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

摘要

The ionosphere at Mars is governed by photo-chemical-equilibrium (PCE) at and below the height of maximum electron density (similar to 130 km), while the topside ionosphere blends PCE with other processes (e.g., plasma dynamics). We studied the altitude profile from 135 to 260 km where PCE conditions transition to the non-PCE domain. Using MAVEN observations of electron density (N-e) and temperature (T-e), CO2 densities, and integrated energy flux F-sun(5-93 nm), we tested the PCE relationship Ne2 proportional to Fsunxcos(SZA)xn(CO2)xTe0.7 (Mendillo et al., 2017, ). We found high correlations coefficients (CC = 80-90) for heights 25-40 km above h(max), and then decreases to CC = 50 (defining partial PCE control). Surprisingly, CCs returned to > 50 in the 160-175 height range. We found the re-establishment of PCE control occurs where the T-e values rise sharply and thereby reduce the plasma recombination rate due to its inverse dependence on electron temperature. Enhancements in the topside ionosphere due to that process were first postulated by Fox and Yeager (2006, ) to explain the "shoulder" seen N-e(h) profiles from radio occultation experiments. MAVEN data now provide the first comprehensive set of in situ observations to relate this enigmatic "bulge" to observed PCE parameters. Moreover, MAVEN observations reveal that additional sources are present to account for the N-e enhancements larger than possible from the thermal process alone.