Structural Variability of the Nightside Martian Ionosphere Near the Terminator: Implications on Plasma Sources

Abstract

With the aid of the Neutral Gas and Ion Mass Spectrometer measurements made onboard the Mars Atmosphere and Volatile Evolution, we examine the morphology of the nightside Martian ionosphere near the terminator at 150–250 km in terms of the extension into darkness for a selected set of ion species: O 2+, NO+, HNO+, N 2+/CO+, CO 2+, and HCO+. Our analysis reveals several interesting characteristics, including the presence of dawn‐dusk and north‐south asymmetries and the variability among different species. A full interpretation of the Neutral Gas and Ion Mass Spectrometer observations relies on the combination of impact ionization by precipitating electrons and day‐to‐night plasma transport, as two important sources of plasma in the nightside Martian ionosphere near the terminator. The former partially contributes to the north‐south asymmetry, whereas the latter favorably explains not only the dawn‐dusk asymmetry but also NO+ and HCO+ as two exceptions which are more extended into darkness as compared to the other species on the dusk side. A comparison between the two plasma sources further indicates that the effect of day‐to‐night transport tends to be suppressed over the southern hemisphere. The above results highlight the impacts of crustal magnetic fields on the structural variability of the nightside Martian ionosphere near the terminator, manifest as the shielding of precipitating electrons and the suppression of day‐to‐night transport, both by the presence of strong magnetic anomalies clustering over the southern hemisphere of the planet.

Energetic Electron Depletions in the Nightside Martian Upper Atmosphere Revisited

“…Such a feature is not only seen well beyond the EUV terminator but also seen in regions where day‐to‐night plasma transport is likely more important than SW electron precipitation as a source for maintaining the nightside ionosphere (e.g., Cui et al, ; Girazian et al, ; Withers et al, ). These observations agree with earlier studies that closed magnetic loops forming near strong crustal anomalies act to hinder both SW electron precipitation and day‐to‐night plasma transport (e.g., Adams et al, ; Cao et al, ). However, NO + behaves as an exception with comparable abundances between regions with and without depletions, likely due to the replenishment of this species on the nightside by ion‐neutral chemistry (Girazian et al, ).…”

“…Despite this, the ion density differences between regions with and without depletions are still prominent, which could be interpreted by the fact that the closed magnetic loops with both footprints on the nightside should not only hinder the precipitation of SW electrons but also hinder the horizontal transport of ionospheric plasma from the dayside. This is consistent with the recent study of Cao et al () revealing that day‐to‐night transport in the Martian ionosphere tends to be suppressed in the presence of strong crustal magnetic fields.…”

Comparison of terrestrial and Martian TEC at dawn and dusk during solstices

“…Nevertheless, there are other processes that dominate at higher altitudes (typically above ∼200 km) for all SZAs, such as vertical transport (e.g., Chen et al 1978;Mendillo et al 2011;Kar et al 1996;Fox 1997Fox , 2009Cravens et al 2017;Wu et al 2019, ), which are not investigated in this study because their contribution to the TEC is smaller. In addition, at high SZAs and far from crustal field regions, dayto-night plasma transport plays an important role in shaping the Martian ionosphere near and beyond the terminator (e.g., Nĕmec et al 2010;Withers et al 2012;Cui et al 2015;Girazian et al 2017;Cao et al 2019).…”

“…Plasma transport into the nightside is not investigated in this study, although it should be noted that the low TEC values beyond 90 • SZA should be affected by this process (e.g., Cui et al 2015;Cao et al 2019). The Huber fits to Martian TEC as a function of SZA in the four SZA regions are shown in Fig.…”

“…At Earth, O + remains the most abundant ion in the F-region, with O + 2 , N + 2 , and NO + playing important roles in the E-region (Schunk and Nagy 2009). The Martian nightside ionosphere near the terminator is maintained thanks to dayto-night plasma transport (e.g., Chen et al 1978;Cui et al 2015) together with impact ionisation by precipitating electrons (e.g., Cao et al 2019), where densities of the major ions decrease with the solar zenith angle (SZA) across the terminator (e.g., Girazian et al 2017;Withers et al 2012). Far from the terminator, electron precipitation is the dominant source of ionisation, which is more important over regions of strong crustal magnetic fields (as will be described in the Section "Planetary environments") (e.g., Nĕmec et al 2010;Lillis et al 2018), and during solar storms (e.g., Sánchez-Cano et al 2019).…”