Global-scale external magnetic fields at Mars measured at satellite altitude

The Martian magnetic field is unique among those of the terrestrial planets. It is the net result of the interaction of the solar wind and the interplanetary magnetic field (IMF) with crustal remnant magnetization and a planetary ionosphere. Internal fields of crustal origin have been the subject of extensive studies; the focus of our work is identification and characterization of contributions from external magnetic fields using the Mars Global Surveyor (MGS) vector magnetic field data. We investigate the magnitude, average spatial structure, and temporal variability of the external magnetic field at MGS mapping altitude of 400 km by first subtracting the expected contributions from crustal fields using existing global crustal field models. We identify periodicities and spatial structure in the field related to the day-night cycle and Carrington rotations, as well as variations corresponding to an annual cycle and short aperiodic signals. We suggest that ionospheric currents driven by upper atmosphere winds contribute to the observed zonal structure in the daily variation in the external magnetic field. Finally, we discuss the potential for magnetic sounding studies using time-varying external fields and surface magnetometry measurements from the InSight mission to be launched in 2018.