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Ch 8 Magnetosphere
• Dungey 1961
– Open magnetosphere model
• Axford and Hines 1961
– Closed magnetosphere model
1
As the magnetized solar wind flows past the Earth, the plasma interacts with
Earth’s magnetic field and confines the field to a cavity, the magnetosphere.
2
The Magnetopause
The solar wind compresses the terrestrial magnetic field into a blunt-nosed
shape on the dayside, forming the dayside magnetopause inside the bow shock.
The location of the magnetopause is
determined by a balance between the
dynamic pressure of the solar wind
and the magnetic field pressure:
The night-side magnetosphere is drawn
out into a long tail structure. The
magnetotail consists of two lobes of oppositely-directed magnetic flux -- S.
hemisphere flux is directed away from the Sun, N. hemisphere towards the
3 Sun.
The shock represents a
discontinuity in the
medium of the solar
wind.
Solar Wind
450 km s-1
The solar wind
is deflected by
the obstacle
presented by
the earth's
field at the
boundary
designated the
magnetopause
The solar wind flows across the bow shock in
front of the earth where it is slowed to
subsonic velocities.
IMAGE
In crossing the
shock, the solar
wind plasma is
slowed down to
~200 km s-1 and
the loss of K.E.
is dissipated as
thermal energy,
increasing
the
temperature to 5
x 106 K. (about
5-10 times hotter
than the solar
wind).
Increased T in
magnetosheath,
=> Cs is higher,
also driving
200 km s-1 to be
subsonic.
The magnetosheath, consisting of relatively hot sub-sonic turbulent plasma.
4
25 March 2000
Vandenberg AFB
Delta II Rocket
5
NASA
SwRI
IMAGE/RPI Sounding in the Polar Cap
Local plasma resonances
X-mode echo trace
Z trace
S/C
7
Polar Cap Plasma Distribution
Kp=6
Kp=0
8
Nsumei et al., JGR, 2003
Empirical Magnetospheric Density Distribution
Measured by IMAGE/RPI
Average
2000-2001
L=7
6
5
March 2001
1200 LT
June 2001
0800 LT
9
Reinisch et al., ASR 2003
The neutral points are the only points that connect the earth's surface
to the magnetopause.
The neutral points are
regions of interest since
this is where solar wind
particles (from the
magnetosheath) can enter
the magnetosphere without
having to cross field lines.
There is experimental
evidence that this does
happen. Particles with
energies typical of the
sheath are observed over
some 5° of latitude around
77°, and over 8 hours of
local time around noon.
These regions, being more extended than an idealized point, are
called the polar cusps or polar clefts.
10
The Magnetotail & Plasma Sheet
Field lines emerging from the polar regions are swept back, away
from the Sun; some of these would have connected on the dayside
in a dipole field. These field lines constitute the magnetotail.
Magnetosheath
Polar
Cusp
The main distinguishing
feature of the plasma sheet
is that it consists of hot
(keV) particles.
Bow
Shock
Radiation
Belts
The
plasma
contained
between the lobes, whether
on closed or open field
lines, is called the plasma
sheet.
Magnetopause
Plasma sheet is a mixture of
particles originating in the
solar
wind
(H+)
and
ionosphere (O+).
11
The boundary of the plasma sheet is determined by a balance between
the magnetic pressure of the tail lobes and the kinetic pressure of the
plasma sheet plasma:
BT2
nkT ~
2 o
where BT = tail field
intensity outside
the plasma sheet.
The tail region is highly dynamic, and is only reasonably represented
by the above description during quiescent periods.
12
Magnetospheric Circulation
P
Sq
Historically, the
current system (as inferred from ground
magnetometers) led to the first speculations about circulation
patterns in the magnetosphere, and theories about solar wind magnetosphere interactions.
Given that electrons
are bound to field
lines in the E-region
(where ionospheric
current flows) and
ions are stationary
by comparison, the
following motions of
field lines (electrons,
opposite to
conventional current
flow) are inferred
from the current
P
Sq
patterns.
Motion of
“feet” of field lines
13
Digisonde drift mesurments at Qaanaaq/Greenland
14
Reinisch, 2002
Magnetospheric Circulation
The generally accepted explanation involves magnetic connection or
merging (between the terrestrial magnetic field and the IMF) on the
dayside and reconnection on the night side. The following figure
(polar plane view) shows the sequences of magnetic merging,
convection and reconnection, where the numbers represent the time
in hours after a field line has merged on the dayside with a
southward-directed IMF (BZ < 0):
1-2
IMF merges with TMF
2-5
The connected field lines are swept back by
the solar wind
6
Reconnection of the TMF occurs
7-9
Field lines convect back to the dayside (this
must happen or an accumulation of magnetic
energy would be implied on the night side)
15
Diffusion (merging)
region
diffusion region
Note southward IMF
16
Magnetic Connection
In the correct geometry, magnetic field lines can interconnect.
