Open Access
Measured and modeled backscatter of ionospheric photoelectron fluxes
Author(s) -
Richards Phil G.,
Peterson W. K.
Publication year - 2008
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2008ja013092
Subject(s) - thermosphere , ionosphere , backscatter (email) , physics , electron precipitation , photoelectric effect , computational physics , atmospheric sciences , earth's magnetic field , magnetosphere , atomic physics , geophysics , optics , plasma , magnetic field , nuclear physics , wireless , telecommunications , quantum mechanics , computer science
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.