroral ionosphere. The actual configuration and response of the oxygen
exosphere has been very poorly known, but considerable
information has recently been obtained by the IMAGE
mission and especially the LENA imager [Moore et al.,
2003; Wilson et al., 2003; Shematovich et al., 2005].
8.4. Resolving Dynamic Escape Processes
[108] The heavy ion plasmas are normally confined to the
F layer proper, similar to the heavy gas atoms. Exceptions
are important in the auroral ionosphere, where oxygen flows
locally in an auroral wind with fluxes greatly exceeding
those of the steadier and more widespread polar wind [Cully
et al., 2003a, 2003b; Strangeway et al., 2005]. The time
variations of auroral wind have been poorly known until
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recently, when considerable information has been obtained
on the drivers of such outflows by the IMAGE/LENA
imager [Wilson and Moore, 2005; Fuselier et al., 2006].
Plasmas in the magnetosphere are heated strongly by
geospace storm events such that they substantially inflate
the inner magnetosphere, producing a ring current that adds
to the Earth’s dipole moment [Moore et al., 2001; Seki et al.,
2001; Cully et al., 2003a, 2003b]. Recently, it has been
found that this ring current is overwhelmingly composed of
O+ from the Earth during the largest geospace storm events.
This surprising result grew out of a collaboration between
the Geotail investigators and the IMAGE LENA Imager
[Nosé et al., 2005; Moore et al., 2007].
8.5. Imaging Missions
[109] The IMAGE mission, at nearly 6 years of age and
long past its design lifetime of 2.5 years, unfortunately
failed a few days before this paper was submitted to
Reviews of Geophysics. After pioneering many aspects of
geospace imaging, including those based on energetic
neutral atoms from the ring current, the Polar Mission is
scheduled to terminate its full solar cycle of operations in
the next month or two. Owing in large part to the transformation of NASA to revive human space flight, no geospace
imaging missions are currently in development or have firm
plans for the future, though several are ‘‘in the wings’’ and
waiting for a suitable opportunity. There is a pressing
need for future magnetospheric and auroral imaging to
better understand the terrestrial atmospheric contribution
to geospace storms. Imaging has proven so fruitful in
understanding how our planetary atmosphere is disturbed
and ablated by solar wind energy inputs that it has become
an essential feature of the toolbin of both space meteorologists and space weather forecasters.
8.6. Extending Observations Beyond the Geosphere
[110] An important priority for the space exploration
initiative will be the exploration of planetary atmosphere
ablation by the solar wind, which is thought to have been an
important factor in the evolution of the Martian atmosphere
[Lundin et al., 2004; Vaisberg et al., 1995, 2005]. As noted
above, Venus is thought to lose atmosphere at a rate
comparable to that of the Earth, but this is based largely
on the Pioneer Venus Orbiter, which did not have instrumentation designed to observe such loss. Moreover, this
comparison needs an update for recent observations of the
Earth’s ablation rate. A number of strategic studies have
placed high priority on aeronomy probes for Mars and
Venus, but the current fascination appears to be with
Mercury, which has a magnetosphere but very little in the
way of an atmosphere. In the past, planetary exploration
missions have understandably focused on the condensed
parts of the planets. Two current missions, the ESA Mars
Express and Venus Express, are now making strides toward
a better understanding of how those planets compare with
the Earth in terms of solar wind coupling. In the future,
missions may devote more resources to the study of effects
that could have a long-term evolutionary impact on those
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planets, now that those effects are better appreciated as a
result of closer study at Earth.
8.7. Extending Observations Beyond the Heliosphere
[111] There is no foreseeable opportunity for in situ
measurements of an extrasolar planet, but the planets being
remotely sensed are slowly becoming more Earth-like as
technology advances and smaller planets become more
detectable. It is hoped that the synergy between extrasolar
planetary studies and in situ measurements of our own
planet and its nearby neighbors will be fully exploited by
researchers. Though the techniques are vastly different at
present, they clearly have the capability to inform each
other.
9.
