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Ann. Geophys., 18, 1216-1223, 2000
www.ann-geophys.net/18/1216/2000/
© European Geosciences Union 2000
Effects of a kappa distribution function of electrons on incoherent scatter spectra
S. Saito1, F. R. E. Forme2, S. C. Buchert1, S. Nozawa1, and R. Fujii1
1Solar-Terrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan
2CETP-Universite Versailles St-Quentin en Yvelines, 10-12, Av. de l'Europe, 78140 Velizy, France
Abstract. In usual incoherent scatter data analysis,
the plasma distribution function is assumed to be Maxwellian. In space plasmas,
however, distribution functions with a high energy tail which can be well
modeled by a generalized Lorentzian distribution function with spectral index
kappa (kappa distribution) have been observed. We have theoretically calculated
incoherent scatter spectra for a plasma that consists of electrons with kappa
distribution function and ions with Maxwellian neglecting the effects of the
magnetic field and collisions. The ion line spectra have a double-humped shape
similar to those from a Maxwellian plasma. The electron temperatures are
underestimated, however, by up to 40% when interpreted assuming Maxwellian
distribution. Ion temperatures and electron densities are affected little.
Accordingly, actual electron temperatures might be underestimated when an energy
input maintaining a high energy tail exists. We have also calculated plasma
lines with the kappa distribution function. They are enhanced in total strength,
and the peak frequencies appear to be slightly shifted to the transmitter
frequency compared to the peak frequencies for a Maxwellian distribution. The
damping rate depends on the electron temperature. For lower electron
temperatures, plasma lines for electrons with a κ distribution
function are more strongly damped than for a Maxwellian distribution. For higher
electron temperatures, however, they have a relatively sharp peak.
Key words: Ionosphere (auroral ionosphere; plasma waves
and instabilities) – Space plasma physics (kinetic and MHD theory)
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