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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 22, issue 1
Ann. Geophys., 22, 237–249, 2004
https://doi.org/10.5194/angeo-22-237-2004
© Author(s) 2004. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 22, 237–249, 2004
https://doi.org/10.5194/angeo-22-237-2004
© Author(s) 2004. This work is distributed under
the Creative Commons Attribution 3.0 License.

  01 Jan 2004

01 Jan 2004

Middle-energy electron anisotropies in the auroral region

P. Janhunen1, A. Olsson2, H. Laakso3, and A. Vaivads2 P. Janhunen et al.
  • 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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