Observations of auroral westward traveling surges and electron precipitations

Abstract

Precipitations of low‐energy electrons above the auroral westward traveling surge were examined from a number of surges observed by the DMSP 32 satellite in the fall of 1974. From the limited surges observed, differences in precipitations were detected, associated with different auroral forms in the vicinity of a surge. To the west of it, over a bright arc (or arcs), the precipitated energy flux was about 10 ergs/cm² s sr, and the differential energy spectrum was very similar to that of quiet evening arcs, but with an occasional shift of the spectral peak from ∼3.2 keV to the next higher energy channel of ∼8 keV. Along the surge front a band of very bright discrete auroras no longer had the precipitation characteristics of quiet arcs, and the energy flux amounted to tens of ergs/cm² s sr. The differential spectrum was highly variable, but sometimes it may be approximated by a power law with index between 1 and 1.5. The most spectacular precipitation feature was observed inside the main body of a surge; an unusually flat differential spectrum was observed. It was harder than dJ/dE ∝ E−1, with an index of about 0.5 extending from 0.2 keV to at least 20 keV. Thus a substantial precipitation of energetic electrons above 20 keV occurred inside a surge. A spectral peak near 8 keV or higher energies was also occasionally observed on this flat background spectrum. To the east of a surge (i.e., behind a westward surge) in the region of nonuniform diffuse auroras the electron precipitation was characterized by a flat spectrum below 8 keV with a sharp cutoff near 10 keV, similar to that of the equatorward diffuse aurora, superposed by a spectral peak near 3.2 or 8 keV, and the energy flux was approximately 1 erg/cm² s sr.