Abstract
In order to further evaluate the behavior of ionospheric variations at Mars, we investigate the Martian ionosphere-thermosphere (IT) perturbations associated with non-migrating thermal tides using over four years of Mars Atmosphere and Volatile Evolution (MAVEN) in situ measurements of the IT electron and neutral densities. The results are consistent with those of previous studies, namely strong correlation between the tidal perturbations in electron and neutral densities on the dayside at altitudes ~150–185 km, as expected from photochemical theory. In addition, there are intervals during which this correlation extends to higher altitudes, up to ~270 km, where diffusive transport of plasma plays a dominant role over photochemical processes. This is significant because at these altitudes the thermosphere and ionosphere are only weakly coupled through collisions. The identified non-migrating tidal wave variations in the neutral thermosphere are predominantly wave-1, wave-2, and wave-3. Wave-1 is often the dominant wavenumber for electron density tidal variations, particularly at high altitudes over crustal fields. The Mars Climate Database (MCD) neutral densities (below 300 km along the MAVEN orbit) shows clear tidal variations which are predominantly wave-2 and wave-3, and have similar wave amplitudes to those observed.
Highlights
An important source of variability in the Martian ionosphere is that because of solar thermal tides in the thermosphere [1,2,3,4,5]
While prior investigations [1,4,14] have noted the consistency of tidal dynamics in IT coupling with photochemical processes, we show here that strong correlations occur between electron and neutral densities above the photochemically controlled altitudes where diffusive processes should dominate
We have done this by analyzing tidal wave perturbations in over two Martian years of Mars Atmosphere and Volatile Evolution (MAVEN) in situ measurement of ionospheric electron density and neutral atmospheric density
Summary
An important source of variability in the Martian ionosphere is that because of solar thermal tides in the thermosphere [1,2,3,4,5]. Thermospheric and ionospheric tides are observable in situ as perturbations in the neutral atmospheric and electron densities, respectively, as a function of longitude, typically over a limited range of local time, solar zenith angle, and latitude [1,6] (see Appendix A). The variations in the neutral atmosphere are driven by an interaction of solar heating and longitudinal/zonal asymmetries of the surface topography [7,8,9,10,11,12]. These tidal perturbations in the neutral atmosphere can give rise to variations in the ionosphere.
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