A Survey of Photoelectrons on the Nightside of Mars

Abstract

Photoelectrons, which are produced by solar Extreme Ultraviolet (EUV) and X-ray ionization of various neutrals, are an important component of the dayside Martian upper atmosphere (e.g. Coates et al., 2011; Fox et al., 2008). The ionization process generates well-defined, unique features in the photoelectron energy distribution, characterized by several distinctive peaks at 22-27 eV related to the He II 30.4 nm line, which is the most intensive EUV emission line in the solar spectrum (e.g. Frahm et al., 2006b; 2006a). In addition, a reduction in photoelectron intensity occurs around 60-70 eV due to the rapid drop in solar radiation at wavelengths shorter than 17 nm (e.g. Peterson et al., 2016; Sakai et al., 2015). The above spectral features have been extensively observed over the past four decades (e.g.

Photoelectron Butterfly Pitch-angle Distributions in the Martian Ionosphere Based on MAVEN Observations

“…The occurrence rate in the eclipse is generally above 1%, and comparable to that reported by Cao et al (2020). These darkness observations are found far away from the source region, indicating that photoelectrons are significantly transported from the dayside to below 300 km at least on the nightside, via long magnetic loops (e.g., Xu et al 2016;Cao et al 2021). Furthermore, the dayside occurrence rate of Martian photoelectron butterfly PADs is generally lower than that observed on the nightside, and presents a prominent altitudinal dependence, varying between 0.5%-1.5% at altitudes above 400 km and decreasing down to <0.2% below 300 km.…”

Cross‐Terminator Variations of the Photoelectron Energy Distribution in the Martian Ionosphere

“…Photoelectrons are an omnipresent population of the Martian ionosphere which are exclusively produced on the sunlit side via solar EUV and X‐ray ionization but are also frequently observed in the darkness due to horizontal transport (e.g., Xu, Mitchell, et al., 2016, Xu, Mitchell, Liemohn, et al., 2017). Previous studies of the deep nightside photoelectrons are mainly focused on their occurrence rates (e.g., Cao et al., 2021). We report here that they also present interesting variations in terms of a significant spectral reshaping.…”

“…The figure suggests that the vertical extension of the spectral hardening is restricted to low altitudes or high density levels when the solar wind dynamic pressure is relatively high. As noted by several studies (e.g., Brain et al, 2020;Cao et al, 2021;Weber et al, 2017), this could be attributed to the fact that under high solar wind dynamic pressures, the closed crustal field lines are compressed and reconfigured, which inevitably modifies the pattern of day-to-night magnetic connectivity on Mars. A similar process is also responsible for the reduced occurrence of energetic electron depletion under high solar wind dynamic pressures, according to Niu et al (2020).…”

“…Despite exclusively produced on the dayside, photoelectrons have also been frequently observed in the deep nightside as the outcome of cross‐terminator photoelectron transport (e.g., Cao et al., 2021; Trantham et al., 2011; Xu, Mitchell, et al., 2016). Existing studies of the nightside photoelectrons mostly focus on their occurrences (e.g., Cao et al., 2021), from which the day‐to‐night magnetic connectivity could be inferred (e.g., Brain et al., 2020; Cao, Cui, et al., 2020; Weber et al., 2017; Xu, Mitchell, Liemohn, et al., 2017, Xu, Mitchell, Luhmann, et al., 2017). It has been well established that the distribution of deep nightside photoelectrons is strongly modulated by the magnetic topology near Mars as well as the upstream solar wind conditions (e.g., Cao et al., 2021; Xu, Mitchell, Liemohn, et al., 2017).…”

“…Existing studies of the nightside photoelectrons mostly focus on their occurrences (e.g., Cao et al., 2021), from which the day‐to‐night magnetic connectivity could be inferred (e.g., Brain et al., 2020; Cao, Cui, et al., 2020; Weber et al., 2017; Xu, Mitchell, Liemohn, et al., 2017, Xu, Mitchell, Luhmann, et al., 2017). It has been well established that the distribution of deep nightside photoelectrons is strongly modulated by the magnetic topology near Mars as well as the upstream solar wind conditions (e.g., Cao et al., 2021; Xu, Mitchell, Liemohn, et al., 2017). These are also the two most important factors controlling the location of the dayside photoelectron boundary near Mars, as reported by Garnier et al.…”