Abstract
Atmospheric photoelectrons are central to the production of planetary ionospheres. They are created by photoionization of the neutral planetary atmosphere by solar EUV and soft X-ray irradiance. They provide the energy to heat the thermosphere. Thermalized photoelectrons permeate magnetospheres creating polarization electric fields and plasma waves as they interact with ions to maintain charge neutrality. Energetic photoelectrons (>1 eV) have a distinctive energy spectral shape as first revealed in data from the Atmosphere Explorer satellites. Energetic photoelectrons escaping the ionosphere follow local magnetic fields illuminating the planet's magnetic topology. Current models using state-of-the-art EUV observations accurately capture their production and transport. However, in spite of 60 years of space research the electron thermalization processes occurring below 1 eV at low altitudes in planetary thermospheres are not understood quantitatively. Results from event analysis of data from the Mars Atmosphere and Volatile Evolution (MAVEN) mission are not consistent with current models of photoelectron thermalization. The lack of quantitative understanding reflects the complexity of the physics and the lack of a large data base of simultaneous neutral, ion, and electron densities and temperatures in lower planetary thermospheres.
Highlights
Electrons in the atmosphere were investigated by Chapman (1931) and by early radio scientists who inferred the existence of an ionized region surrounding the Earth created by the absorption of radiation from the sun
Techniques have been developed to use the unique energy spectral shape of energetic photoelectrons to tease out details of planetary magnetic topology
Current models of photoelectron thermalization are based on the heat equation which quantifies balance of electron heating and cooling (e.g., Matta et al, 2014)
Summary
Electrons in the atmosphere were investigated by Chapman (1931) and by early radio scientists who inferred the existence of an ionized region surrounding the Earth created by the absorption of radiation from the sun. These emission features are the result of the production of Auger electrons from atomic oxygen by soft X rays from the sun These distinct features in the photoelectron energy spectra allow investigators to use them to trace magnetic field lines, determine global magnetic topology, determine spacecraft potential, infer potential drops along magnetic field lines, and monitor variations in solar extreme ultraviolet (EUV) irradiance. They demonstrated that during the MAVEN deep dip interval in October 2015, at the lowest altitudes sampled (∼120 km), the neutral (110 ◦K), ion (250 ◦K), and electron (500 ◦K) temperatures were far from thermal equilibrium and warmer than expected
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