J. P. Doering

Characteristics of 1-500 eV electrons observed in the earth's thermosphere from the photoelectron spectrometer experiment on the Atmosphere Explorer satellites

Low energy electrons play a significant role in a variety of atmospheric, ionospheric, and magnetospheric processes. Those produced in the thermosphere by photoionization represent the primary source of ion and electron heating in this and higher-altitude regions. Photoelectron produced in a sunlit hemisphere can, under certain circumstances, connect along geomagnetic field lines to a nonsunlit conjugate point in the opposite hemisphere, causing phenomena such as predawn heating. Secondary electrons produced by streams of high energy electrons which bombard the auroral atmosphere play an important role in the dynamics of the high latitude ionosphere. Low energy electrons are believed to be principal carriers of large-scale field-aligned, or Birkeland currents which are a basic element in the three-dimensional system coupling the magnetosphere with the lower atmosphere and ionosphere. The characteristics of low energy electrons are important to an understanding of the complex plasma processes that convey the solar wind plasma down through the magnetospheric cusps to the lower ionosphere. Low energy electrons precipitate over the earth’s polar regions and can be used as an important tracer of the complicated acceleration mechanisms which occur in the distant magnetospheric boundaries and tail region.

Observations of low-energy electrons from AE-C in the south polar cusp during the geomagnetic storm of September 21, 1977

The AE-C spacecraft skimmed through the southern polar cusp at a 400 km altitude during a large geomagnetic storm on September 21, 1977. This period has been designated as a special IMS period, and the AE-C data were acquired close to the times that data were acquired by the DMSP satellite at nearly the same location over the southern polar cap, and by the GEOS satellite located near the noon-meridian in the northern hemisphere. Low energy electrons (1-500 eV) were measured with the photoelectron spectrometer experiment experiment onboard AE-C. This instrument was operated in the mode which measured precipitating electron fluxes and backscattered electron fluxes in alternating 4 s intervals with two sensors. A region of intense precipitating electron fluxes was observed near 0924 UT on September 21, 1977 extending from 69 degree invariant latitude at 1100 MLT to 72 degree invariant latitude at 1152 MLT. From the spectra of the precipitating electrons, this region is identified as the southern polar cusp. Since the K p equals 7— during this time, the displacement of the cusp down to these low latitudes is not unreasonable. Particle data obtained from the DMSP satellite on orbits close to AE-C, confirm that the position of the cusp was rapidly changing during this period, and was displaced to latitudes equatorward of the quiet time position. A second region of intense fluxes of precipitating electron was observed by AE-C at approximately 0933 UT from 69 degree invariant latitude near 1700 MLT to 66 degree invariant latitude near 1730 MLT. This region of low energy electron fluxes is characterized by slightly harder energy spectra and is interpreted as being the afternoon auroral zone. The remarkable and fortunate location of the AE-C, DMSP, and GEOS spacecraft during this special IMS period will allow future correlative studies aimed at the determination of the shape of the magnetosphere during very disturbed conditions.