The effects of crustal magnetic fields and the pressure balance in the high latitude ionosphere/atmosphere at Mars

Abstract

The strongest crustal magnetic fields at Mars are located in certain regions in the Southern hemisphere and lead to the formation of large-scale mini-magnetospheres. In the Northern hemisphere, the crustal fields are rather weak and usually do not prevent direct interaction between the SW and an ionosphere/atmosphere. Exceptions occur in the isolated mini-magnetospheres formed by the crustal anomalies. Electron density profiles derived from radio occultation data obtained by the Radio Science Mars Global Surveyor (MGS) experiment have been compared with the crustal magnetic fields measured by the MGS Magnetometer/Electron reflectometer (MAG/ER) experiment. The effective scale-height of the electron density for two altitude ranges, 145–165 km and 165–185 km have been derived for each of the profiles studied. For the regions outside of the mini-magnetospheres the thermal pressure of the ionospheric plasma for the altitude range 145–185 km has been derived. The pressure balance in the high latitude ionosphere at Mars has been studied. In the Northern hemisphere average pressure at the altitude 160 km p 160 is practically independent of SZA. In the Northern hemisphere B 2/8π can exceed p 160 in half the cases at altitudes 170–180 km, and magnetic forces can drive effective convection. In large scale mini-magnetospheres in the Southern hemisphere the ratio H 145–165/ H 165–185 on average, larger and more variable than in the Northern hemisphere. This suggests that plasma convection at altitudes above 165 km is effective. Within the Martian mini-magnetospheres plasma convection has to be primarily a drift of charged particles across the strong crustal magnetic fields and in this regard Martian mini-magnetospheres are similar to the terrestrial magnetosphere.