The altitude patterns of photo-chemical-equilibrium conditions in the Martian ionosphere

Mars has primary and secondary ionospheric layers at ~130 km and ~110 km, respectively. These layers are produced by solar EUV and X-ray ionizing radiation, and they are lost by plasma recombination reactions. Plasma dynamics is minimal within these layers and thus electron densities (N_e) are governed by the balance of production and loss—i.e., by photo-chemical-equilibrium (PCE). Such PCE conditions exist because the background neutral atmosphere is so dense (CO₂ >> N_e). In the topside ionosphere, however, the neutrals are far less abundant and thus plasma dynamics (internal diffusion and solar wind coupling) competes with local chemistry to violate pure PCE control. Here we study the altitude profile of PCE vs. non-PCE correlations using Mars Atmosphere and Volatile EvolutioN (MAVEN) data sets spanning altitudes 130-400 km. PCE conditions are defined as N_e² ~ n(CO₂) * F_sun * cos(SZA) *T_e^0.7 (Mendillo et al., 2017), where electron densities and temperatures come from the Langmuir Probe and Waves instrument, CO₂ densities from the Neutral Gas and Ions Mass Spectrometer, and integrated solar irradiance from the Extreme Ultraviolet Monitor. The correlation of N_e² with the PCE factor (F_PCE) is typically 90% from N_max at h_max up to ~ 170 km. The F_PCE correlation with N_e² then decreases rapidly with height. We use a correlation value of 50% to define non-PCE control and find, surprisingly, that there is not a monotonic decrease from PCE to non-PCE conditions. We examined 200 orbits during three Deep Dip campaigns to search for the parameter (solar, neutral gas and plasma) responsible for the return to PCE control. The key finding was that it occurs at the height where the T_e values rise sharply in the topside ionosphere. This T_e-induced return to PCE dominance was first postulated by Fox and Yeager (2006) to explain the "shoulder" seen in topside patterns of N_e(h) profiles from radio occultation experiments. MAVEN data now provide the first set of in-situ observations to relate this enigmatic "bulge" (Peter et al., 2014) to observed PCE parameters.

References: Fox & Yeager, JGR, 2006; Mendillo et al., JGR, 2017; Peter et al., Icarus, 2014.