ELECTRON VELOCITY DISTRIBUTION FUNCTION IN A PLASMA WITH TEMPERATURE-GRADIENT AND IN THE PRESENCE OF SUPRATHERMAL ELECTRONS - APPLICATION TO INCOHERENT-SCATTER PLASMA LINES

The plasma dispersion function and the reduced velocity distribution f unction are calculated numerically For any arbitrary velocity distribu tion function with cylindrical symmetry along the magnetic field. The electron velocity distribution is separated into two distributions rep resenting the distribution of the ambient electrons and the supratherm al electrons. The velocity distribution function of the ambient electr ons is modelled by a near-Maxwellian distribution function in presence of a temperature gradient and a potential electric field. The velocit y distribution function of the suprathermal electrons is derived from a numerical model of the angular energy flux spectrum obtained by solv ing the transport equation of electrons. The numerical method used to calculate the plasma dispersion function and the reduced velocity dist ribution is described. The numerical code is used with simulated data to evaluate the Doppler frequency asymmetry between the up- and downsh ifted 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 f or high-frequency radars, while for low-frequency radars the Doppler a symmetry depends more on the presence of a suprathermal population. It is also seen that the full evaluation of the plasma dispersion functi on gives larger Doppler asymmetry than the heat how approximation for Langmuir waves with phase velocity about three to six times the mean t hermal velocity. For such waves the moment expansion of the dispersion function is not fully valid and the full calculation of the dispersio n function is needed.