FUTURE THEORY REQUIREMENTS
[112] Theory and modeling progress in ionospheric outflows, as with other phenomena, is in large part driven and
inspired by new observational findings, such as those
projected in the previous section. For example, advances
in spacecraft potential control may enable new comprehensive findings on the characteristics and behavior of <2 eV
ions in regions such as the high-altitude polar cap, tail lobes,
and plasma sheet. This will inspire and suggest new
directions of theory and simulation to understand and
replicate those observational discoveries. Some of the areas
of theory and simulation of ionospheric outflows that appear
to be ready for near-term rapid advances include the
following.
[113] 1. Auroral field-aligned plasma transport can be
better simulated by incorporating ionospheric plasma and
energy production and loss and auroral and kinetic processes
[e.g., Wu et al., 1999, 2002; Tu et al., 2004, 2005]. This
would be done by implementing more sophisticated, selfconsistent treatments of auroral processes involving finestructured parallel electric fields, for example, kinetic
Alfvén waves and solitary waves [e.g., Ergun et al., 1998;
Singh, 2002] and waves which cause transverse heating,
including ion cyclotron and lower hybrid waves [e.g., André
and Yau, 1997].
[114] 2. Molecular ion outflows in the auroral regions
[e.g., Peterson et al., 1994; Wilson and Craven, 1999] have
thus far not been systematically treated with transport
models, but can readily be added to multifluid codes.
[115] 3. Plasmaspheric drainage plumes, involving substantial ionospheric F-region plasma densities, are observed
convecting toward the cleft region and across the polar caps
following large magnetic storms [e.g., Foster et al., 2002].
These should be incorporated into ionospheric plasma
transport models of the cleft ion fountain and global
magnetosphere. Foster [2005] has suggested that such large
F-region densities in these drainage plumes could be
sources of plasma content, in particular heavy ions, for
the cleft ion fountain at these times.
[116] 4. Improved ionospheric plasma transport models
and processes can be incorporated as ionospheric outflow
plasma sources in global magnetospheric circulation and
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dynamics models. At the present time, the global magnetospheric dynamics model of Winglee et al. [2002] has
pioneered the incorporation of ionospheric plasma sources,
but more and more researchers are reporting efforts to move
in a similar direction. There is room for improvement in the
manner in which ionospheric plasma outflows occur in such
models. As a first step, it would be possible to use empirical
specifications of ionospheric response to magnetospheric
inputs [Strangeway et al., 2005; Zheng et al., 2005], as
reported by Moore et al. [2007] and Fok et al. [2006].
Ultimately, we will need an embedded model of the
ionosphere and thermosphere, with all appropriate physics
included, within a global dynamical simulation.
[117] 5. The inner magnetosphere is centrally important to
terrestrial space weather. It is common for global simulation
codes to omit the inner magnetosphere from the simulation
space, setting an inner boundary at a radius of 3 –4 RE. This
is mainly for reasons of resolution, though curvilinear and
adaptive calculation grids have largely rendered that reason
obsolete, and indeed, one of the codes now extends down
all the way through the ionosphere proper [Maynard et al.,
2001], albeit with a single fluid that can originate in either
the ionosphere or the solar wind. While this cannot readily
distinguish the separate roles of plasma from the ionosphere and solar wind, it allows for realistic momentum
and energy flows, which may ultimately be the most
important consideration.
10. CONCLUSIONS AND OUTSTANDING
PROBLEMS
[118] In this review we have focused upon observations
of the expansion of our ionized upper atmosphere into
geospace, and the theories that have been developed to
explain those observations as well as their consequences.
We assert that observations and theory have reached sufficient convergence to form a strong basis for predicting the
behavior of similar systems under different conditions and
in different contexts. In particular, the knowledge we have
reviewed can be used to anticipate and predict the behavior
of the diverse other planets throughout our own solar
system, and for conditions around other stars, to the degree
that their stellar winds and intrinsic magnetic fields and
atmospheres have observed or inferred properties. This
includes, for example, the practical exercise of anticipating
terrestrial space weather changes during the next geomagnetic field reversal, which may well be underway in view of
recent trends in the internal field of Earth.
