An overview of Earth’s
magnetosphere and its
coupling with the solar wind
Scot R. Elkington
LASP, University of Colorado
(scot.elkington@lasp.colorado.edu)
REU Summer Program
June 11, 2009
S. Elkington, June 11, 2009
Magnetic fields
Magnetism is familiar to all of us, usually from permanent
magnets and compasses.
In its simplest form, magnetism comes in the form of
a dipole, with a northern and southern ‘poles’.
Plasma: the 4th State of Matter
solid (ice)
liquid (water)
gaseous (steam)
Plasma
The charged particles (electrons and ions) of the plasma are glued to the
magnetic field and move around it in circular orbits. Lorentz stated this
force of nature as ….
F=qV x B
V
B
(positive charge q)
… where F is the force acting on a particle with charge q and velocity V in a
magnetic field B.
The sun and the solar wind
The sun is continually ejecting portions of its atmosphere
into interplanetary space in the form of a solar wind.
The solar wind is in the plasma state, and accelerates as it moves outward from the
sun. At the Earth, the solar wind speed is typically ~400 km/s, but may exceed
1000 km/s during solar disturbances.
S. Elkington, June 11, 2009
The sun and solar wind
The Sun has an intrinsic magnetic field. The
action of the solar wind is to sweep the field
out away from the sun into space, where it
forms the Interplanetary Magnetic Field, or IMF.
The plasma moves out radially. Because sun
rotates with a ~27 day period and the field
lines are constrained by the plasma, the
simplest configuration has the IMF in the
form of a Parker spiral.
S. Elkington, June 11, 2009
The sun and solar wind
Activity on the sun modifies this simple picture, providing the
IMF with either a northward or southward component.
S. Elkington, June 11, 2009
Active sun: CMEs, etc.
In addition to the steady-state activity
described previously, the sun is capable of
violent outbursts. Coronal Mass Ejections
describe large ejections of solar material and
fields into interplanetary space.
S. Elkington, June 11, 2009
Earth’s magnetic field
Earth also has an intrinsic magnetic field, similar to that produced by a bar magnet.
The solar wind deforms the magnetosphere, compressing the front and sweeping
the back antisunward.
S. Elkington, June 11, 2009
The magnetosphere
The cavity carved out in space by the Earth’s
magnetic field is known as the magnetosphere,
bounded by the magnetopause.
Within the magnetosphere,
there are a zoo of distinct
regions, each with
characteristic plasma
populations affected by
different dynamical processes.
S. Elkington, June 11, 2009
Magnetic Reconnection
A process that:
• changes the field topology
by “breaking” and “mending”
individual field lines in a local
region.
• converts magnetic energy to
a jetting plasma
Energy from the solar wind:
reconnection
S. Elkington, June 11, 2009
Energy from the solar wind:
reconnection
Reconnection not only provides a means of getting
energy and mass from the solar wind into the
magnetosphere, it also sets up large scale convective
motion within the magnetosphere.
S. Elkington, June 11, 2009
Open, closed, and interplanetary
magetnetic fields
One may identify three types of magnetic field
lines in near-Earth space:
• The interplanetary field is that originating
with the sun.
• ‘Open’ field lines have recently reconnected
with the IMF… one end connects to the
Earth, the other to the IMF.
• ‘Closed’ field lines have not reconnected…
both ends of the field line originate on Earth.
The magnetospheric polar cap defines the ionospheric separatrix
between the open and closed field lines of the magnetosphere,
and may be seen in terms of ionospheric currents, electric fields,
and plasma flows. Particles precipitating at the edge of the
polar cap forms the auroral oval.
S. Elkington, June 11, 2009
Particle physics in space: basic particle motion
A charged particle will move at constant velocity in a straight
line unless acted on by a force. In space, the most important
forces for charged particles arise from electric and magnetic
fields.
•Electric fields (E) will accelerate particles in the direction of the field.
