Characterization of the M1 and M2 layers in the undisturbed Martian ionosphere at a variety of solar conditions with MAVEN ROSE

We utilize data from the MAVEN Radio Occultation Science Experiment ( Withers et al., 2020 ) - with unprecedented coverage in solar zenith angle - to isolate the effects that local time and season induce on the photochemical ionosphere of Mars around solar minimum, leading to solar maximum. 185 out of the 1228 electron density profiles of the Martian undisturbed ionosphere collected by MAVEN ROSE between July 2016 and December 2022 show a distinct M1 layer below the M2 layer. We define undisturbed here as conditions when there are no solar events or dust storms to influence the ionosphere. This allowed us to study the behavior of both the M2 and M1 peak densities and altitudes as a function of solar zenith angle, and, for the first time, to be able to separate these trends by dusk and dawn local time, as well as by southern spring and summer versus southern fall and winter. We find that the M1 layer at small SZA can occur at altitudes lower than 100 km; that the peak altitudes and densities of both the M2 and M1 layers at dawn change more with season than they do at dusk; and that the M2 peak density decreases at a faster rate than the M1 with SZA. • There is more seasonal variability in the dawn ionosphere than the dusk ionosphere. • The M1 layer is present at small SZA and, when present, is located ∼ 20 km below the M2 layer. • The ratio of M1 over M2 peak densities increases with SZA, and the vertical distance between the two layers increases.