ANALYSIS OF F-REGION INCOHERENT-SCATTER SPECTRA DURING PERIODS OF HIGH ELECTRIC-FIELDS

In the presence of high electric fields in the auroral ionosphere, the ion distribution functions depart from their usual Maxwellian shape. The incoherent scatter radars have observed the signature of this phen omenon in the form of distorted spectra which have lost their double-h umped characteristics. In consequence, the classical procedure of spec trum analysis, which assumes Maxwellian distributions to fit the plasm a parameters, is no longer valid and has to be modified to account for a non-Maxwellian plasma. The observations of the European incoherent scatter radar EISCAT were at the origin of substantial developments, w hich are still in progress. We briefly review the main steps in the ex perimental and theoretical approaches of the non-Maxwellian behaviour of the ionospheric plasma, and we discuss particularly the most recent ly published results.