•Magnetic fields (B) will accelerate particles in a direction
perpendicular to the both the B field and the particles motion.
Thus a magnetic field will cause a particle to execute some kind
of gyromotion.
q
F  qE  v  B
c
L  c
mv cp

qB
qB
qB
L 
.
mc
“Gyroradius”
“Gyrofrequency”
S. Elkington, Feb 22, 2009
Particle physics in space: basic particle motion
If a particle gyrating in a magnetic field is acted on by an
external force, it will cause the particle to drift perpendicular to
the external force and the local magnetic field.
An electric field perpendicular to the local magnetic field will
cause such a drift:
S. Elkington, Feb 22, 2009
Particle physics in space: basic particle motion
Similarly, if the magnetic field is nonuniform in a direction perpendicular
to the local magnetic field, a drift results:
On the other hand, a magnetic field that is nonuniform in a
direction parallel to the magnetic field will cause a particle to
experience a force away from the regions of strong magnetic
field:
Fm   || B
S. Elkington, Feb 22, 2009
Charged particle motion in the Earth’s
magnetosphere
The Earth has an intrinsic magnetic field that is
roughly a dipole. Charged particles moving under
the influence of the Earth’s magnetic field
therefore execute three distinct types of motion.
•Gyro: ~ millisecond
Characteristic •Bounce: ~ 0.1-1.0 s
time scales:
•Drift: ~ 1-10 minutes
S. Elkington, Feb 22, 2009
Regions: the bow shock and magnetopause
At Earth, the solar wind is
supersonic (and superAlfvenic).
The Earth forms an obstacle in the
solar wind, thus producing a bow
shock upstream of Earth.
The boundary between the
magnetosphere and the IMF is defined
by the magnetopause current or ChapmanFerraro current, which can be
simplistically understood in terms of
the basic particle motion as shown.
S. Elkington, June 11, 2009
Regions: ring current and radiation belts
Drift motion in closed-field regions of the magnetosphere leads to currents in space
encircling the Earth. The ring current is formed by energetic electrons and ions
gradient-drifting across field lines in opposite directions about the Earth.
The Dst index measures the energy content of the ring current by measuring the
magnetic perturbation at Earth caused by this current. Negative excursions in
Dst characterize geomagnetic storms.
S. Elkington, June 11, 2009
Regions: the Van Allen Radiation Belts
The high-energy component of the
ring current forms the radiation belts.
These are comprised of relativistic
electrons and protons, with MeV
energies (as opposed to the keV ring
current populations).




Discovered by James Van Allen in 1958
via a Geiger counter on Explorer I.
Trapped electrons and ions drifting in
orbits encircling Earth.
Two spatial populations: inner zone and
outer zone.
Energies from ~200 keV to > several
MeV.
S. Elkington, June 11, 2009
Regions: the plasmasphere
Plasmas flowing out from the Earth’s ionosphere form
a cold, dense population that corotates with the Earth.
The boundary of the
plasmasphere is the plasmapause,
and is defined by the competing
effects of Earth’s corotation and
magnetospheric convection.
Convectively-driven plasmas
Co-rotating plasmas
S. Elkington, June 11, 2009
Regions: the plasmasheet and lobes
Field lines which have reconnected at the magnetopause are swept
back into the tail, forming the northern and southern lobes of the
magnetotail.
Reconnection in the tail creates a
closed-field region near the
magnetic equator called the central
plasma sheet. This region is
dominated by convection.
The boundary of the
plasmasheet is the Alfven
layer, and is defined by the
competing effects of
gradient-curvature drift and
magnetospheric convection.
S. Elkington, June 11, 2009
Southward IMF: geomagnetic storms
During periods of extended southward IMF, the energy input into the
magnetosphere can cause (among other things) intensifications in auroral
activity, amplification of magnetospheric currents, and a depression of the
local magnetic fields strength measured at Earth. These periods are known as
geomagnetic storms.
