A model for IS spectra for magnetized plasma with arbitrary isotropic velocity distributions

Abstract

The plasma line in the incoherent scatter spectrum is known to provide information about the state of the ionosphere. However, it is weak in signal strength and therefore difficult to measure reliably and consistently. When high-energetic electrons (suprathermal electrons) are present in the ionosphere the plasma line echo power is enhanced and detectable by more radars. Recent measurements made by the Arecibo radar show an altitude and aspect angle (angle between the radar beam and the magnetic field line) dependence on the returned echo power of the plasma line. This was assumed to be due to enhancements in the suprathermal electron velocity distribution but has neither been confirmed through theory nor numerical analysis.

The theory describing the plasma line in the incoherent scatter spectrum due to scattering off thermal electrons has been known for a long time. This theory includes radar measurements at large angles to the magnetic field but a similar general derivation has not been formulated where suprathermal electrons are included in the distribution.

In this work a derivation of the dielectric function which is a fundamental part of the derivation of the incoherent scatter spectrum was carried out for an arbitrary isotropic velocity distribution. Further, a program calculating the spectrum using the derived dielectric function was developed. The program was used to model the incoherent scatter spectrum for different electron velocity distributions and the echo power in the plasma line as a function of aspect angle and electron number density. It was shown that the enhancements found in the suprathermal distribution map to the structures found in the plasma line echo power, in line with the proposed explanation based on measurements. These findings support an aspect angle formula relating energy and received plasma resonance frequency based on the assumption that the main contributing factor to the resonance frequency are the electrons with velocity close to parallel to the magnetic field line.