Ion‐Neutral Coupling in the Upper Atmosphere of Mars: A Dominant Driver of Topside Ionospheric Structure

Abstract

The structure of the upper atmosphere of Mars provides insights into the physical mechanisms that drive escape of species into outer space. Deviations in plasma density profiles with altitude from the theoretical exponentially decaying formulation have been routinely observed for decades yet remain largely unexplained. Proposed mechanisms driving this variability have focused primarily on plasma‐specific processes, as limited by past plasma‐only observations. The Mars Atmosphere and Volatile Evolution mission's Neutral Gas and Ion Mass Spectrometer data set has recently provided unprecedented planetographic coverage for both ions and neutrals in the Martian upper atmosphere. Ion, electron, and neutral density profiles with altitude, collected on the sun‐lit inbound portion of the spacecraft orbit have been analyzed. It was found that neutral species, measured between ~160 and 200 km, behave consistently with the bulk atmosphere and that variations in ion density profiles follow neutral profile variations at the same altitudes in 70% of the observations. In the remaining 30%, additional structure was apparent in the ionized species' profiles that were found to preferentially lie in regions of strong crustal field or to be measured near dawn. A 1‐D ionospheric model was used to show that many observed features in plasma profiles are directly driven by neutral atmospheric features, providing strong evidence for ion‐neutral coupling in the atmosphere of Mars.

Solar and Magnetic Control of Minor Ion Peaks in the Dayside Martian Ionosphere

“…The results presented here are useful toward establishing an overall picture of the structural variability of minor ion distribution in the dayside Martian ionosphere, highlighting the roles of solar illumination and magnetic field configuration as controlling factors. These results are well suited for follow‐up comparisons with realistic photochemical model calculations (e.g., Fox, 2009, 2015; Fox & Yeager, 2006; Krasnopolsky, 2002; Matta et al, 2013, 2014, 2015; Mayyasi et al, 2019; Shinagawa & Cravens, 1989). Withers, Vogt, Mayyasi et al, (2015) recently presented a detailed comparison between the dayside NGIMS ion density data and various model results.…”

Tidal Wave-Driven Variability in the Mars Ionosphere-Thermosphere System

“…To show the occurrence rate of statistically significant (positive) correlations between the tidal perturbations in electron and neutral densities, the percentage of time this is observed, per altitude, is given in Figure 7 for both the day (7–19 h SLT) and night sides (19–5 h SLT) separately, the extra hour of SLT included in the dusk side of the dayside range is to include regions of the atmosphere illuminated above the surface. The local times of 5–7 have been removed as a precaution because of the complex nature of the dawn terminator region [ 16 , 23 ]; though we find including this region does not significantly change the results. No distinction in the sorting has been made in L S or latitude.…”

“…The LPW measured electron densities and NGIMS closed source neutral carbon dioxide (CO 2 ), oxygen (O), diatomic nitrogen (N 2 ), carbon monoxide (CO), and argon (Ar) abundances are selected along the inbound trajectory of MAVEN’s orbit from 300 km to periapsis (typically ~150 km, and ~130 km for deep dip and aerobraking campaigns) over ~twenty-day intervals and sorted by altitude and longitude. Only the inbound segments are used in order to avoid effects of adsorption of gas on the NGIMS spectrometer cell wall [ 16 , 22 ]. NGIMS argon coverage routinely extends to higher altitudes than do the other neutral species; so, we use both neutral argon density and calculated neutral mass density.…”

“…Evaluation of the dataset is done separately for the night and day because the photo-ionization is occurring on the dayside. Statistics on the average tidal wave amplitude and correlation coefficient between the electron and neutral density variations in altitude are determined for the night (19–5 solar local time (SLT)) and day (7–19 SLT) side wave fit profiles separately (excluding the dawn terminator, 5–7 SLT, where the thermosphere exhibits more complex behavior, e.g., larger amplitude gravity waves [ 23 ] and the ionosphere exhibits plasma-specific structures [ 16 ]) and finding the mean absolute wave amplitude in each altitude range, of the full wave fit amplitude (over all longitudes). Experiments with calculating the mean RMS wave amplitude per altitude gave similar amplitudes and curve shape in amplitude-altitude space.…”

“…They found that the electron profile was characterized by a Chapman profile below ~200 km, but that at higher altitudes plasma processes where also involved. Mayyasi et al [ 16 ] compared dayside ion and neutral measured densities in the Martian ionosphere-thermosphere system between ~150 km and 200 km and showed the plasma and neutral atmosphere to be strongly correlated in 70% of the density-altitude profiles studied.…”