Mapping Mercury’s magnetic topology with solar energetic electrons
Thursday, 17 December 2015: 08:30
2007 (Moscone West)
The inner heliosphere is bathed in MeV electrons during solar energetic particle (SEP) events. These relativistic electrons can enter the magnetosphere of Mercury via open magnetic field lines, nearly instantaneously filling the magnetospheric lobes and polar cap regions. Because of its thin shielding, the Fast Imaging Plasma Spectrometer on the MErcury, Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft provides highly sensitive measurements of these energetic particles. From data during 11 SEP events that span 33 MESSENGER orbits, we find that sharp changes in observed energetic particle flux coincide precisely with topological boundaries in the magnetosphere such as the magnetopause, polar cap, and central plasma sheet. Precipitating electrons have been measured over the entire polar cap area, demonstrating that electron space weathering of Mercury’s surface is not limited to the dayside cusp region. Although no direct measurement of low energy is made by MESSENGER, the physics elucidated by these observations suggests that suprathermal electrons from the nominal solar wind create a “polar rain” at Mercury, regularly showering the majority planet’s surface with ~107–109 cm-2 s-1 fluxes of ~100 eV electrons. Finally, we observe enhanced MeV electron fluxes within the central plasma sheet that appear ordered by magnetic latitude rather than invariant latitude. Such structure implies that although these particles cannot form a stable ring current around the planet, their non-adiabatic motion nonetheless results in a quasi-trapped population at low latitudes in the magnetotail.