A Sporadic Topside Layer in the Ionosphere of Mars From Analysis of MGS Radio Occultation Data

Abstract

The topside ionosphere of Mars is subject to dynamic interactions between the planet and the surrounding space environment. Significant variations in its structure have been observed that have not been consistently investigated or explained. All 5,600 ionospheric profiles obtained by the Mars Global Surveyor Radio Science experiment are analyzed here to systematically characterize a topside ionospheric layer that is visible in ~50% of the observations. For simplicity, we refer to this layer of plasma as the M3 layer and find that it is located, on average, ~ 40 km above the main ionospheric peak and reaches average maximum concentrations of ~ 9 × 103 cm−3. The properties of this topside feature, derived from the Mars Global Surveyor (MGS) radio occultation data set, are compared with existing findings of topside plasma enhancements at Mars observed in other data sets. Newly found characteristics in the topside layer properties are shown to support that solar radiation is not the main production source of this layer, in contrast to the behavior expected for the lower ionospheric photochemical layers at Mars. The analysis presents a set of trends that must be explained by theory for the formation mechanisms. These trends do not support some popular explanations such as Kelvin‐Helmholtz instabilities and crustal magnetic field localization. Occurrence rates suggest a more constant source, such precipitation from the solar wind, and possible subsequent heating. These findings shed light on the enigmatic behavior of the topside Martian ionosphere and its surrounding dynamics.

On the Altitude Patterns of Photo‐Chemical‐Equilibrium in the Martian Ionosphere: A Special Role for Electron Temperature

“…Follow-up studies using multiparameter observations from MAVEN, plus 1-D modeling, indicated two types of N e enhancements at the height of the M3-layer: (1) photo-ionization of localized enhancements in the neutral atmosphere, and (2) enhancements due to other processes (e.g., field-aligned-currents, electron precipitation, solar wind interactions, and plasma instabilities). The first category identified by Mayyasi et al (2018) is classic PCE physics involving photo-ionization of the neutral atmosphere (more CO 2 →more N e )-referred to as the "coupled" case. The second category was called "uncoupled" because there is no CO 2 (h) signature simultaneously evident with the observed N e (h) feature.…”

“…The first category identified by Mayyasi et al. (2018) is classic PCE physics involving photo‐ionization of the neutral atmosphere (more CO 2 →more N e )—referred to as the “coupled” case. The second category was called “uncoupled” because there is no CO 2 (h) signature simultaneously evident with the observed N e (h) feature.…”

“…Major reviews of past studies, new analyses, and modeling of these features appear in Mayyasi et al. (2018, 2019). The reader is referred to those studies for a comprehensive list of references.…”

“…This has been a region of investigation spanning the full history of observations and modeling of Mars' ionosphere. Major reviews of past studies, new analyses, and modeling of these features appear in Mayyasi et al (2018Mayyasi et al ( , 2019. The reader is referred to those studies for a comprehensive list of references.…”

“…The Mayyasi et al. (2018) analyses of radio occultation detections of the M3‐layer led to the preliminary conclusion that a quasipermanent topside layer could be plasma produced by solar wind impact of the thermosphere above h max . Follow‐up studies using multiparameter observations from MAVEN, plus 1‐D modeling, indicated two types of N e enhancements at the height of the M3‐layer: (1) photo‐ionization of localized enhancements in the neutral atmosphere, and (2) enhancements due to other processes (e.g., field‐aligned‐currents, electron precipitation, solar wind interactions, and plasma instabilities).…”