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Ring Current Behavior as Revealed by Energetic Proton Precipitation
F. Søraas, K. Aarsnes and D.V. Carlsen
Department of Physics and Technology, University of Bergen, Norway.
K. Oksavik
Department of Physics, University of Oslo, Norway.
D.S. Evans
NOAA Space Environment Center, Boulder, Colorado, USA.
The precipitation of energetic ions and electrons into the upper atmosphere is a
direct manifestation of their acceleration and pitch angle scattering in the magnetosphere. Electric fields inject/convect the particles from the tail plasma sheet
towards the earth, and when closer to the Earth they spread in local time due to
magnetic field forces. The electrons drift towards the morning sector and the ions
towards the evening sector thus creating the ring current. Certain aspects of the ring
current behavior can be revealed by the precipitating energetic protons. From these
particles a proxy for the energy injection rate into the ring current can be estimated,
and a ring current index which correlates highly with the pressure corrected Dst*
can be calculated. The pitch angle distribution of the precipitating ring current protons is either isotropic with a filled loss cone, or anisotropic with an almost empty
loss cone. In the isotropic zone the ring current protons are stable to wave growth.
In the anisotropic zone, however, the protons are unstable to wave growth. Thus,
there exists a fairly wide L-value interval equatorward of the isotropic zone with
ample conditions for EMIC (electromagnetic ion-cyclotron) wave generation.
In the anisotropic zone a number of wave-particle phenomena linked to the precipitating protons take place: enhanced proton pitch angle scattering manifested as
intensity peaks at mid-latitudes, SAR arc formation, Pc1 and IPDP wave generation, and increased loss of relativistic electrons. An important decay process for the
ring current protons is through charge exchange. The ENAs (Energetic Neutral
Atoms) from this process create a well defined belt or region of ENA and protons
observed at low altitudes along the geomagnetic equator. This belt reveals important aspects of the ring current such as: the ring current injection region, the drift
of ring current particles, and convection losses of the ring current particles to the
dayside magnetopause, and its asymmetric and symmetric behavior.
Magnetosphere Modeling
Geophysical Monograph Series XXX
Copyright 2004 by the American Geophysical Union
10.1029/XXXGMXX
1
2 LOW ALTITUDE PROTON OBSERVATIONS
1. INTRODUCTION
The aurora is created when energetic electrons and ions
excite the atoms and molecules in the upper atmosphere.
These atoms and molecules then emit light and aurora is displayed on the dark sky. The energetic charged particles in
the magnetosphere are governed/take part in a number of
processes. All these processes imprint their mark on the
particle population. Thus by examining the particles, their
energy and pitch angle distribution and their behavior in time
and space, it is possible to gain some insight into the physical mechanisms that are operating.
The main physical cause for the ground magnetic perturbations at low latitudes, Dst, is the variability of the ring current
composed of energetic ions and electrons encircling the Earth
at altitudes of several Earth radii. Due to their larger mass the
ions contribute by far the most to the ring current energy density. There have been many suggestions on how the ring current particles are injected. For a review see Tsurutani and
Gonzalez [1997] and McPherron [1997]. It is generally
believed that the ring current is created by a combination of
substorm particle injection and enhanced convection. When
injected from the plasma sheet into the ring current, ions will
be accelerated and precipitate to the atmosphere. During their
injection ions are subjected to intense pitch angle scattering
in the region of strong field line curvature that scatters or
“demagnetizes” the protons [Sergeev et al., 1983]. An isotropic pitch angle distribution with a filled loss cone is the
result. This process is most intense in the midnight/evening
sector. Using the “splash-catcher” model advocated by
O’Brien [1964], where the loss cone is populated by some of
the protons which are injected into the ring current, Søraas
et al. [2002] used the intensity of these precipitating protons
as a measure for the ring current energy injection rate.
Low altitude satellite observations give a magnified view
of processes taking place within and close to the atmospheric
loss cone. In the following we will concentrate on protons
and discuss how observations of these particles at low altitudes can reveal information on the ring current (RC).
