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 phenomenon in the form of distorted spectra which have lost their double-humped characteristics. In consequence, the classical procedure of spectrum analysis, which assumes Maxwellian distributions to fit the plasma parameters, is no longer valid and has to be modified to account for a non-Maxwellian plasma.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, which are still in progress. We briefly review the main steps in the experimental and theoretical approaches of the non-Maxwellian behaviour of the ionospheric plasma, and we discuss particularly the most recently published results.