Using Magnetic Topology to Probe the Sources of Mars' Nightside Ionosphere

Abstract

We combine thermal electron densities in Mars' ionosphere with magnetic topology information to investigate the sources of the nightside ionosphere. Thermal electron density is measured in situ by the Langmuir Probe and Waves experiment onboard Mars Atmospheric and Volatile EvolutioN, while magnetic topology is simultaneously inferred from suprathermal electron energy‐pitch angle distributions measured by the Solar Wind Electron Analyzer and the Magnetometer. Topologically closed regions inhibit electron impact ionization, allowing us to isolate the effects of plasma transport from the dayside, which exhibits a dawn‐dusk asymmetry. Pressure gradient forces on open magnetic field lines connected to the dayside ionosphere source the high‐altitude nightside ionosphere, resulting in higher densities. Regions that are topologically open to the nightside ionosphere allow us to assess in situ production by electron impact ionization, which is responsible for ~50% of the nightside ionosphere below ~160 km and ~25% above ~220 km (on average).

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

“…The scenario of electron impact ionization appears to be able to account for the NGIMS observations over the southern hemisphere, whereas the plasma transport scenario has to be invoked to interpret the observations over the northern hemisphere. Since day-to-night transport is well known to be an effective plasma source of the nightside Martian ionosphere near the terminator (Adams et al, 2018;Cui et al, 2015;Duru et al, 2011;Nemec et al, 2010;Withers et al, 2012;Zhang et al, 1990), our analysis presented here essentially implies that such a mechanism is suppressed over the southern hemisphere, which we propose to be caused by the presence of strong crustal magnetic fields (e.g., Connerney et al, 1999).…”

“…To interpret the remaining characteristics revealed by the NGIMS data, the scenario of day‐to‐night plasma transport is invoked, which is known to be a viable mechanism supporting a significant nightside ionosphere on Mars (Adams et al, ; Cui et al, ; Duru et al, ; Girazian et al, ; Nemec et al, ; Withers et al, ; Zhang et al, ). Such a scenario is able to account for not only the dawn‐dusk asymmetry in ion density but also the presence of NO + and HCO + as two exceptions which are significantly more extended into darkness as compared to the remaining species on the dusk side.…”

“…The above discussions call for an alternative plasma source which not only accounts for the dawn‐dusk asymmetry in the nightside Martian ionosphere near the terminator but also predicts a more rapid depletion in ion density beyond the terminator, as compared to both photoionization and electron impact ionization. We propose that such an alternative plasma source is the transport of dayside ions to the nightside (e.g., Adams et al, ; Cui et al, ; Duru et al, ; Girazian et al, ; Nemec et al, ; Withers et al, ). For the simplest scenario, ions observed on the dusk side are direct remnants from the dayside, whereas ions observed on the dawn side are dayside remnants that pass through the dusk terminator and then the midnight.…”

Spectral Analysis of Accelerated Electron Populations at Mars

“…The influence of this complex geometry on the Martian ionosphere is critical on the nightside where the solar extreme ultraviolet photons, as the dominant dayside ionization source, are absent. In the absence of photoionization, the high recombination rates in the Martian ionosphere (e.g., Fowler et al, 2015, and references therein) imply that the nightside ionosphere is maintained either via plasma transport from the dayside (e.g., Adams et al, 2018;Cui et al, 2015;Girazian, Mahaffy, Lillis, Benna, Elrod & Jakosky, 2017) or via impact ionization by precipitating electrons (e.g., Girazian, Mahaffy, Lillis, Benna, Elrod, Fowler, et al, 2017;Fillingim et al, 2007Fillingim et al, , 2010Lillis et al, 2011Lillis et al, , 2018. Understanding the characteristics and energetics of the nightside ionosphere, therefore, will benefit from understanding the details and extent to which the various ionizing particle populations gain access to this region.…”

Variability of the Martian ionosphere from the MAVEN Radio Occultation Science Experiment

“…On the nightside, the Martian ionosphere is patchy and sporadic, with electron precipitation generally thought to be the dominant source of ionization (e.g., Verigin et al, ; Safaeinili et al, ; Fowler et al, ; Girazian et al, ). In practice, energetic electrons capable of ionizing the nightside Martian atmosphere could be either solar wind electrons or photoelectrons produced on the dayside and transported to the nightside under favorable ambient magnetic field configurations (Xu SS et al, , ; Weber et al, ; Adams et al, ). In addition, horizontal transport has been suggested as a viable mechanism supporting a nightside ionosphere on Mars, especially near the terminator (e.g., Zhang et al, ; Němec et al, ; Duru et al, ; Withers et al, ; Cui J et al, ; Girazian et al, ).…”