1. Observations of Electrons Accelerated Upwards
to keV Energies at 1000 km
Wayne Keith, David Winningham, Olle Norberg, Goran Marklund, and Tomas Karlsson
Observed by the MEDUSA instrument aboard the Swedish Astrid-2 Satellite
MEDUSA characteristics:
360° FOV in spin plane
22.5° azimuthal resolution
5° elevation acceptance
31 energy steps
16 electron spectra per second
8 ion spectra per second
460 meter electron resolution
920 meter ion resolution
Energy range ~ 1 eV to 22 keV
Very fast sweeps give sub-km resolution in low Earth orbit.
Normal mode returns all sectors all the time
Select mode returns only 3 sectors, which are chosen for each sweep to be those closest to the parallel, perpendicular, and antiparallel velocity vectors of the particles with respect to the magnetic field.
MEDUSA modes:
Normal Mode
16 azimuthal sectors
Sectors co-rotate with
satellite
Select Mode
3 azimuthal sectors
Sectors chosen to be
closest to B field parallel,
perpendicular, and antiparallel

2. Miniaturized Electrostatic Dual-top-hat Spherical Analyzer
Cutaway view
Electron and ion top hat analyzers top-to-top
Aperture (orange area) has 360 degree view in spin plane
Accumulation and data selection (in select mode) done in DPU
R1 = R2 = 16 R3 = mm

3. Astrid-2 Coordinate Systemx z
Spin direction
Rotation rate about 6 seconds per revolution, or 10 RPM
Sun-pointing, with MEDUSA on the anti-sunward side
Aperture plane wobbles + / - 3 degrees from spin plane. This is to prevent the opposite corner of the upper platform from blocking the field of view.
Electric field booms also in spin plane
3 axis magnetometer located on star-camera boom
Sun is in –z direction
MEDUSA is always anti-sunward
Aperture plane ± 3° of spin plane
E-field measured in spin plane

4. Possible Electric Field Configurations
Candidate Electric field configurations associated with upward accelerated electrons. Further analysis is needed to determine which one applies to this event.
Upper panel is parallel (downward) electrons
Lower panel is anti-parallel (upward) electrons
Red line plot is electric field, upper panel is E parallel to B, lower panel is E perpendicular to B.
Color bar range is from 2000 counts per second to 500,000 counts per second
Red intense area centered at about 5 keV, duration is about ½ second. Entire event is about 1 second, or about 7 km along the orbit.
Red 1/2 second area covers about 3.7 km along the orbit, or 12.7 arc minutes as seen from the ground.

8. Plot of energy of the peak intensity for each sweep.
Throughout the event it shows no clear V shaped pattern.
Plot is for entire event. Red bar indicates high intensity (red) region
Peak Energy fluxes are about ergs/cm2/s

9. Conclusions
Upward electron acceleration as seen by Freja at about 1750 km are observed
by MEUDSA to occur below 1000 km with keV intensities. This raises the
intensity of the electric field required for such accelerations and lowers the
altitude at which acceleration occurs.
Work is ongoing to fine-tune the measurements, model the physical processes involved, and locate similar events in this and other data sets.