In particular, changes in the magnetospheric ring current will cause a decrease in
the horizontal component of the magnetic field measured at Earth, and is
characterized by the Dst index.
S. Elkington, June 11, 2009
Magnetic storms and auroral activity
As reconnection proceeds at the magnetopause,
more of the magetospheric field lines become
‘open’. The polar cap increases its size, and the
auroral oval is driven to more southerly
latitudes.
Aurorae
S. Elkington, June 11, 2009
Expansion phase:
few minuts
Growth phase:
~1-2 hours
Geomagnetic storms: substorms
In contrast to the simple picture of steady
reconnection and convection, sometimes energy is
stored in the tail and then released episodically. Such
events are known as substorms.
Energy is stored, and the tail becomes very
stretched. The plasmasheet begins to thin.
Field lines reconnect at the NENL, releasing the
energy stored in the tail.
Energy flows away from the reconnection site. Field
lines near Earth go from stretched to dipole-like, and
particles are injected from the reconnection site both
down the tail and into the inner magnetosphere.
S. Elkington, June 11, 2009
Magnetic storms: auroral substorms
Particles injected into the inner
magnetosphere during a substorm can
cause intense, dynamic auroral activity.
S. Elkington, June 11, 2009
Substorm simulation animations?
S. Elkington, June 11, 2009
Storm effects on the plasmasphere
Increased convective activity in the
magnetosphere can strip away plasma
from the plasmasphere, reduce the
plasmapause, and introduce spatial and
temporal features in the plasmasphere.
Convectivelydriven plasmas
Co-rotating
plasmas
S. Elkington, June 11, 2009
Storm effects on the radiation belts
Adiabatic Heating/Loss
Local Heating/Loss
S. Elkington, June 11, 2009
‘Space Weather’
Damage to
spacecraft
Power grids, transformers
Polar airline routes
Polar Airline
Routes
P
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arP
1 ol
ar P
ol
aPol
r ar 4
3
Hon 2
g
Kon
g
No
Radiorth
Blackout
Pol
During
e Events
Particle
C
hi
c
a
g
o
Hazards to human
activity in space
S. Elkington, March 2, 2006
Summary
• The sun expels its atmosphere and fields in the form of a solar wind and IMF.
• Earth’s intrinsic magnetic field carves out a cavity in the solar wind known as the
magnetosphere.
•Compressed sunward, stretched antisunward
•May reconnect with the solar wind IMF
• Various processes and particle populations define fundamental regions of the
magnetosphere:
•Bow shock and magnetopause
•Ring current and radiation belts
•Plasmasphere
•Tail lobes and plasmasheet
•Etc.
• The solar wind and IMF can cause dynamic activity in the magnetosphere: solar
storms
•Enhanced convection, substorms
•Auroral activity
•Plasmasphere erosion
•Radiation belts
•Space Weather!
S. Elkington, June 11, 2009
Parking Lot
S. Elkington, June 11, 2009
Density (cm-3)
Magnetic field
strength (nT)
Convection
speed (km/s)
Tail Lobes
1
25
5
Plasma sheet
0.01
10
4
Plasmasphere
1000
>100
<1
Earth magnetic field strength at the poles (equator): 62000 (31000) nT
Density (cm-3)
Magnetic field
strength (nT)
Convection
speed (km/s)
Solar Wind
5
5
450
Magnetosheath
10
10
200
Magnetotail
lobe
0.01
25
4
Forces on plasma due to magnetic field
dv
m
 q( E  v  B )
dt
du

 j  B  p
dt
  ni mi  ne me
Equation of motion
Momentum fluid equation
u  ni mi ui  ne meue  
j  ni qi ui  ne qeue
Magnetospheric regions and processes
Global magnetosphere
Plasma and
MHD waves
Energetic
particles
Basic plasma
processes
S. Elkington, June 11, 2009
ESD Damage
HA-2700 surface damage in the C2 MOS capacitor
(Courtesy of JPL)
175X
4300X
S. Elkington, June 11, 2009