Mars Express Pickup Ion Observations as an Interplanetary Magnetic Field Proxy

Without a global dipole magnetic field, the interaction between the solar wind and Martian atmosphere generates an induced magnetosphere. The upstream interplanetary magnetic field (IMF) largely controls the spatial distributions of plasma, currents, and electromagnetic fields of the induced magnetosphere. The ESA Mars Express (MEX) spacecraft has been observing Martian induced magnetosphere since 2004. However, without a magnetometer onboard, some studies of Martian ion loss and plasma environments based on MEX data have been limited by the lack of IMF information.

In order to better utilize MEX data to understand Martian induced magnetosphere, we developed an IMF clock angle proxy based on MEX measurements of heavy pickup ions that are accelerated by the motional electric field (E = -V_SW x B_IMF). The direction of these heavy pickup ion fluxes tells us about the IMF direction in the plane perpendicular to the solar wind (i.e. clock angle). Our preliminary results based on MEX heavy pickup ion measurements compared with the simultaneous IMF measurements by the magnetometer on the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft show that this IMF clock angle proxy has an error ≲30 degree for most of the cases. The energy and position of these heavy pickup ions can be used to derive the magnitude of the IMF component perpendicular to the solar wind. Furthermore, the IMF cone angle (angle between IMF and solar wind) can be derived from the pickup protons when a ring distribution is observed (Yamauchi et al., 2015). Therefore, the full upstream IMF information: magnitude and direction, may be obtained from MEX pickup ion observations under certain conditions. We will present our most up-to-date results on the development of the IMF proxy based on MEX ion data.

This proxy can be applied to the analysis of over a decade of MEX data prior to the MAVEN mission and later time periods when MAVEN is not in the upstream region to measure IMF, which can help to improve our understanding of the long-term variation of the induced magnetosphere and ion loss of Mars.