In general, the field lines must be anti-parallel and there must be
an electric field as shown.
The electric field causes
plasma and field lines to
drift into the merging
region (black arrows
pointing along Z)
After merging, the B
field lines have a
component directed
along Z, causing a drift
perpendicular to the
new B direction and to
the E field. The new
flow is along X as shown.
Magnetic connection occurs in the
magnetotail and at the dayside
magnetopause.
17
Recall from earlier notes that in a magnetoplasma an applied E-field
results in a plasma drift
E B
VP 
2
B
E and Vp are equivalent in highly conducting plasmas such as the
magnetosphere and solar wind.
E
B
Vp
18
Conversely, if a magnetoplasma moves at a velocity VP with respect
to a stationary observer, the observer will measure an electric field
E  VP  B
The above is just a restatement of the Lorenz force on the
particle. This provides a useful way to understand the concept of a
field line; that is, the motion of a field line is such that an observer
moving with it detects zero electric field.
From a stationary
observer's viewpoint, the dynamics of the magnetosphere may be
understood in terms of electric fields instead of moving field lines.
From this point of view, then, the depicted antisunward flow over
the polar cap in previous figures can be interpreted in terms of a
dawn-to-dusk electric field (for an earth-fixed observer). Typically,
the magnitude is about 6 mV m-1 which corresponds to a cross-cap
potential difference of 60 kV.
19
Magnetospheric Circulation - Equatorial Plane View
+ +
+Dawn+
+
+
E
E  
- -
Dusk
-
-
-
-
The cross-cap potential is often used as a measure of intensity of solarwind/magnetosphere interaction, and can reach values of order 200 kV during intense
20
magnetic disturbances.
21
Polar Cap Electric Potential - Southward IMF
(e.g., Cannon and Reinisch, JGR, 1999)
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Modern Ionosonde and Transmit Antenna
The UMASS Lowell Digisonde
Transmit antenna
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College
Gakona
King Salmon
Point Arguello
Norilsk
Quaanaaq
Sondrestrom
Tromso
Lerwick
Narssarssuaq
Fairford
Goose Bay
Chilton
Juliusruh
Hanscom
Dourbes
Millstone Hill
Paris
Rome
Roquetes
San Vito
Wallops Is.
Athens
Arenosillo
Dyess
Bermuda Is.
Eglin
Ramey
Sao Luis
Fortaleza
Jicamarca
Tucuman
Yakutsk
Irkutsk
Anyang
Beijing
Hokkaido
Islamabad
Osan
Kokubunji
Okinawa
Karachi Wuhan
Chung-Li
Hainan Is.
Kenya
Pontianak
Ascension Is.
Cachoeira Paulista
Louisvale
Madimbo
Santa Maria
Wyndham Kalkaringi
Darwin
Curtin Base
Elliott
South Hedland
Learmonth
Alice Springs
Grahamstown
La Trobe
Melbourne
Stanley
Zhong-Shan
Casey
Earth photograph © 1990 Tom Van Sant, Inc. / the GeoSphere Project
Lowell Digisonde Locations
as of August 14, 2001
Digisondes with INTERNET Access to the Real Time Data
Digisondes with NIPRNET Access to the Real Time Data
Other Digisondes
Planned Digisonde locations
24
25
Polar Cap Electric Potential - Northward IMF
(e.g., Cannon and Reinisch, JGR, 2001)
26
27
28
Principal Plasma Populations in Earth’s Magnetosphere
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Main Zones of Magnetospheric Plasma Precipitation
Boundary between
Open field lines at
High latitudes and
Closed field lines at
Low latitudes
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The Plasmapause
The outer boundary of the plasmasphere, at about 4 RE, is where
the plasma density undergoes a sudden drop.
This is the
plasmapause.
Ring Current
However, the plasmapause
boundary is very dynamic,
and varies between about
3 to 6 RE, sometimes
getting as low as 2 RE.
Although not depicted as
such in the previous
figure, note that the
plasmasphere
overlaps
with a considerable part of
the radiation belt region.
However, these represent
two
entirely
different
particle
(energy)
populations.
31
Plasmasphere Depleting and Refilling
DST
full
full
depleted
Lpp,storm
full
32
Reinisch et al., JGR, 2003
Plasmapause Boundary
Now, the co-rotating plasmasphere sets up a "co-rotation"
electric field:
E   R   B
Outside the plasmapause the plasma is not co-rotating, and the
circulation there is determined by the cross-tail potential.
Essentially, the plasmapause represents the boundary where
these two electric fields are of the same order:
BE
E T ~ 3 LR E 
L
where BE = equatorial magnetic flux density at the surface, L =
distance in RE, and RE = radius of earth.
