On improving the accuracy of electron density profiles obtained at high altitudes by the ionospheric sounder on the Mars Express spacecraft
Abstrakt
We attempt to improve the standard inversion routine used to obtain electron density profiles from the data measured by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) topside radar sounder on board the Mars Express spacecraft. Artificial ionospheric traces corresponding to how the MARSIS instrument would see model electron density profiles are constructed, and they are inverted using the standard routine.
Ideally, this should result in the original electron density profiles. However, it is found that discrepancies between the original and resulting electron density profiles may be significant, in particular at larger solar zenith angles and for higher spacecraft altitudes.
This is due to a gap of MARSIS time delay measurements at low-sounding frequencies, and a necessity to interpolate electron density profiles in this plasma frequency range. Although the ionospheric scale height likely increases with the altitude, the standard inversion routine uses an exponential interpolation with a constant scale height.
We suggest a new inversion method, which takes into account expected shapes of ionospheric profiles in the interpolation region. We verify the performance of this method both on the model electron density profiles and on real MARSIS data.
We demonstrate that it seems to perform considerably better than the standard routine, in particular for higher spacecraft altitudes. Finally, we use the new inversion method to obtain a revised dependence of the peak altitude on the solar zenith angle.