2. INSTRUMENTATION
The present study uses observations from the MEPED
instrument on board the NOAA 12 and 15 satellites. Protons
with angles 10° and 80° with the local vertical are measured
in three energy channels ranging from 30 to 800 keV, and
electrons above 30, 100 and 300 keV are measured at the
same angles. The MEPED instrument cannot distinguish
between different ion types. We therefore use the term protons in this study. The NOAA 12 satellite DOME instrument
can detect relativistic electrons with energies above about
1.5 MeV whenever high energy protons with energies above
13 MeV are not dominant. The orbits of both spacecraft are
circular at an altitude of about 850 km. At this altitude the
atmospheric loss cone above 50° geographic latitude is
around 50°. The 10°detector thus looks well into the loss
cone, while the 80°detector looks at trapped protons. A full
description of the satellites and their instrumentation are
given by Raben et al. [1995] and Evans and Greer [2000].
3. PROTON INJECTION AND THE RING
CURRENT INDEX
Figure 1 shows data from a typical NOAA satellite pass
during the recovery phase of a storm in March 1998. Protons
with angles 10° (solid line) and 80° (dashed line) to the local
vertical are each measured in three energy channels 30-80,
80-250 and 250-800 keV.
Both the 10° and 80° protons have a well defined poleward
boundary, where the particle intensities drop two orders of
magnitude when the satellite enters the polar cap. On the equatorward side the 10° protons exhibit a clear boundary. The
region where the proton intensities at 10° and 80° are about
equal is well defined and marks the isotropic precipitation
zone. The equatorward border of this zone is called the
isotropic boundary (IB). Protons in the isotropic zone are subjected to intense pitch angle scattering and those inside the loss
cone will penetrate into the atmosphere and be lost. The protons thus exhibit two precipitation zones, one poleward zone
where the pitch angle distribution is isotropic, and an equatorward one where the pitch angle distribution is anisotropic, that
is with a partly filled loss cone. This precipitation pattern has
been described in a number of papers [Hauge and Søraas,
1975; Lundblad and Søraas, 1978; and others].
Plate 1 shows the intensity (color-coded) of protons precipitating into the evening sector from October 7 to
November 6 in 1998. The measured proton intensities in each
NOAA 15 pass are plotted vs. ILAT (invariant latitude) and
time. The three top panels show protons well within the loss
cone for three different energy channels. The three next panels show the same energy channels for the 80° detector, and
the bottom panel shows Dst. The data refer to evening/midnight, as this is the MLT (magnetic local time) sector that
most directly exhibits the injection of protons into the ring
current.
The time period considered contains one large (−111 nT)
and two smaller (−60 and −55 nT) geomagnetic storms. The
intensity and latitudinal extent of the protons correlates with
the Dst. Note that during every negative deviation in Dst there
is increased proton precipitation and an equatorward movement of the precipitation zone, indicating a deeper injection
of protons into the magnetosphere. As the size of the disturbance increases the equatorward border moves to lower
latitudes and the intensity of the precipitation increases.
SØRAAS ET AL. 3
Figure 1. Typical dusk-to-dawn NOAA pass in the Northern Hemisphere. Protons with angles 10° and 80° with the vertical are
measured in three energy channels ranging from 30 to 800 keV. The intensity of the 80° protons is shown by a dashed line, and the
intensity of the 10° protons is shown by a solid line.
As seen from the three top panels the precipitating
protons exhibit a clear IB. Poleward of this boundary the
precipitation is isotropic, and equatorward of it the proton
pitch angle distribution is anisotropic. Plate 1 gives a clear
picture of the two-zone structure of the protons observed at
low altitudes.
From Plate 1 it is seen that the intensity for both the 10°
and 80° detectors exhibit a daily modulation, most prominent
in the 80° detector. This behavior is due to the magnetic field
control of the particles and that a satellite in low orbit
observes particles on the same L-shell at different magnetic
field strengths during the course of the day [Berg and Søraas,
1972].
