Intense fluxes of low‐energy electrons at geomagnetic latitudes above 85°

Abstract

A systematic survey was performed to determine the frequency of occurrence of intense fluxes of electrons above the level of the polar rain at geomagnetic latitudes ≥85° and their relationship to the orientation of the interplanetary magnetic field, the density and velocity of the solar wind, and geomagnetic activity. These intense fluxes we identify as the “polar showers” and “polar squalls” reported by Winningham and Heikkila (1974). For the study, 452 orbits of data from the SSJ/3 sensor on the DMSP F2 satellite were used. The orbits selected satisfied the criteria that the satellite attained a geomagnetic latitude ≥85° during the orbit and that interplanetary magnetic field (IMF) and solar wind data were available both for the hour in which the satellite passed over the northern hemisphere and for the previous hour. The key result of the study is that in 50% of the cases studied an electron flux exceeding the level of the polar rain was found in both hemispheres at geomagnetic latitudes ≥85° and that such occurrences are dependent predominantly, if not exclusively, on the Bz component of the IMF being positive. Unlike visible polar cap arcs, a positive value of the IMF Bz component appears to be both a necessary and a sufficient condition for the occurrence of such fluxes above 85° geomagnetic latitude. Occurrence is independent of the IMF Bx and By components. The occurrence is strongly skewed toward Kp = 0, but an occurrence exceeding 30% is found up to a Kp = 4. The occurrence is independent of the number density or velocity of the solar wind. The values of the peak energy flux and number flux observed above 85° are roughly logarithmically distributed over the ranges from 3×106 to 3×109 keV/cm² s sr and 107 to 1010 el/cm² s sr, respectively. Average energies of the peak fluxes are typically between 50 and 300 eV. The maximum energy flux and number flux above 85° appear to lie above a threshold dependent on the IMF Bz component magnitude but are independent of the solar wind flux nV. The maxima in integral number flux and energy flux above 85° fall in a continuum of values consistent with visible polar cap arcs being cases near the maximum of the continuum.