The high latitude D-region and mesosphere revealed by the EISCAT incoherent scatter radars during solar proton events

Abstract

Energetic protons precipitating into the polar atmosphere during solar proton events cause increased ionisation rates at mesospheric heights, producing a target for incoherent scatter radars at altitudes not normally accessible by that technique. Such measurements fall into two general classes; they are obtained either fortuitously, when the event erupts during an ongoing radar operation, in which case the experiment mode may not be optimal for low altitude studies, or a suitably-designed experiment is initiated on notification of the event. The main parameters derived from these observations are the electron concentration, spectral width of the backscattered signal (from which aeronomic information may be derived) and Doppler shift (from which neutral winds may be derived). We present a review of the results obtained from the EISCAT incoherent scatter radars during solar proton events. Particular emphasis is given to data obtained during the most recent solar cycle maximum when strong events were observed in the years 1989–1991. The review concentrates on the height range 50 to 90 km, and covers topics including the height- and time-variation of electron concentration and negative ions, wave-like behaviour of the neutral winds, comparisons of measured spectral widths with model calculations, and effective recombination coefficients.