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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 16, issue 3
Ann. Geophys., 16, 298–302, 1998
https://doi.org/10.1007/s00585-998-0298-0
© European Geosciences Union 1998
Ann. Geophys., 16, 298–302, 1998
https://doi.org/10.1007/s00585-998-0298-0
© European Geosciences Union 1998

  31 Mar 1998

31 Mar 1998

Field-aligned conductance values estimated from Maxwellian and kappa distributions in quiet and disturbed events using Freja electron data

A. Olsson1 and P. Janhunen2,1 A. Olsson and P. Janhunen
  • 1Swedish Institute of Space Physics, Uppsala Division, Uppsala, Sweden
  • 2Finnish Meteorological Institute, Geophysical Research, Helsinki, Finland

Abstract. We study the question of what difference it makes for the derived field-aligned conductance (K) values if one uses Maxwellian or kappa distributions for the fitting of low-orbiting satellite electron flux spectra in the auroral region. This question has arisen because sometimes a high-energy tail is seen in the spectra. In principle, the kappa fits should always be better, because the kappa distribution is a generalization of the Maxwellian. However, the physical meaning of the parameters appearing in the Maxwellian is clearer. It therefore makes sense to study under which circumstances it is appropriate to use a Maxwellian. We use Freja electron data (TESP and MATE) from two events. One of the events represents quiet magnetospheric conditions (stable arc) and the other represents disturbed conditions (surge). In these Freja events, at least, using kappa rather than Maxwellian fitting gives a better fit to the observed distribution, but the difference in K values is not large (usually less than 20). The difference can be of either sign. However, sometimes even the kappa distribution does not provide a good fit, and one needs a more complicated distribution such as two Maxwellians. We investigate the relative contributions of the two Maxwellians to the total field-aligned conductance value in these cases. We find that the contribution of the high-energy population is insignificant (usually much less than 20). This is because K is proportional to nEc, where n is the source plasma density and Ec is the characteristic energy.

Key words. Field-aligned conductance · Maxwellian distribution · Kappa distribution · Freja electron data

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