Søraas et al. [2002] showed that the protons precipitating
within the isotropic zone in the evening/midnight quadrant
can be used as a proxy for the particle injection into the ring
current. The injection rate Q(t) is not based upon solar wind
parameters but directly on the observed proton precipitation
rate. This is done under the assumption that the proton intensity in the loss cone follows the rate at which protons are
injected into the ring current. The protons in the loss cone do
not merely represent a loss from the ring current but are, in
fact, a measure of the proton injection rate into the ring current. From the precipitating protons, Søraas et al. [2002]
derived a RC index (ring current index) which is not influenced by magnetic fields generated by magnetopause, fieldaligned, and tail currents (as Dst is). The RC index should
give a “true” picture of the proton energy content in the ring
current.
We will now show how the low altitude observations
obtained during the larger October 1998 storm relate to Dst
and the solar wind parameters measured by the Wind
satellite. The relations between the different parameters are
illustrated in Plate 2.
4 LOW ALTITUDE PROTON OBSERVATIONS
Plate 1. Intensity of protons precipitating into the evening local time sector from October 7 to November 6 in 1998. Each NOAA 15
orbit is plotted vs. ILAT and the observed proton flux is color-coded. The three top panels show precipitating protons in the energy
channels 30 to 80 keV, 80 to 250 keV and 250 to 800 keV, and the following three channels give the same information for the mirroring protons. The bottom panel shows Dst during this time.
SØRAAS ET AL. 5
Plate 2. Parameters related to the larger storm taking place on day 292 in 1998; in the top panel the Dst* and the ring current index,
in panel 2 the proton injection rate Q(t), and in panel 3 the AE index. In the next four panels the interplanetary electric field component Ey, the IMF Bz component, the solar wind density Nsw and velocity Vsw are shown.
6 LOW ALTITUDE PROTON OBSERVATIONS
In the top panel of Plate 2 the Dst* (the pressure corrected
Dst) and the RC index are shown. The correlation between the
two ring current estimates is 0.94 for this 20-days period. The
RC index is normalized to have the same average value as the
Dst*. The two indices follow each other very well, and the six
injection events taking place in the storm recovery phase are
clearly seen in both indices.
In panel two the injection rate Q(t) as estimated from
NOAA 15 satellite observations is shown. There is a small
increase in Q(t) on day 290 coinciding with a small decrease
in Dst*. One day later there is a similar increase in the injection rate and a decrease in Dst*. On day 292 there are two
large increases in the injection rate, and the Dst* decreases to
-111 nT in the storm main phase. The storm has an initial fast
recovery back to -50 nT during this period when the injection
rate is reduced. It could be argued that this initial fast recovery of the Dst* reflects a contribution from the cross tail current, which has a rapid dynamic [Alexeev et al., 1996].
Subsequently the storm exhibits a slow recovery with multiple proton injections each followed by a decrease in Dst*.
During the 10-days recovery there are six distinct injection
events with increased Q(t), each followed by a decrease in
Dst*. The slow recovery of this storm is due to a HILDCAA
(High Intensity Long Duration AE Activity) event [Tsurutani
and Gonzalez, 1987], characterized by the prolonged
AE activity displayed in panel three. This event and its
HILDCAA behavior have been discussed by Sandanger et al.
[2004]. The AE activity is often taken as a measure for substorm activity. The AE activity and the injection rate Q(t)
have very similar time development and both increase for
every one of the six decreases in Dst* during the storm
recovery phase.
In the next four panels of Plate 2 the interplanetary electric
field component Ey, the IMF Bz component, the solar wind
density Nsw, and velocity Vsw are shown. Notice the large
positive Ey component and the intense AE activity during the
storm main phase. This increased electric field and high substorm activity transport particles from the plasma sheet and
deep into the magnetosphere creating the main-phase ring
current.
