Use of Martian Magnetic Field Topology as an Indicator of the Influence of Crustal Sources on Atmospheric Loss

Mars lacks a global magnetic field, and therefore the solar wind interacts directly with the atmosphere over much of the planet. Over some regions, however, Mars' crustal magnetic field is strong enough to locally stand off the solar wind to altitudes >1000 km, shielding the enclosed atmosphere. Like Earth's polar cusp regions, Martian crustal fields do not act as perfect shields for the atmosphere. At times, crustal magnetic field lines connect with the interplanetary magnetic field (IMF), providing conduits for charged particle exchange between the solar wind and lower ionosphere. These open field lines have been observed by spacecraft and predicted by models. The volume of atmosphere protected by crustal magnetic fields as well as the extent and variability of open field lines has implications for atmospheric escape to space and the energetics of the upper atmosphere. Electron and magnetic field data from the Mars Global Surveyor MAG/ER instrument provide a means of measuring the influence of crustal sources on nonthermal escape processes. The angular distribution of electrons observed by the electron reflectometer (ER) can be used to determine where and when crustal magnetic field lines are connected to the IMF. Using four years of data from the MGS mapping orbit, we have classified ER observations of electrons at 190 eV according to the topology of the magnetic field. This extended data set allows us to quantify the effects of a number of parameters that control magnetic field topology at 400 km. We will show where open and closed field lines are likely to occur, and how changes in solar wind pressure or the direction of the interplanetary magnetic field affect the locations of open field regions. From these results we will calculate the fraction of Mars' atmosphere that is shielded from the solar wind under different conditions, and the fraction of the lower ionosphere accessible through cusps. We will relate these results to the present day atmospheric escape rates in order to quantify the effect that crustal magnetic fields have on the efficiency of solar wind related atmospheric loss processes.