Fourier Analysis of Magnetic Fields in the Ionosphere of Mars

The ionosphere of Mars is almost always magnetized. The two sources of magnetic field are the solar wind interaction with the planet, and, mainly in the southern hemisphere, crustal magnetization. The magnetic fields measured in the ionosphere by the magnetometer (MAG) onboard NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft and by the magnetometer onboard the Mars Global Surveyor (MGS) display a wide range of spatial scales, from the global (i.e., L ≈ 103 km) to mesoscale (L ≈ 102 km) to small-scale (L < 10 km). Hamil et. al. (2022) used MAVEN magnetometer data to study the details of the structures, and suggested that they might be advected into the ionosphere from the solar wind. For this talk we carry out a Fourier analysis of the fields and interpret the resulting power spectral density (PSD) profiles. This analysis shows that the main determinant of ionospheric magnetic structure outside crustal field regions is the solar wind. The spectral density frequency profiles are similar to those from the solar wind (i.e., power law with index of about -2), but shifted in frequency by a factor of 100 or so. From this comparison and the observed frequency shift, we deduce that the ionospheric plasma, carrying magnetic field with it, moves downward with at about 10 to 20 meters per second. The derived power spectra also show hints of extra power at a spatial scale of about 10 km, which might be due to the evolution of magnetic structures within the ionosphere itself.