3.1. Summary Ring Current Index
The calculated RC index gives a good estimate for the
Dst*. The correlation between the two quantities is high, indicating that they vary in concert and that the ring current
energy is for a large part due to protons being injected from
the plasma sheet. It further shows that the corrected Dst* is
a direct measure of the particle energy content in the ring
current. The absolute energy content in the ring current,
however, cannot be calculated by our method. In order to
obtain the same average value as Dst* our estimate has to be
normalized. Nevertheless, the normalizing factor calculated
for different times is almost invariant. This indicates that the
ring current and the proton precipitation are intimately connected. Due to the good correlation between the estimated
and measured Dst, it would be possible to use observations
from the NOAA satellites to derive a space-based equivalent
of the Dst index in near real-time without many of the problems associated with the ground-based Dst currently in use
[Søraas et al., 2002].
4. THE STORM TIME EQUATORIAL BELT
The production of ENAs by charge exchange of ring current ions with neutral hydrogen in the geocorona was predicted by Dessler et al. [1961] and is an important loss
process for the ring current. The energy range 20-200 keV
includes the carriers of the major part of the ring current
energy. The first observational data on ENA precipitation
giving rise to low latitude protons was obtained in 1969 and
1970 from the AZUR satellite [Moritz, 1972; Hovstadt et al,
1972]. Moritz [1972] suggested that these ions at low L-values
near the equatorial plane come from ENAs born in charge
exchange of ring current ions. The ENAs originating from
higher L-values may reach low altitudes where they are reionized by charge exchange and become energetic ions
trapped by the magnetic field. Tinsley [1981] has given a
comprehensive review of these equatorial ions and some of
their consequences for the equatorial atmosphere.
Søraas et al. [2003] studied the STEB (Storm Time
Equatorial Belt) for a number of geomagnetic storms. In this
report we will consider the STEB behavior during a major
geomagnetic storm. The intense (Dst = −358 nT) geomagnetic storm starting on March 31, 2001, is an example of
such a storm. Figure 2 display the observations of the particles measured at the magnetic equator arranged by the local
time (LT) of observation.
In the main phase of the storm the low L-value particles
exhibit a clear LT asymmetry. The intensity in the midnight/evening sector is markedly higher than in the postnoon/morning sector (two top panels). A delay in the
appearance of the particles with LT is also apparent. The (30
to 80 keV) particles appear simultaneously at 02 and 19 LT.
There is, however, a delay before they appear in the morning
sector (14 and 07 LT). As seen from the Dst index, the March
storm exhibits the double main phase depression typical for
large storms. This double structure is also clearly seen in the
intensity of the low-latitude particles. It appears that the ring
current injection region is more widespread in LT during
the first RC injection than in the second one. The intensity
in the first injection is about equal at 02 and 19, but the
second injection is more concentrated around 19 LT. In the
bottom panel the ratio between the intensities measured at
SØRAAS ET AL. 7
that there is a continuous injection of energy into the ring
current also during the storm recovery phase.
4.1. Summary STEB
In Plate 3 the different aspects of our STEB observations
have been illustrated. It shows how the particles in the storm
main phase are injected from the tail and gradient/curvature
drift through the midnight/evening sector to produce the
asymmetric part of the ring current. While they drift and
charge exchange, some ENAs are sent towards the low-altitude equatorial region of the Earth, where they are ionized
forming the STEB. During this phase of the storm the ring
current is subjected to heavy convection loss. As time progresses the convection field disappears and the ring current
develops into a symmetric belt that decays through charge
exchange and wave-particle interaction. The ENA flux is a
function of ring current intensity and its spatial distribution.
The protons at low L-values will experience a rapid decay
due to charge exchange processes in the dense geocorona.
Because of their fast decay and slow azimuthal drift the proton intensity in the STEB is determined by their source, the
charge exchange of ring current protons at larger L-values.
