High‐latitude daytime absorption spike events

Abstract

Imaging riometer observations of cosmic noise absorption intensities have revealed a new class of ionospheric radiowave absorption events, the high‐latitude daytime absorption spike events. The occurrences have been examined on the basis of imaging riometer observations from Sondrestrom at an invariant latitude of 73.7°. Representative examples are presented in this report. The spike absorption events, typically, have durations of 1–2 min, and are limited in spatial extent to a region of typical dimensions of 50–100 km. They are rather weak, typically 0.2–0.3 dB, rarely up to l dB at the observing frequency of 38 MHz. Their local magnetic signatures are weak, there is no clear relation to the planetary magnetic activity level (Kp) or to specific interplanetary field variations. The bulk of these events occur at or within a few degrees equatorward of the convection reversal boundary at the afternoon convection cell for typical two‐cell polar ionospheric convection patterns. We suggest that the absorption events relate to the sudden precipitation of high‐energy (30–300 keV) magnetospheric electrons. It is further suggested that the precipitation mechanism for a major part of the afternoon cases is associated with dynamic inverted‐V potential structures developed through a magnetosphere‐ionosphere feedback process acting through near‐saturated upward region 1 field‐aligned currents at the afternoon convection reversal. The consequences derived from the suggested mechanism match very well the observed characteristics of absorption spike events.