A Survey of Photoelectrons on the Nightside of Mars

Last updated：2021-12-20

Photoelectrons are an important population of a planetary upper atmosphere that plays a crucial role in ionization, heating, radiation, and escape. Despite that photoelectrons are ideally produced by solar radiation on the dayside only, they have also been observed on the nightside of a planet such as Mars. This study is devoted to a statistical survey of photoelectrons in the nightside Martian upper atmosphere, based on the state-of-the-art measurements made by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. A surprisingly large fraction of photoelectron energy spectra is identified on the nightside of Mars, occupying nearly 30% of the available nightside measurements. Meanwhile, substantial variations in the observations of nightside photoelectrons are suggested by the data, revealing that (1) their occurrence rate declines rapidly near and beyond the terminator; (2) their velocities are more likely parallel to the ambient magnetic field lines; and (3) they are less likely observed under high solar wind dynamic pressures. These observations could be interpreted by photoelectron transport along large-scale magnetic field lines from their dayside source regions to the nightside, a scenario that is controlled by both internal and external conditions and thus provides a useful diagnostic of the ambient magnetic field configuration.

The occurrence rates of photoelectron measurements for the field-aligned only category (a), the perpendicular only category (b), and the all-direction category (c). The former two categories refer to the cases with photoelectron signatures only identified at 0 – 45° or 135 – 180° and 45 – 90° or 90 – 135°, respectively.
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The variation of the photoelectron PA distribution during day-to-night transport as predicted by test particle Monte Carlo calculations. (a) The imposed magnetic field line along with the full crustal magnetic field map of Morschhauser et al. (2014) at an altitude of 150 km shown in the X–Y plane. (b) The model results with the initialization of 105 electrons with isotropic PA distribution starting at point A, with the dashed line indicating the edge of the loss cone. (c) The model results with the initialization of 105 electrons with isotropic PA distribution starting at point B. (d) The variation of the magnetic strength along the imposed field line.