The LT extension and intensity of the low latitude belt is thus
an “image” of the ring current. From observations of the
STEB the ring current injection region, its evolution through
the evening/afternoon and into the morning sector, its asymmetry during the initial and main phases, and its development
into a symmetric ring current in the recovery phase of the
storm can all be clearly seen and followed.
Observations of STEB at several local times provide
unique opportunities to follow the build up and decay of the
ring current. Multiple satellites makes it possible to get a fair
time resolution of the STEB and thus of the ring current.
Figure 2. The top four panels show how the intensity of trapped
protons in the energy range 30 to 80 keV varies with time at low
L-values through the March 31 storm. Each panel refers to a different local time (LT), and data from both NOAA 15 and 16 are shown.
The Dst index is in the fifth panel, and the bottom panel gives the
ratio of the intensities measured at LT 19 and LT 07. To guide the
eye a straight line is drawn at a ratio of 1 in the bottom panel.
19 and 07 LT is shown. This ratio (an asymmetry indicator)
exhibits large values at 06 and 18 UT on day 90 concurrent
with the injection event seen in the Dst index. This occurs
simultaneous with the large values of the asymmetry indicator shown in panel five. The STEB thus responds to the
asymmetric ring current. In the storm recovery phase the
intensities at all local times decay in a similar fashion with
about equal intensities, even though the evening intensity is
slightly above the morning intensity. This probably indicates
5. WAVE-PARTICLE INTERACTION
During the recovery of geomagnetic storms, highly localized regions of enhanced proton (ion) precipitation can
appear at mid-latitudes well inside the anisotropic precipitation zone. The particle pitch angle distribution in these
enhanced regions is most often anisotropic with maximum
intensity perpendicular to the magnetic field [Søraas et al.,
1999].
In Figure 1 an example of increased proton intensity at
mid-latitudes is shown. Such enhanced regions of precipitation embedded in the anisotropic zone are most likely due to
protons scattered by resonant wave-particle interaction at or
near the plasmapause, as discussed by Lundblad and Søraas
[1978], and represent a loss of ring current particles in the
recovery phase of a geomagnetic storm. They further showed,
using data from the ESRO I satellite, that these regions with
8 LOW ALTITUDE PROTON OBSERVATIONS
Plate 3. Schematics of the ring current behavior during the different phases of a geomagnetic storm, as revealed by the STEB. The LT
for the NOAA 15 and 16 orbits are shown.
SØRAAS ET AL. 9
increased proton precipitation were closely connected with
SAR arc formation.
Yahnina et al. [2003] have done studies relating these midlatitude enhancements to geomagnetic pulsations of Pc1 and
the IPDP (Intervals of Pulsations with Diminishing Periods)
types. Both Pc1 and IPDP pulsations are believed to be the
electromagnetic ion-cyclotron waves generated in the equatorial plane by the unstable proton angular distribution. These
waves scatter energetic protons in pitch angle, so those
authors concluded that the precipitation patterns at mid-latitudes are the particle counterparts of ion-cyclotron waves.
These observations of particles and waves support the
Cornwall et al. [1970, 1971] theory for the generation of ioncyclotron waves and SAR arc formation. It was suggested by
Thorne and Kennel [1971] that ion-cyclotron waves can precipitate relativistic electrons in the E ~1 MeV range. If this
theory is applicable, one would expect to observe relativistic
electrons collocated with the increased proton precipitation
taking place in the anisotropic zone.
An example of such a collocation of protons and relativistic electrons is shown in Figure 3. The event took place on
day 207 during the recovery phase of a −50 nT geomagnetic
storm that had its main phase on day 204. The three panels in
the figure display from the top: electrons with energies
>30 keV, relativistic electrons with energies >1500 kev, and
protons in the energy range 30 to 80 keV. The protons shown
in the bottom panel exhibit the mid-latitude enhancement
typical for the storm recovery phase. This enhancement
occurred in both hemispheres at the same ILAT (63°) and at
approximately the same MLT (~ 06).