33
Putting in numbers,
14.4
ET ~ 2
L
mVm-1
~ 1 mVm-1 at 4 RE
Put another way, the plasmapause represents the boundary
between the "inner magnetosphere" and "outer magnetosphere"
plasma circulation patterns. The former is co-rotating, and the latter
is strongly influenced by the solar wind interaction (see following
figure):
Viewed this way, one expects intensification of the outer
magnetospheric circulation to lead to a contraction of the
plasmasphere (inward movement of the plasmapause). This indeed
happens (see subsequent figures).
In fact, it is thought that the intensified outer circulation
leads to a peeling off of outer layers of the plasmasphere, which are
then lost as detached plasma chunks in the magnetotail and solar
wind.
34
Satellite observations of ion density, showing
the plasmapause at several Kp levels
L
35
Magnetosheath
Entry
Layer
Details of the
Cusp Region
Low-Latitude Boundary Layer
36
Note: since ~1028-1029 particles/s impact the dayside
magnetopause, and ~ 1026 particles/s are estimated to enter the
plasma sheet, only 1% efficiency of this process is required.
37
Particle Flow in the Merging - Reconnection - Convection Process
“Dipolarization” of the B-field
During the return
flow, the particles are
energized in their
attempt to satisfy
the first adiabatic
invariant,
2
1 mv  = const

2 B
As particles convect towards the earth, B increases, therefore
the particle energies increase. The energy comes from the E-field.
38
Producing the Aurora
Auroras are produced by electrons and
protons striking Earth’s atmosphere.
When oxygen and nitrogen atoms are
hit by these energetic particles, they
become excited. As they relax to their
original state, they emit light of a
characteristic color
Green = Oxygen
Red = Oxygen (lower energy electrons)
Blue = Nitrogen
Also emitted in UV and X-ray
39
Electron Precipitation & the Aurora
40
Early Auroral Studies
41
The Phases of the Aurora
Quiet
Growth
Onset
42
Expansion
Maximum Area
Recovery
MAGNETOSPHERIC CURRENTS
The combination of plasma and electric fields in the
magnetosphere allows electric current to flow.
Several current
systems have been identified:
• magnetopause current (A)
• Birkeland (field-aligned) currents
• tail current (B)
• ring current (C)
The strong deviations of the magnetosphere from a dipole shape are in fact due to the
first three (A, B, C) of the above current systems. These are now discussed in turn.
43
Plasma Populations and Current systems of the Magnetosphere
44
MAGNETOPAUSE CURRENT
If we ignore any magnetic or electric field in the solar wind, the
origin of the magnetopause current and the corresponding
modifications of the magnetic field can be grossly understood as
follows:
Consider a small section of the dayside magnetopause with the
solar wind normal to it (see following figure). The ions are deflected
one way and the electrons the other, causing a current to flow
(consisting mostly of ions due to their greater penetration depth). The
current flow at the magnetopause is such that its magnetic field
cancels the geomagnetic field outside the boundary.
Similarly, earthward of the boundary the field strength is
doubled -- this is essentially the "compression" of the dayside
magnetosphere that we have alluded to before.
45
Currents and Fields at the Magnetopause Boundary
46
It turns out that the force produced by this current (the so-called
Lorentz or J X B force) balances the momentum force of the solar
wind, which is another way of stating the "dynamic pressure" vs.
"magnetic pressure" balance we discussed before.
When the solar wind intensifies, the magnetopause current is
increased:
--
This further "compresses" the dayside magnetosphere;
--
The ground magnetic signature of this sudden current
increase associated with the compression is called a
"sudden impulse" (SI), or if it is connected with the
beginning of a storm, it is called a "sudden storm
commencement" (SSC).
47
TAIL CURRENT
The down-wind extension of the magnetosphere into a tail
indicates the presence of a current system as follows:
View from Earth
48
This is what keeps the dark-side magnetic field from assuming its
dipolar form. The energy to do this comes from the solar wind. The
magnetic flux in a current loop is
BT  o iT
Since BT ~ 20 nT, then iT ~ .016 A/m. Assuming something reasonable
for the tail length, iTail ~ 108 A. For a cross-tail potential of 60 kV, the
power extracted from the solar wind ~ 6,000 GW !!
1
j=

3
0
The neutral sheet separates the northern
lobe from the southern lobe.
 B
2
B
 j =  B =  2 iˆ
o1
x 1
3
1
•
j
•
•
•
•
Current out of page
•
•
"neutral sheet" here refers to the magnetic
field, and the region where currents flow so
that reconnection is inhibited, similar to the
heliospheric current sheet.
49
Currents and Magnetic Fields in the Geotail
50
Energy is Imparted to the Plasma During Reconnection
51
RING CURRENT
Under magnetic storm conditions the magnetic field of the
earth at low latitudes may be depressed ~ 1-2% for a day or two (main
phase). This is due to a westward ring current which we have already
discussed in relation to particle drift (gradient-curvature drift) on
curved field lines with the magnitude of B increasing towards the
earth.
