Precipitating electron interaction with the atmosphere: 2. The dayside cusp region

Abstract

Interaction of precipitating low‐energy magnetosheath electrons with the atmosphere in the dayside cusp region has been studied. Both pitch angle and energy distributions of the fluxes as well as excitation functions for selected N2 and O UV emission were obtained by numerically solving the multiangle equations of electron transport. The angular dependence of the elastic scattering was treated more precisely—in contrast to its treatment in previous studies via limiting approximations involving either sharply forward peaked or isotropic shapes. Pitch angle distributions with broad peaks around 90° were encountered, albeit in a very narrow energy, band, in the upper atmosphere above the collision‐dominated lower atmosphere where isotropic pitch angle distributions predominate. Pitch angle distributions in the upper atmosphere at energies below this narrow band are approximately mirror images of the distributions at higher energy in the sense that the high and low values of the two distributions are located in diametrically opposite pitch angle directions. In our energy regime, it is difficult to accurately infer the spectral characteristics of the incident flux from the spectral characteristics of the flux measured at 350–250 km because the densities at even these altitudes are sufficient to substantially alter the spectral characteristics of low‐energy incident electrons. In contrast, the incident flux in the keV region travel through this altitude region almost unaffected. For low precipitating fluxes on the order of (1–2) × 108 el cm−2 s−1, photoelectrons may constitute a significant fraction of the measured flux for solar zenith angles as large as 100°. Influence of the photoelectrons decreases if either the precipitating flux increases or the solar zenith angle increases. There is some possibility that atmospheric emissions may be used for remote measurements of incident soft energy flux, because the ratios of molecular to atomic emission line intensities in the low‐energy region are quite different from those in the high‐energy region.