Does the Precipitation of Solar Wind Plasma Cause the Ionospheric Upwellings Detected by MARSIS on the Dayside of Mars?

Abstract:

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) onboard the Mars Express spacecraft possesses an ionospheric mode, which is used for local and remote sounding measurements of the Martian topside ionosphere. Ideally, the sounding pulse transmitted by MARSIS gives a vertical reflection from the horizontally stratified ionosphere, down to the ionospheric peak. In practice, this is usually the case, however oblique reflections are also detected. These oblique reflections are often found in regions where the remanent crustal magnetic field is nearly vertical and the sources of reflections are often at a higher apparent altitude than the surrounding ionosphere for the same electron density level. There are recurring observations of such ionospheric upwellings during repeated passes of Mars Express above certain regions over time periods of tens of days. An increased ionospheric scale height seems to create these plasma bulges. A possible cause is a localized heating of the neutral atmosphere due to the entrance of solar wind plasma through the magnetic cusps. We test this explanation by using in situ measurements of electron energy distributions made by the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) onboard Mars Express. The statistical study considers dayside oblique echoes (solar zenith angle ≤ 90°) with spacecraft altitude ≤1100 km, for orbits with sufficient MARSIS data coverage and corresponding to a criterion of repetitive passes above a given region. We keep only oblique echoes which are no further below than 10 km in apparent altitude compared to the surrounding ionosphere (most of the cases), to ensure the echoes most likely come from near the vertical direction, at the time of closest approach. Finally we take the oblique echoes with simultaneous ASPERA-3 data, with short time intervals (up to 2 minutes) before and after the time of closest approach. This leaves 761 oblique echoes. The intervals are then manually checked for the fraction of electron spectra having features typical of the magnetosheath and magnetic pileup region, such as differential energy flux intensity at energies >50 eV. The results are a classification of the oblique echoes into: solar wind only, no solar wind and mixing of both, which we can use to characterize the generation of ionospheric bulges.