Large-scale geometry and temporal variability of the Martian external magnetic field

The martian magnetic field is unique among the terrestrial planets, as it results from the interaction of fields caused by crustal remnant magnetization and a planetary ionosphere with the solar wind and the interplanetary magnetic field. Internal fields of crustal origin have been subject to extensive studies, whereas the focus of our work deals with average spatial structure and time variability in the martian external magnetic field. We use the Mars Global Surveyor (MGS) vector magnetic field data to investigate the large-scale geometry and magnitude of such external fields. We analyze the day-time and night-time magnetic signature for the duration of the MGS mission in mapping orbit (2000-2006). We use along-track vector field measurements to estimate the day-time and night-time external fields after the subtraction of predicted crustal magnetic fields at spacecraft altitudes. We also examine day/night differences (i.e., the daily variation) in external fields; these are independent of crustal fields. Because the external fields are modified by the crustal fields, we investigate their structure as a function of latitude in the local time frame and as a function of both latitude and longitude in the body-fixed frame. In the body-fixed-frame B_Θis generally dominant in magnitude with a day/night variation described to first order by a zonal degree-2 spherical harmonic structure. B_r is strongly correlated with the crustal magnetic field. B_Φ shows variable spatial behaviour during both night and day. Seasonal variations are observed as stronger average magnetic fields in the hemisphere pointing towards the sun. Additional shorter time scale variations in the global external field structure are observed.