[119] Thus this review should be understood in the
general context of coupling between a stellar atmosphere
and the volatile matter embedded within it as part of
a planetary system. Prior to stellar ignition, the overall
situation is dominated by gravitational accretion with rotational dynamics reflecting the initial angular momentum of
the system. Once stellar ignition occurs, however, the
emanated UV energy flux in general ionizes these atmospheres into fully or partially ionized plasmas, with higher
charge states and complete stripping of electrons from
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Figure 15. A schematic plasma flowchart of the heliosphere-geosphere interaction. Flow color coding
runs from dotted line (energy) to solid line (mass), with dashed line for both (mass and energy). Flow
dependences on IMF are indicated by labels above the arrows.
atoms emitted from the star itself. Energetic photon fluxes
then extend throughout the planetary system and ionize
much of the diffuse gas surrounding the more condensed
objects. Once the system contains a substantial amount
of plasma, electromagnetic effects and magneto-plasma
dynamic forces become important in coupling the entire
system together, and we expect to see telltale signs of
nonequilibrium activity, including reconnection, plasma
heating, and superthermal particle acceleration, together
with neutral atom and photon emission.
[120] As an aid to visualizing the global circulation of the
various plasmas in the geosphere, we offer Figure 15 as a
schematic diagram or plasma flowchart. The flows begin
with the upstream solar wind and the creation of an
ionosphere and light ion plasmasphere by solar photons.
The solar wind boundary layers carry the bulk of solar wind
around the geosphere, but entry occurs in the cusp/cleft and
low-latitude boundary layer regions. The high- and lowlatitude features are divided top to bottom in this schematic
to fit the three-dimensional structure on the page without
adding unnecessary complexity. The overall circulation is
controlled by IMF orientation via the distribution and effect
of dayside magnetic reconnection. Northward IMF Bz
interrupts the normal flow of boundary layer flow away
from the subsolar region, interrupting and diverting the
high-latitude flow cell to the low-latitude boundary layers,
closing open flux from the polar lobes and shrinking the
polar caps, while also thickening and drawing plasma from
the plasma sheet, as indicated by arrow labels. This also
interrupts the supply of the plasma sheet from the highlatitude or auroral wind circulation, switching the source to
the low-latitude circulation cells, which are fed by solar
wind plasmas. Southward IMF has the reverse effect of
strengthening the high-latitude flow cells in comparison
with the low-latitude cells.
[121] Over the past 2 decades, considerable progress has
been made in the study of atmospheric ablation by heliospheric influences, leading to the following conclusions.
[122] 1. Both the solar wind and polar wind contribute
light ion plasmas to the magnetospheric coupling region.
During typical conditions (average solar wind properties;
spiral IMF with small Bz) a substantial contribution of
ionospheric plasma with significant heavy ion content
(mainly O+ but also N+ and molecular species during high
activity) is supplied via the auroral wind, to a degree that
varies in response to energy dissipation as a strong function
of solar wind dynamic pressure.
[123] 2. For events with strong northward IMF, the
auroral wind is reduced and circulated into the low-latitude
boundary layers and downstream rather than over the poles
into the magnetotail.
[124] 3. For events with strong southward IMF, the
eroded plasmasphere (supplied by polar wind) supplies
significant amounts of plasma to the dayside afternoon
magnetopause region, resulting in an interaction with
magnetospheric global circulation as driven by reconnection.
[125] 4. Substorm effects in the near-Earth midnight
sector significantly energize and enhance the auroral wind
plasmas that return to the inner magnetosphere via the
midnight plasma sheet. The solar wind and polar wind
proton plasmas are less influenced by substorm effects.
[126] 5. The storm time ring current becomes increasingly
auroral wind dominated with increasing ring current energy
density, implying that these enhancements of the quiet time
ring current are supplied with charge carriers largely
through atmospheric ablation by solar wind energy. Essen-
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tially, the storm time ring current is produced by the
pressure of heated ionospheric plasmas seeking to escape
from magnetospheric confinement.
[127] 6. The expanding ionosphere is lost into the downstream solar wind in both the dayside and nightside reconnection regions, where closed magnetospheric flux tubes
are first opened up to the solar wind, stretched into the
tail with acceleration of the plasmas residing in them, and
then reconnected after substantial plasma expansion has
occurred, beyond the nightside reconnection region.