The >1500 keV electrons exhibit increased precipitation in
the region of anisotropic proton precipitation. The relativistic
electron precipitation starts immediately equatorward of the
IB (isotropic boundary) in both hemispheres and is confined
within the region of anisotropic proton precipitation. In the
Southern Hemisphere the relativistic electrons exhibited a
very narrow peak displaying a sudden intensity increase of
nearly an order of magnitude above its uniform background
Figure 3. Morningside NOAA 12 pass from north to south (crossing the Equator). The three panels display from the top; electrons
with energies >30 keV, omnidirectional relativistic electrons with energies >1500 keV, and protons in the energy range 30 to 80 keV.
In panels one and three the flux of 80° particles is shown with a dashed line, and the 10° particles with a solid line.
10 LOW ALTITUDE PROTON OBSERVATIONS
precipitation level. In the Northern Hemisphere the relativistic electrons also exhibit a peak in precipitation, but not as
pronounced as in the conjugate hemisphere. The increased
precipitation of relativistic electrons is coincident with the
increased loss of protons. The >30 keV electrons show no
direct spatial resemblance to the protons or the >1500 keV
relativistic electrons.
5.1. Summary Waves and Particles
Relativistic electrons execute nearly circular drift orbits
around the Earth in the L-value range 3 to 7. These orbits are
within the region of the ring current protons. Both the
isotropic and the anisotropic zones are traversed by these
electrons during their drift. Observations support the conclusion that there is an almost one-to-one correspondence
between the precipitation (loss) of relativistic electrons and
the anisotropic proton precipitation.
In the anisotropic zone the protons are unstable to wave
growth. The instability causes pitch angle diffusion, particle
precipitation and the generation of ion-cyclotron waves. Thus
there exists a fairly wide L-value range equatorward of the
isotropic zone with ample conditions for EMIC wave generation. According to Thorne and Kennel [1971] one should
expect relativistic electrons to be parasitically scattered into
the loss cone by ion-cyclotron waves generated at or near the
plasmapause by the unstable proton population. It is in the
midnight/evening MLT sector that the proton drift paths
intersect the plasmapause and they are expected to be
strongly unstable. Our observations show that there can be
intense precipitation of relativistic electrons collocated with
the proton precipitation peak also in the morning sector, and
it is interesting to observe that the relativistic electron precipitation can take place over the whole anisotropic proton
zone; a fairly wide region in L-value.
Proton precipitation in the anisotropic zone can also exist
without any evidence of relativistic electron precipitation,
but on the other hand, whenever relativistic electron precipitation is present it is always embedded in the anisotropic
proton zone. The close correspondence between protons
and relativistic electrons suggests that the electrons are
scattered into the loss cone by EMIC waves. Spatial peaks in
proton precipitation have been shown to be associated with
SAR arc formation [Lundblad and Søraas, 1978], the
plasmapause, and Pc1 and IPDP wave generation [Yahnina
et al., 2003]. During the recovery phase of the storm the
location of the proton peak moves towards higher ILAT in
accordance with the plasmapause [Søraas et al., 1999].
This later observation further strengthens the connection
between precipitation of relativistic electrons and EMIC
waves generated at the plasmapause and in the anisotropic
proton zone.
Acknowledgments. The authors thank the Norwegian Research
Council for financial support. Furthermore, we thank the Wind
spacecraft team and CDA Web for providing solar wind data.
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F. Søraas, K. Aarsnes, and D.V. Carlsen, Department of Physics
and Technology, University of Bergen, AllÈgaten 55, N-5007
Bergen, Norway. (E-mail: [email protected]; kjell.aarsnes@
ift.uib.no)
K. Oksavik, Department of Physics, University of Oslo, P.B. 1048
Blindern, N-0316 Oslo, Norway. (E-mail: kjellmar.oksavik@
fys.uio.no)
D.S. Evans, NOAA Space Environment Center, 325 Broadway,
Boulder, CO 80305, USA. (E-mail: [email protected])