1. ASEN 5335 Aerospace Environments -- Magnetospheres
Electron Precipitation & the Aurora
ASEN 5335 Aerospace Environments -- Magnetospheres

2. ASEN 5335 Aerospace Environments -- Magnetospheres

5. ASEN 5335 Aerospace Environments -- Magnetospheres
video
Aurora from
the Ground
video
Visible
Wavelengths
ASEN 5335 Aerospace Environments -- Magnetospheres

6. ASEN 5335 Aerospace Environments -- Magnetospheres
Aurora from Space
video
video
Ultraviolet
Wavelengths
video
ASEN 5335 Aerospace Environments -- Magnetospheres

7. ASEN 5335 Aerospace Environments -- Magnetospheres
Aurora from POLAR Satellite during Bastille-day Storm
video
ASEN 5335 Aerospace Environments -- Magnetospheres

8. ASEN 5335 Aerospace Environments -- Magnetospheres
Auroral Potential Structure
ASEN 5335 Aerospace Environments -- Magnetospheres

9. ASEN 5335 Aerospace Environments -- Magnetospheres
Diffuse Aurora
Broad, nondescript auroral emission
Extended 1000’s of kilometers in longitude
Extended km in latitude
Pitch-angle diffusion of plasma sheet particles
Ee < 1 keV
Discrete Aurora
Structured emission in arcs, bands, rays
Extended 100 to 1000 km in longitude
Extended 100m to 1km in latitude
Acceleration process
Ee > 1 keV and typically 10 keV
ASEN 5335 Aerospace Environments -- Magnetospheres

11. Geomagnetic Substorm

12. Themis launched Feb 17, 2007!

13. THEMIS Space-Based Element
Mission Characteristics:
Five probes, 2 year lifetime baseline mission.
PI Institution: UC Berkeley
During tail-phase (winter):
Probes line up linearly down tail every ~4 days
Apogees align over North America.
Ground-based determination of auroral onset.
Launched: February 17th, 2007

14. THEMIS Ground-Based Element
You all know the mission premise – 5 satellites on 1, 2, and 4
Sidereal day orbits synched for apogee conjunctions over central Canada –
primary mission objective to parse the magnetotail evolution during substorms –
GB work is essential to mission success. We need to do better GB than ever before, however, and that is the objective of THEMIS-Canada.

15. Scale Sizes of the Aurora
Geographic Coordinates
Geomagnetic Coordinates
COUNTS
1000
Ground Allsky Imager (Rick Doe)
IMAGE Spacecraft (Stephen Mende)
High resolution camera (Josh Semeter)

16. Ionosphere response Electron Density Altitude (km) Universal Time
Incoherent Scatter Radar

17. ASEN 5335 Aerospace Environments -- Magnetospheres
HST Images of Aurora
ASEN 5335 Aerospace Environments -- Magnetospheres

18. ASEN 5335 Aerospace Environments -- Magnetospheres
Jupiter Aurora
NASA and Prof. John Clarke (Boston University) HST-STIS
ASEN 5335 Aerospace Environments -- Magnetospheres

19. ASEN 5335 Aerospace Environments -- Magnetospheres

20. ASEN 5335 Aerospace Environments -- Magnetospheres

21. ASEN 5335 Aerospace Environments -- Magnetospheres
Current Systems of the Magnetosphere
ASEN 5335 Aerospace Environments -- Magnetospheres

22. 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) • tail current (B)
• Birkeland (field-aligned) currents • 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.
ASEN 5335 Aerospace Environments -- Magnetospheres

23. ASEN 5335 Aerospace Environments -- Magnetospheres
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.
ASEN 5335 Aerospace Environments -- Magnetospheres

24. ASEN 5335 Aerospace Environments -- Magnetospheres
Currents and Fields at the Magnetopause Boundary
ASEN 5335 Aerospace Environments -- Magnetospheres

25. ASEN 5335 Aerospace Environments -- Magnetospheres
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 as 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).
ASEN 5335 Aerospace Environments -- Magnetospheres

26. ASEN 5335 Aerospace Environments -- Magnetospheres
TAIL CURRENT
The down-wind extension of the magnetosphere into a tail indicates the presence of a current system as follows:
View from Earth
ASEN 5335 Aerospace Environments -- Magnetospheres

27. ASEN 5335 Aerospace Environments -- Magnetospheres
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
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 !!
The neutral sheet separates the northern lobe from the southern lobe.
Particle motion along a neutral sheet
• 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.
View of Tail from Earth
ASEN 5335 Aerospace Environments -- Magnetospheres

28. ASEN 5335 Aerospace Environments -- Magnetospheres
Currents and Magnetic Fields in the Geotail
ASEN 5335 Aerospace Environments -- Magnetospheres

29. ASEN 5335 Aerospace Environments -- Magnetospheres
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"
-- hence gradient drift is not important for "cold"particles like those populating the ionosphere and plasmasphere; these particles co-rotate.
ASEN 5335 Aerospace Environments -- Magnetospheres

30. ASEN 5335 Aerospace Environments -- Magnetospheres
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 keV (see figure below).
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.
Energy and Number of Protons in the Ring Current at L = 4 during a Magnetic Storm
ASEN 5335 Aerospace Environments -- Magnetospheres

31. ASEN 5335 Aerospace Environments -- Magnetospheres
Particles are energized and injected into the ring current, as reflected in the Dst index
(100’s keV)
ASEN 5335 Aerospace Environments -- Magnetospheres

32. 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.
ASEN 5335 Aerospace Environments -- Magnetospheres