We investigated 7 years worth of data from the electron reflectometer and magnetometer aboard Mars Global Surveyor to quantify the deposition of photoelectron and solar wind electron populations on the nightside of Mars, over the strong crustal field region located in the southern hemisphere. Just under 600,000 observations, each including energy and pitch angle distributions, were examined. For solar zenith angles (SZA) less than 110°, photoelectrons have the highest occurrence rate; beyond that, plasma voids occur most often. In addition, for SZA >110°, energy deposition of electrons mainly occurs on vertical field lines with median pitch angle averaged energy flux values on the order of 107–108 eV cm−2 s−1. The fraction of downward flux that is deposited at a given location was typically low (16% or smaller), implying that the majority of precipitated electrons are magnetically reflected or scattered back out. The average energy of the deposited electrons is found to be 20–30 eV, comparable to typical energies of photoelectrons and unaccelerated solar wind electrons. Median electron flux values, from near‐vertical magnetic field lines past solar zenith angle of 110°, calculated in this study produced a total electron content of 4.2 × 1014 m−2 and a corresponding peak density of 4.2 × 103 cm−3.

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Multiple studies have reported either isotropic or trapped pitch angle distributions of high‐energy (>100 eV) electrons on closed crustal field lines on the dayside of Mars. These pitch angle distributions are not to be expected from collisional scattering and conservation of adiabatic invariants alone. We use 2 years of data from the Mars Atmosphere and Volatile EvolutioN mission to analyze the pitch angle distributions of superthermal electrons on dayside‐closed crustal magnetic fields and compare to results from an electron transport model. Low‐energy electrons (10–60 eV) have pitch angle distributions in agreement with modeling results, while high‐energy electrons (100–500 eV) do not. High‐energy electrons have a flux peak at perpendicular pitch angles which suggests there is a ubiquitous energization process occurring on crustal fields. Wave‐particle interactions seem to be the most likely candidate. Trapping of high‐energy electrons may impact the nightside ionosphere dynamics.

Mars has a unique space environment relative to other planets in our solar system. The Interplanetary Magnetic Field's interaction with the ionosphere of Mars sets up an induced magnetosphere, further complicated by the presence of localized crustal fields that rotate with the planet. Superthermal electrons, electrons with energies ranging from 1 to 1,000 eV, populate these crustal field lines during their time in the dayside hemisphere. These electrons primarily consist of photoelectrons, produced from photoionization of atmospheric neutrals, with peak production occurring around 130 km. Below the photoelectron exobase, found to be ∼150 km by S. Xu et al. (2016), collisions dominate and the electrons are lost locally. Above the photoelectron exobase, the particles are magnetized and can travel to high altitudes, eventually reaching the conjugate foot point of the crustal magnetic field. The magnetic field strength decreases with altitude and the electron's pitch angle becomes more field aligned as it travels to higher altitudes due to the conservation of the first adiabatic invariant. This source cone distribution is more pronounced for higher energy electrons as Coulomb collisions are proportional to 1/E 2 , where E is the energy of the electron. Figure, 1 of Shane et al. (2019) details this pitch angle distribution (PAD) evolution along a field line as a function of energy. Data from Mars Global Surveyor (MGS) and the Mars Atmosphere and Volatile EvolutioN (MAVEN; Jakosky et al., 2015) mission have shown that our assumptions of the PADs of superthermal electrons on the Martian crustal fields are incorrect and that more physics are involved than just collisions and single particle motion. Liemohn et al. (2003) looked at a case study between MGS data and modeled electron fluxes and found that MGS measured isotropic distributions for electrons with energies greater than 100 eV. Brain et al. (2007) looked at the PADs of 115 eV electrons measured with MGS and showed that isotropic and two-sided loss cones are the most common distributions on closed crustal field lines. Shane et al. (2019) performed a statistical study using MAVEN data of superthermal electron PADs on closed crustal field lines. They showed that electrons with energies less than 60 eV have PADs that are in agreement with modeling results and that adiabatic invariants and collisions describe the evolution of their distribution. Electrons with energies greater than 60 eV were not in agreement and electrons with energies between 100 and 500 eV

The unique and dynamic magnetic field environment of Mars offers a fascinating laboratory to study space physics. Crustal magnetic fields cover the surface of the planet and rotate in and out of interaction with the solar wind. The strongest crustal fields are in the southern hemisphere and have a structure similar to coronal arcades on the surface of the Sun. In between these mini-magnetospheres are cusp regions allowing the solar wind access to the upper atmosphere of Mars. A myriad of plasma processes have been studied on the crustal fields including magnetic reconnection (e.g.,