1. BIRKELAND (FIELD-ALIGNED) CURRENTS
Magnetometers carried on satellites have detected persistent magnetic perturbations 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 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 energy to the neutral atmosphere; the magnetotail;
• developing a current system capable of • transferring momentum dissipating energy through ohmic losses; to the neutral atmosphere.
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2. ASEN 5335 Aerospace Environments -- Magnetospheres
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.
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3. ASEN 5335 Aerospace Environments -- Magnetospheres
Region-1 Current System
The magnetic field lines are highly conducting, and so it is natural that the magnetosphere seeks some closure of current through an ionospheric route. In fact, the so-called Region 1 currents are necessary if we are to require the polar ionosphere to convect with the magnetic field lines:
MHD Dynamo
Region 1 Configuration
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4. ASEN 5335 Aerospace Environments -- Magnetospheres
Ionosphere Closure Currents
Pedersen Currents
The electric field impressed upon the ionosphere by the solar wind-magnetosphere dynamo moves the ions and electrons in the ionosphere in an ExB direction. Below about 180km the ions experience neutral collisions and move nearly parallel to E.
The ion current in the direction of E is divergent ( ,here "H" refers to "horizontal“) and cannot close in the ionosphere and so there must be some current linkage with the magnetosphere
-- This ion current is called the Pedersen current whose current carriers are ions
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5. ASEN 5335 Aerospace Environments -- Magnetospheres
Ionosphere Closure Currents
Hall Currents
At altitudes above about 90km, electrons move with the field lines in a quasi-horizontal plane in the ExB direction. This flow differs from the ion motion creating a current that is essentially divergence-free.
(Here "e" refers to "electrons"; the above is zero since ). These currents are called Hall currents whose current carriers are electrons.
Together, the Hall and Pedersen currents form the ionospheric current system.
(see following figure)
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6. ASEN 5335 Aerospace Environments -- Magnetospheres
Field-Aligned and Closure Currents in the Ionosphere
Horizontal View
Vertical View
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7. ASEN 5335 Aerospace Environments -- Magnetospheres
Region-2 Current System
The Region-2 current system maps to the equatorial plane of the magnetosphere at distances of L~7-10 and the currents are supplied by an excess distribution of charge
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8. Region-2 Current System

9. ASEN 5335 Aerospace Environments -- Magnetospheres
The origin of the Region 2 currents is connected with a longitudinal asymmetry in the ring current induced by pressure gradients maintained by the dawn to dusk transpolar electric field.
This results in a decrease in the ring current as it flows from midnight --> dusk --> noon; it must therefore discharge current (into the conducting ionosphere) on the dusk side.
decrease
increase
Similarly the ring current increases as it flows from noon --> dawn --> midnight; it must therefore draw current from the ionosphere in the dawn sector.
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10. The Substorm Current Wedge
McPherron et al. (1973)