[128] 7. Ionospheric plasmas that are retained in the inner
magnetosphere are ultimately lost, mostly by escaping
outward after being freed by charge exchange.
[ 129 ] Among the outstanding problems stemming
from the expansion of ionospheric plasma into space,
the following pose the highest priority requirements for
improved observations and theory: (1) continuing awareness of the importance of relatively cold plasma populations
in the magnetosphere, as reflected in attention to spacecraft
floating potential measurement and control; (2) a global
theoretical and/or empirical model of ionospheric topside
response to solar wind electromagnetic energy and particle
precipitation inputs, including auroral potential structures
associated with circulation shear and field aligned currents;
(3) theory and observations of the dynamic role of
the extended ionosphere in mass loading, wave propagation,
and particle acceleration within the magnetosphere;
(4) theory and observations of the extended ionospheric
pressure that inflates the storm time magnetosphere and
escapes during the largest storms; and (5) theory and
observations of the role of extended ionospheric density in
moderating plasma and particle acceleration, and enhancing
particle losses from the magnetosphere, through both classical processes and plasma effects such as wave-particle
interactions.
[130] Beginning with the surprising discovery of O+ ions
in the magnetosphere [Shelley et al., 1972], the SolarTerrestrial Physics community has undergone a paradigm
shift that transformed our understanding of the storm time
plasmas of the magnetosphere. Once thought of as the result
of entry of the intense solar wind, storm-enhanced plasmas
are now seen to result from the dissipation of solar wind
energy in the terrestrial atmosphere and ionosphere, with
resultant expansion. The key to this understanding began
with ideas about ionospheric expansion in the polar wind
[Banks and Holzer, 1969], but early suggestions that the
magnetosphere was filled with hot plasmas accelerated from
the polar wind were found wanting [Hill, 1974]. With the
understanding that a substantial auroral wind blows as a
result of solar wind energy inputs, a spirited debate began
between those who advocated [Chappell et al., 1987]
and those who questioned [Ashour-Abdalla et al., 1992;
Borovsky et al., 1998] a substantial contribution of ionospheric material to the hot magnetospheric plasmas. With
more and more observations both of large auroral wind
fluxes and substantial pressures of auroral wind O+ in the
ring current, it gradually became clear that global circulation
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models would need further development, since the ionospheric load is not confined to a thin boundary layer.
Accurate description of a magnetosphere in which the
ionosphere extends throughout the system will require
models with ionospheric fluids or particles as active dynamic
components.
[131] We have emerged from this transformation with
ample evidence and community acceptance that the ionosphere expands to the magnetospheric boundaries and
escapes continually into the downstream solar wind, its
composition and partial pressure varying with solar wind
drivers. Updated ionospheric models now produce the
observed heavy ion outflows from solar wind energy inputs.
We also have promising new or revised global circulation
models that incorporate the ionosphere as an extended load
within the system, and we are learning that this load can be
felt all the way out to the boundary layer reconnection
regions. There is much additional work to be done to work
out the consequences of this shift in our understanding, but
we are now well equipped for this work and also well
equipped to use our new simulation tools to predict and
understand the behavior of diverse other planetary systems
within and beyond our solar system.
[132] ACKNOWLEDGMENTS. We are indebted to Manfred
Cuntz of the University of Texas at Arlington for valuable inputs to
the section on extrasolar atmospheric ablation. This work was
supported by Polar/TIDE under UPN 370-08-43, IMAGE/LENA
under UPN 370-28-20, and by NASA ROSES under UPN 432-01-34
at Goddard Space Flight Center, by NASA grant NNG05GF67G,
and NSF grant ATM-0505918 to the University of Texas at
Arlington.
[133] The Editor responsible for this paper was Peter Riley. He
wants to thanks two anonymous technical reviewer and one
anonymous cross-disciplinary reviewer.
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J. L. Horwitz, Department of Physics, University of Texas at
Arlington, 502 Yates Street, Box 19059, Arlington, TX 76019, USA.
(horwitz@uta. edu)
T. E. Moore, Heliospheric Physics Branch, Code 612.2, NASA
Goddard Space Flight Center, Greenbelt, MD 20771, USA. (tmoore@
pop600.gsfc.nasa.gov)
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