Martian nonmigrating atmospheric tides in the thermosphere and ionosphere at solar minimum

Abstract

The response of Martian nonmigrating tidal perturbations in the thermospheric and ionospheric densities to solar minimum, as contrasted with their behavior in the solar declining phase, is investigated using five years of data collected by the Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite. We find that the average tidal amplitudes, expressed as the percent variation of the neutral and electron densities from their respective average background values, are larger (by a factor of ~1.3 above 170 km) during solar minimum compared to the declining phase of the solar cycle. These larger amplitudes, on the dayside, may be due to a decrease in the atmospheric scale height, which for neutral argon averaged over a Martian year, goes from 12.5 km during declining phase, to 10.5 km for solar minimum. An additional controlling factor is that the average vertical dissipation length scale becomes shorter, going from ~34 km during the declining phase to ~29 km at solar minimum. On the nightside, the amplitude of the neutral atmospheric tides is similar between the declining phase and solar minimum. There are changes in the length scales, with the average night side scale height becoming somewhat greater during solar minimum, as well as an increase in the dissipation length scale. Dayside tidal perturbations in the ionosphere at ~145 km to ~170 km exhibit the expected amplitude relation predicted by simplified photochemistry for the observed neutral perturbation amplitudes as well as strong correlation with the longitudinal neutral density perturbations. Interpretation of the data are faced with the complication of limited coverage between season, latitude, and local times together with long temporal baselines that may mix different tidal modes; discussion on the possible role of the latitude effect on amplitude are discussed. The results of the study are put into context of a hypothesis on how tidal amplitudes might be expected to change in response to solar minimum.