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Ann. Geophys., 22, 237-249, 2004
www.ann-geophys.net/22/237/2004/
© European Geosciences Union 2004
Middle-energy electron anisotropies in the auroral region
P. Janhunen1, A. Olsson2, H. Laakso3, and A. Vaivads2
1Finnish Meteorological Institute, Geophysical Research, Helsinki, Finland
2Swedish Institute of Space Physics, Uppsala Division, Uppsala, Sweden
3ESTEC, Space Science Department, Noordwijk, The Netherlands
Abstract. Field-aligned anisotropic electron distribution functions of
T∥ > T⊥ type are observed on auroral field lines
at both low and high altitudes. We show that typically the anisotropy
is limited to a certain range of energies, often below
1keV, although sometimes extending to slightly higher energies
as well. Almost always there is simultaneously an isotropic electron
distribution at higher energies. Often the anisotropies are up/down
symmetrical, although cases with net upward or downward electron flow
also occur. For a statistical analysis of the anisotropies we divide the energy range
into low (below 100eV), middle (100eV–1keV) and high (above 1keV)
energies and develop a measure of anisotropy expressed in
density units. The statistical magnetic local time and invariant latitude
distribution of the middle-energy anisotropies obeys that of the
average auroral oval, whereas the distributions of the low and high
energy anisotropies are more irregular. This suggests that
it is specifically the middle-energy anisotropies that have something
to do with auroral processes. The anisotropy magnitude decreases
monotonically with altitude, as one would expect, because electrons have high
mobility along the magnetic field and thus, the anisotropy properties spread rapidly
to different altitudes.
Key words. Magnetospheric physics (auroral phenomena).
Space plasma physics (wave-particle interactions; changed
particle motion and acceleration)
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