11. between the plasma sheet
The flow of current through the ionospheric circuit is consistent with the collapse of the tail at the neutral point; the latter can only occur if the cross-tail current is substantially reduced, as it is when being diverted along the field lines into the ionosphere.
Region 1 currents flow
between the plasma sheet
and the ionosphere
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.
About ~1 hour after storm onset, the Region-2 currents intensify in the ionosphere and set up a counter-potential field, shielding the lower latitudes from electro-dynamic coupling from high latitudes.
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12. Under Disturbed Conditions Magnetic Merging Occurs Near ~100 RE,
and a Plasmoid is Ejected out the Magnetotail
Neutral line location under quiescent conditions ~1000 RE
Thinning and pinching of plasma sheet upon onset of intensified coupling
expel plasmoid
Return to pre-substorm conditions
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13. ASEN 5335 Aerospace Environments -- Magnetospheres
VENUS AND MARS
Venus and Mars have very weak magnetic fields, and the interaction of the solar wind with these planets is governed by different processes.
without an IMF, the flow
around the planet is
similar to that of fluid
flow around a sphere.
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14. ASEN 5335 Aerospace Environments -- Magnetospheres
With an IMF that is not steady (as in the solar wind), the interaction of the changing IMF with the conducting ionosphere generates currents that keep the field from penetrating through the body by generating a canceling field:
(Faraday's law -- time changing magnetic field induces currents in a conductor).
A steady IMF would eventually diffuse into the body (time constant depends on ionospheric conductivity)
A bow shock is expected, since the supersonic solar wind plasma is diverted around the body.
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15. ASEN 5335 Aerospace Environments -- Magnetospheres
For Venus, pressure balance considerations similar to those for a magnetosphere apply, except with the ionospheric pressure replacing the magnetic pressure.
An ionopause is formed when the solar wind pressure balances the ionospheric pressure.
The subsolar ionopause of Venus is quite low -- about 350 km, as compared with about 1,000 km at the terminator.
The bow shock is at about 2,000 km.
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16. ASEN 5335 Aerospace Environments -- Magnetospheres
Ionospheric ions can gyrate around the flowing IMF lines, and thus be "picked up" by the solar wind.
Ion
pickup
Integrated over the 4.5 billion year lifetime of the planet, this scavenging by the solar wind has probably had an impact on the evolution of the atmospheres of Mars and Venus.
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17. ASEN 5335 Aerospace Environments -- Magnetospheres
bow shock
Solar wind
O+, O2+ scavenging
IMF
ionopause
NASA Mission:
MAVEN – Mars Atmosphere and Volatile EvolutioN
To study the loss processes of the Martian atmosphere
Launch in late 2013
rV2 ~ npkTp
crustal magnetism
H+ scavenging
Solar wind
IMF
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18. ASEN 5335 Aerospace Environments -- Magnetospheres
IMF
A Scenario for the Loss of Water on Mars
H+, O+, O2+, etc.
scavenging
photo-
ionization
 H+, O+, etc.
photo-
dissociation
H2O + hn  H + OH
H2 + O
Vertical Transport
(large-scale dynamics and diffusion)
H2O
Lower Atmosphere
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19. Solar Wind and IMF Interactions with Mars’ Ionosphere
Courtesy NASA/Nagoya University
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20. ASEN 5335 Aerospace Environments -- Magnetospheres
As shown below, this ion removal process becomes evident in comparisons of measured ionospheric profiles with those obtained from models which do not take into account this process.
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21. ASEN 5335 Aerospace Environments -- Magnetospheres
Mercury
Mercury has a significant magnetic field, but no atmosphere; interaction of the solar wind with Mercury forms a magnetosphere that shares many gross characteristics with that of earth.
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22. Schematic of Mercurian Magnetosphere and Anticipated Processes
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23. ASEN 5335 Aerospace Environments -- Magnetospheres
Launched August 2004
Orbit insertion March 2011
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24. ASEN 5335 Aerospace Environments -- Magnetospheres
Jupiter and Io
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25. ASEN 5335 Aerospace Environments -- Magnetospheres
Deep within Jupiter's interior, at extreme pressures and temperatures, hydrogen changes from molecular liquid to a state called liquid metallic hydrogen, an excellent electrical conductor.
The liquid metallic hydrogen and the planet's rapid rotation (9 hours 55 minutes) produce dynamo electric currents that generate Jupiter's magnetic field, which is more than 10 times stronger than that of Earth.
Scales
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26. ASEN 5335 Aerospace Environments -- Magnetospheres
NASA New Frontiers Mission:
JUNO – To study Jupiter’s atmosphere and magnetosphere
Launch – August 2011
Jupiter arrival – 2016
Previous Jupiter orbiting mission: GALILEO Launch – October 1989
Jupiter arrival – December 1995
Jupiter demise – September 2003
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27. ASEN 5335 Aerospace Environments -- Magnetospheres
Europa
Ganymede Io
Callisto
1 ton / sec
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28. ASEN 5335 Aerospace Environments -- Magnetospheres
The weak atmosphere of Io is maintained by continual volcanic eruptions.
Neutral atoms in the cloud become ionized.
As the magnetosphere rotates with Jupiter, it sweeps past Io, stripping away about a ton of matter per second and forming a torus—a doughnut-shaped ring around Jupiter
The torus is predominantly composed of electrified oxygen and sulfur glowing in the ultraviolet.
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29. ASEN 5335 Aerospace Environments -- Magnetospheres
Jupiter’s Io
The reason for the larger value of Jupiter’s subsolar magnetopause is that Io provides a significant source of mass to the Jovian magnetosphere, which in turn accelerates this mass to high velocities because of the rapid rotation of the planet.
The resulting centrifugal force and pressure of the plasma pushes out against the solar wind, inflating the magnetosphere and leading to a more distant standoff distance.
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30. ASEN 5335 Aerospace Environments -- Magnetospheres
For Jupiter, the actual subsolar magnetopause is found to be over 100 RJ , as opposed to the value of 45 RJ expected from pressure balance arguments. This is due to the effects of the plasma spewing from Io.
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31. ASEN 5335 Aerospace Environments -- Magnetospheres
As the heavy ions from Io migrate outward, their pressure inflates the magnetosphere to more than twice its expected size. Some of the more energetic ions fall into the atmosphere along the magnetic field to create Jupiter's auroras. Io
Infrared
The Io Aurora
IR, UV spot images
Text -> implications
- energy does get to ionosphere
- but not just at Io footprint
energetic particles bombard
atmosphere
‘wake’ emission extends
halfway around Jupiter
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32. ASEN 5335 Aerospace Environments -- Magnetospheres
Clarke et al.
Aurora
Polar storms
Io wake
Main Oval
Io footprint
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33. ASEN 5335 Aerospace Environments -- Magnetospheres
NASA Cassini Mission to Saturn Launched in October 1997 and arrived in July 2004
Saturn Magnetosphere
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34. ASEN 5335 Aerospace Environments -- Magnetospheres

35. ASEN 5335 Aerospace Environments -- Magnetospheres
Additional Slides
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36. ASEN 5335 Aerospace Environments -- MagnetospheresIo
300 km
Amirani
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37. Io’s Volcanoes & Geysers
Pilan Plume
Prometheus
Pilan 5 months apart
Pele IR
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38. ASEN 5335 Aerospace Environments -- Magnetospheres
Nightside of Io - Visible
Glowing Lava
Plume Gas & Dust + Airglow
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39. ASEN 5335 Aerospace Environments -- Magnetospheres
Ground-based telescopic observations of scattered sunlight from neutral sodium atoms produced by charge exchange of (cold) torus ions
Sodium
Mendillo et al.
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40. ASEN 5335 Aerospace Environments -- Magnetospheres
Cassini/MIMI observations of hot neutral sulfur from charge-exchange of radiation belt particles S*
S+ + O -> O+ + S*
Krimigis et al.
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