Mars Global Surveyor radio science electron density profiles: Some anomalous features in the Martian ionosphere

We have analyzed some 807 Mars Global Surveyor electron density profiles that are confined to the northern high latitudes and thus are relatively free of the effects of crustal magnetic fields. These profiles have shown some anomalous features in the Martian ionosphere, and one of these is the noticeable variability in number density (Nm) and height (hm) of the primary ionospheric peak on the same day when solar conditions and solar zenith angle have remained the same, a feature not expected from a photochemically controlled layer. We study this feature by generating longitudinal plots of Nm and hm for the 807 profiles and by applying a least squares spectral fit consisting of wave number 1, 2, and 3 components to these data sets. We find some significant relationship between the two parameters, with the troughs in Nm coinciding with the ridges in hm (and vice versa) on the longitudinal scale. An examination at fixed solar zenith angles shows a significant anticorrelation between the two parameters recorded over a period of about 3 months. However, theoretical considerations would support a positive correlation expected in response to changes in the EUV flux that occurred during this period. Further, we observe a large variability in electron density at 160 and 180 km, altitudes in the topside ionosphere, where photochemistry is expected to dominate. This is an additional anomalous feature. No such variability is observed in the topside ionosphere of Venus. We discuss plausible mechanisms like neutral atmosphere dynamics and solar wind interaction to explain some of the features.