Energetic Electron Depletions in the Nightside Martian Upper Atmosphere Revisited

Abstract

Energetic electron depletions are a notable feature of the nightside Martian upper atmosphere. In this study, we investigate systematically the variations of the occurrence of depletions with both internal and external conditions, using the extensive Solar Wind Electron Analyzer measurements made on board the Mars Atmosphere and Volatile Evolution. In addition to the known trends of increasing occurrence with decreasing altitude and increasing magnetic field intensity, our analysis reveals that depletions are more easily observed in regions with near horizontal magnetic fields and under low solar wind (SW) dynamic pressures. We also find that below 160 km, the occurrence increases with increasing CO 2 density, a trend mostly visible in weakly magnetized regions. These observations have important implications on the formation of electron depletions: (1) Near strong magnetic anomalies, closed magnetic loops preferentially form and shield the atmosphere from direct access of SW electrons, a process that is modulated by the upstream SW condition; and (2) in weakly magnetized regions, SW electrons precipitate into the atmosphere unhindered, but at sufficiently low altitudes, they are either "absorbed" due to inelastic collisions with ambient neutrals or shielded again in response to a change in magnetic connectivity from open to closed. Our analysis further reveals that both the ionospheric plasma content and thermal electron temperature are reduced in regions with depletions compared to regions without, supporting SW electron precipitation as an important source of external energy driving the variability in the deep nightside Martian upper atmosphere and ionosphere. the Solar Wind Electron Analyzer (SWEA) on board the recent Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft (Jakosky et al., 2015). Steckiewicz et al. (2017) performed a comprehensive analysis of nightside energetic electron precipitation near Mars combining the measurements made by the Mars Global Surveyor (MGS), the Mars Express (MEx), and MAVEN, identifying more than 134,500 depletions between 125 and 900 km in altitude. The fractional

A Survey of Photoelectrons on the Nightside of Mars

“…Lillis et al, 2018) and, within the context of this study, increasing the portion of solar wind electrons contained in the observed suprathermal electrons. This effect was also supported by the observed modulation of energetic electron depletions in the nightside Martian ionosphere by the upstream solar wind condition (Niu et al, 2020). On the other hand, a higher solar wind dynamical pressure can compress the magnetic field lines that connect the dayside and CAO ET AL.…”

“…Lillis & Brain, 2013;Lillis et al, 2018;Xu et al, 2016b). Here we compare in Figure 4 the spatial distribution of the photoelectron occurrence rate between high and low upstream solar wind conditions, defined with the respective solar wind dynamic pressures above and below 0.65 nPa, respectively, where the solar wind dynamic pressure from each individual MAVEN orbit is adapted from Niu et al (2020) based on the MAVEN SWIA measurements (Halekas et al, 2015). Within the sunlight regions, the occurrence rate of photoelectron measurements is near 100% under both high and low solar wind conditions.…”

Mars Express Observations of Cold Plasma Structures in the Martian Magnetotail

“…Moreover, crustal fields appear to be key in determining the occurrence of electron depletion regions in the nightside ionosphere at altitudes greater than ∼170 km, while no dependence on the crustal field morphology was observed for lower altitudes (Steckiewicz et al., 2015, 2017). Electron depletions are also apparently correlated with the solar wind, with a higher occurrence rate observed for low solar wind dynamic pressure (Niu et al., 2020). Crustal magnetic fields are a key factor for shaping the plasma structures in the nightside ionosphere and have been shown to substantially modulate the precipitation of energetic electrons and result in ionization (Lillis & Brain, 2013; Lillis et al., 2011).…”

Superthermal Electron Deposition on the Mars Nightside During ICMEs

“…More details of how to determine topology are in the methodology section in Xu et al (2019). These basic principles to identify topology have been used in many studies (e.g., Brain et al, 2007; Duru et al, 2011; Niu et al, 2020; Steckiewicz et al, 2015; Weber et al, 2017; Xu et al, 2017, 2019).…”