Increasing activity on the Sun as next summer’s sunspot maximum approaches is allowing MESSENGER to make its first observations of Mercury’s magnetosphere under extreme solar wind conditions. At Earth, interplanetary shock waves and coronal mass ejections produce severe “space weather” in the form of large geomagnetic storms that affect telecommunications, space systems, and ground-based power grids. The primary physical process that powers magnetospheric space weather is “magnetic reconnection.” It does so by allowing magnetic fields to reconfigure in such a manner that solar wind energy is channeled deep into the magnetosphere to energize charged particles and drive intense electrical currents.  In the case of Mercury, the primary effect of extreme space weather is on the ability of its weak global magnetic field to shield the planet from the solar wind. Direct impact of the solar wind on the surface of airless bodies like Mercury results in space weathering of the regolith and the sputtering of atomic species like sodium and calcium to high altitudes where they contribute to a tenuous, but highly dynamic exosphere. MESSENGER observations indicate that during extreme interplanetary conditions the solar wind plasma gains access to the surface of Mercury in three main regions: 1. The magnetospheric cusps, which fill with solar wind plasma and move equatorward.  2. The subsolar magnetopause, which is compressed and eroded to very low altitudes where the natural gyro-motion of solar wind protons about the planetary magnetic field results in their direct impact on the surface.  3. The magnetotail, where highly energized plasma jetting sunward impacts the nightside of Mercury. The possible implications of these new observations at Mercury for the understanding of magnetic reconnection and our ability to predict space weather at Earth and other planets will be described. 

About the speaker:
James A. Slavin is Professor of Space Science and Chair, Department of Atmospheric, Oceanic, and Space Sciences at the University of Michigan, Ann Arbor.  He was at Goddard from 1987--2011, in the Laboratory for Extraterrestrial Physics and then as Director of the Heliospheric Sciences Division.