Polar cap X‐rays and electrons under low density solar wind conditions: Coordinated PIXIE and DMSP observations on 11 May 1999

Abstract

X‐ray images from 11 May 1999 typically show emissions filling a region above about 75°–80° magnetic latitude with the emitting region centered a few degrees toward mid‐morning from the magnetic pole during a period when the solar wind reached unusually low values. Ionospheric particle measurements show the entire northern polar cap illuminated by precipitating electrons during much of this time, while the southern polar cap was mostly “dark.” The precipitating electrons had multicomponent spectra, one component with characteristic energy ∼200 eV throughout the observation period, and others with time‐varying characteristic energies ranging between ∼3.5 keV and 10 keV, all components with spatial characteristics common to polar rain. Measurements in the solar wind also show similar multicomponent spectra and are relatively well correlated with observations of solar radio bursts and the polar‐cap x‐ray flux. We conclude that the higher energy components were associated with electrons accelerated in solar flares or coronal flare‐like events.