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Ann. Geophys., 16, 1226-1240, 1998
www.ann-geophys.net/16/1226/1998/
© European Geosciences Union 1998
Electron velocity distribution function in a plasma with temperature gradient and in the presence of suprathermal electrons: application to incoherent-scatter plasma lines
P. Guio1, J. Lilensten2, W. Kofman2, and N. Bjørnå1
1The Auroral Observatory, University of Tromsø, N-9037 Tromsø, Norway, Fax: +47 77 64 62 80; e-mail: patrick@phys.uit.no
2CEPHAG Domaine Universitaire, BP 46, F-38402 St-Martin-D'hères, France
Abstract. The plasma dispersion function and the
reduced velocity distribution function are calculated numerically for any
arbitrary velocity distribution function with cylindrical symmetry along the
magnetic field. The electron velocity distribution is separated into two
distributions representing the distribution of the ambient electrons and the
suprathermal electrons. The velocity distribution function of the ambient
electrons is modelled by a near-Maxwellian distribution function in presence of
a temperature gradient and a potential electric field. The velocity distribution
function of the suprathermal electrons is derived from a numerical model of the
angular energy flux spectrum obtained by solving the transport equation of
electrons. The numerical method used to calculate the plasma dispersion function
and the reduced velocity distribution is described. The numerical code is used
with simulated data to evaluate the Doppler frequency asymmetry between the up-
and downshifted plasma lines of the incoherent-scatter plasma lines at different
wave vectors. It is shown that the observed Doppler asymmetry is more dependent
on deviation from the Maxwellian through the thermal part for high-frequency
radars, while for low-frequency radars the Doppler asymmetry depends more on the
presence of a suprathermal population. It is also seen that the full evaluation
of the plasma dispersion function gives larger Doppler asymmetry than the heat
flow approximation for Langmuir waves with phase velocity about three to six
times the mean thermal velocity. For such waves the moment expansion of the
dispersion function is not fully valid and the full calculation of the
dispersion function is needed.
Key words. Non-Maxwellian electron velocity
distribution · Incoherent scatter plasma lines · EISCAT · Dielectric response
function
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