Measured and modeled backscatter of ionospheric photoelectron fluxes

Abstract

[1] This paper examines the backscatter of ionospheric photoelectrons from the thermosphere using photoelectron measurements made on closed field lines in the magnetosphere from 1997 to 2004. The satellite made simultaneous measurements of both the precipitating photoelectrons from the sunlit conjugate hemisphere and the backscattered electrons from the dark thermosphere. The backscatter ratios of the observed fluxes are in very good agreement with photoelectron model calculations for energies ranging from a few eV to several hundred eV and for different levels of solar activity. The backscattered electron flux consists of a direct elastic backscatter component that is a weak function of energy and a much stronger cascade component that greatly increases with decreasing electron energy so that the backscatter ratio ranges from 0.2 at high energies to greater than one at low energies. Although the photoelectron fluxes can vary by up to an order of magnitude over the solar cycle, the backscatter ratios show only a small variation above 20 eV. There can be a factor of 2 or more variability in the backscatter ratios for energies below 20 eV due to thermal electron Coulomb interactions. The calculations show that 55-60% of the precipitating flux energy is backscattered from the thermosphere back along the field line to the conjugate hemisphere. The model-data comparisons show that photoelectrons are able to travel the long journey from the sunlit hemisphere to the satellite without significant degradation indicating that pitch angle scattering and trapping of photoelectrons in the magnetosphere may be small.

Electron conic distributions produced by solar ionizing radiation in planetary atmospheres

“…This fraction is reported in Table 1. The 30-40% mirrored flux for the highest shadow heights for 40-60 eV electrons is consistent with the electron reflection coefficients reported by Richards and Peterson (2008). The observations presented above are also consistent with those obtained from photoelectron production codes.…”

Solar Cycle Changes in the Photochemistry of the Ionosphere and Thermosphere

“…The FLIP model uses the HEUVAC irradiances to calculate the photoelectron fluxes. These photoelectron fluxes agree well with measurements by the FAST satellite [Richards and Peterson, 2008;Peterson et al, 2009]. The ionospheric photoelectron flux is determined by solar EUV irradiances with wavelengths below 400 Å.…”

Photoelectrons as a tool to evaluate spectral variations in solar EUV irradiance over solar cycle timescales

“…These photoelectron models are based on the two‐stream model introduced by Nagy and Banks [1970]. Richards and Peterson [2008] used FAST upward and downward fluxes to demonstrate the validity of the two‐stream model. In addition to TIMED/SEE Version 9 Level 3 irradiances, we use 0–50 nm solar spectra from HEUVAC [ Richards et al , 2006], and the Flare Irradiance Spectral Model (FISM) [ Chamberlin et al , 2007, 2008].…”