Recall that the gradient drift depends on the particle
"magnetic dipole moment "
BB
vg  
2
eB
2
mv
1 

2 B
-- hence gradient drift is not important for "cold"
particles like those populating the ionosphere
and plasmasphere; these particles co-rotate.
52
However, it is also true that it is not the energetic Van Allen particles
that are the main contributors to the ring current. The fluxes of
these particles are too small. The main ring current particles are
protons of 20 - 100 keV (see following figure).
The ring current is located between 4 and 6 RE, close to the
inner edge of the plasma sheet and and outer edge of the trapping
zone.
Where do the ring current particles come from?
--
magnetospheric convection, after reconnection,
accelerates particles inward, but they eventually
mirror in the stronger field near the earth, and
this is where the inner edge of the plasma sheet
forms.
53
BIRKELAND (FIELD-ALIGNED) CURRENTS
Magnetometers carried on board satellites have detected persistent
perturbations of the earth's magnetic fields over the auroral zones which can
only be interpreted as resulting from currents flowing into and out of the
ionosphere.
The following figure shows the average locations of these currents for two
levels of magnetic disturbance as determined from measurements on the TRIAD
satellite.
The solar wind/magnetosphere interaction provides energy and momentum to
the magnetosphere system; the magnetospheric circulation is determined by
redistributing its plasma and fields in a way that allows for dissipation of this
energy. This dissipation occurs in the form of:
• energizing particles which give up their • dispelling blobs of plasma out the
energy to the neutral atmosphere;
magnetotail;
• developing a current system capable of
dissipating energy through ohmic losses;
• transferring momentum
to the neutral atmosphere.
54
Region 1 and Region 2 Current Systems
R2
R2
R2
R1
R1
R2
Quiet
Active
Current flow is also consistent with the requirement for dissipation of the energy
deposited into the magnetosphere by the solar wind; ohmic dissipation of currents in
the ionosphere is one way of doing this.
55
Region 1 Current System
Sun
B
Dusk
Dawn
JE  0
56
• Concurrent with the flow of Region-1 currents into the ionosphere at
the onset of a substorm, currents and electric fields spread throughout
the conducting ionosphere; even at equatorial latitudes the signatures
of “penetration” electric fields are seen.
• After ~1 hour, the Region-2
currents close in the ionosphere
and set up a counter-potential
field, shielding the lower latitudes
from electro-dynamic coupling
from high latitudes.
• The magnetosphere can be
viewed as a voltage generator
imparting the same potential
across N. and S. polar regions;
the currents that flow depend
upon the conductivity of the
ionosphere.
57
Auroral Potential Structure
58
Major Magnetic Storms
A major magnetic storm generally follows an extended period of southward IMF.
However, although severe and nearly continuous substorms always accompany
major magnetic storms, a magnetic storm is not just a collection of magnetic
substorms.
The main distinguishing feature is the build up of an enhanced radiation environment
in the inner magnetosphere.
Consequences of Major Magnetic Storms
59
Consequences of Major Magnetic Storms
60
Consequences of Major Magnetic Storms
61
Consequences of Major Magnetic Storms
62
COMPARATIVE MAGNETOSPHERES
63
The outer planets -- the "gas giants" (Jupiter, Saturn, Uranus, Neptune) provide
interesting comparisons with the earth's magnetosphere.
The magnetospheres of the outer planets have been explored by the
Pioneer, Voyager, and Voyager 2 missions.
V2
V2
64
Some of the factors accounting for differences between the
magnetospheres of the outer planets and that of earth include:
• Properties of the solar wind change as we move outward,
affecting the coupling of energy flux from the solar wind to
the magnetospheres.
• Magnetic fields of the outer planets are generally much
larger than earth's.
• Rapid rotation of the outer planets provide centrifugal
forces large compared to those of earth.
• Jupiter's moon Io represents a dominant source of plasma
to the Jovian magnetosphere.
• The outer planets have rings capable of absorbing trapped
radiation (especially in the case of Saturn).
65
VARIATION IN SOLAR WIND PROPERTIES
The solar wind number density and the radial component of
the IMF decrease (inverse square) as distance from the sun increases.
(electron and ion temperatures of the solar wind plasma also decrease
with distance). The solar wind velocity, on the other hand, increases
(slightly) with distance.
The increased Mach numbers consistent with
the above imply stronger shock fronts at the
outer planets.
The weaker dynamic pressure suggests larger
magnetospheres for comparable B-values.
Weaker IMF suggests merging and reconnection
not so important.
66
MAGNETOSPHERIC SIZEsolar wind dynamic pressure.
The following table provides data on the relative sizes of the
magnetospheres (in terms of subsolar magnetopause distances) for
earth and the four outer planets:
67