Energetic Particles in Mercury's Magnetosphere During MESSENGER's Second Flyby

The MESSENGER spacecraft made the first direct observations of Mercury's magnetosphere in over 30 years during its first flyby on 14 January 2008. The interplanetary magnetic field (IMF) was northward immediately prior to and following MESSENGER's equatorial passage through this small magnetosphere. The Energetic Particle Spectrometer (EPS) did not observe the intense bursts of energetic electrons reported by Mariner 10 in 1974. EPS, which responds to electrons from ~15 keV to ~1 MeV and ions from ~15 keV to ~3 MeV, did not see any increases in particle intensity above instrumental background (a generous upper limit is 0.1% of those reported by Mariner 10) during its passage through Mercury's magnetosphere. In contrast, a low- energy, multi-component plasma was measured throughout the magnetospheric traversal. MESSENGER's next flyby will be on 6 October 2008. Although the Sun has been very quiet during the months leading up to the second flyby with almost no sunspots or active regions visible on the solar disk, strong high-speed solar wind stream structures indicative of solar minimum have been observed for many solar rotations. Under an extended period of southward IMF, high-speed solar wind enhances reconnection, which in turn is expected to lead to energetic particle acceleration events. If there is significant solar flare activity, solar energetic particles should have easy access into Mercury's magnetosphere. Even in the absence of solar flare activity during MESSENGER's second flyby, however, high-speed solar wind may produce bursts of energetic particles in Mercury's magnetosphere. Direct observations of these bursts could resolve the controversy over the true composition of these particles that has existed for over 30 years because of questions about the response of the Mariner 10 instrument.