The computer session on auroral signatures makes use of data from the
Fast Auroral SnapshoT (FAST) Explorer satellite. Comprehensive information on
the satellite, data, and processing software is available at the FAST web site. The orbit 1827 selected
below has been used by Stenbaek-Nielsen et al.
(1998) for a study of auroral arcs in conjunction with optical data.
Tasks in italics are
somewhat more challenging.
1. Bring up the data from FAST orbit
1827 (1997/02/06) from about
The plots, from top to bottom show:
a) electron differential energy flux,
in eV/cm2-s-sr-eV, as a function of energy and time;
b) electron differential energy flux
as a function of pitch angle (0 º indicates downward) and time;
c) ion differential energy flux as a
function of energy and time, all ions;
d) ion differential energy flux as a
function of pitch angle and time, all ions;
e) ion differential energy flux as a
function of energy, H+;
f) ion differential energy flux as a
function of energy, O+;
g) DBz (perpendicular to the satellite
track, roughly East–West);
h) Electric field component in the
(rotating) satellite system;
i) Power spectral density for the
electric field fluctuations, (mV/m)2/Hz, in the range 0 – 16 kHz
(the upper value on the y axis is wrong, 512 in not kHz but the number of
frequency channels);
j) Power spectral density for the
magnetic field fluctuations, nT2/Hz,
in the range 0 – 16 kHz (same note as above);
k) Power spectral density for the
electric field fluctuations in the range 0–1 MHz;
l) Power spectral density for the
magnetic field fluctuations in the range 0-1 MHz.
2. Identify the times of the satellite
crossings of the polar cap boundary, both on the evening and morning side.
3. Zoom in on the evening side auroral
oval. What boundaries can you identify in the electron and ion data (both
regarding energies and pitch angles), and the electric and magnetic field data?
Give the times and try to identify the nature of the boundaries. Can you for
example identify the boundary between the Region 1 and Region 2 currents?
4. Identify three inverted-V events
(start and stop times). Can you see any related change in the B-field? Give
your interpretation. How wide is the loss-cone typically? For one of the
inverted-V’s estimate its width, and how
wide the corresponding auroral arc would be at 100 km altitude. (Hint: the spacecraft velocity is roughly 6
km/s and the geomagnetic field at the satellite level is about one fourth of
the ground magnetic field).
5. Of your
three inverted V’s, which one do you think would create the most intense
aurora? Check if your guess agrees with the results in the paper by Stenbaek-Nielsen
et al. (1998).
6. List the times when the FAST
satellite crosses the border of the auroral acceleration region. How did you
identify these crossings? What happens with the electric field at and close to
these crossings? Draw a sketch of what
you think the geometry of the lower boundary of the acceleration region looks
like. (In order to do this it is helpful to consider the energies of the ion
beams and the electrons.)
7. Identify two ion conics, one
associated with an upward current, and one with a downward current. Do you see
any difference in their properties? If
so, can you explain them?
8. There is a strong wave emission at
around 400 kHz during the evening auroral crossing. Can you guess what kind of
waves these are? From the lower cutoff,
estimate the magnetic field strength and altitude of the source region of the
waves. (Hint: the geomagnetic field at the Earth surface is approximately
50 000 nT in the auroral region.)
9. Relate the properties of the low-frequency wave emissions to the
field-aligned current, and speculate on the nature of these waves.
10. Let us now if you have any
questions or comments.