Regional-Scale Surface Magnetic Fields and Proton Fluxes to Mercury’s Surface from Proton-Reflection Magnetometry

Tuesday, 16 December 2014
Reka M Winslow1, Catherine L Johnson1, Brian J Anderson2, Daniel J Gershman3, Jim M Raines4, Robert J Lillis5, Haje Korth6, James A Slavin4, Sean C Solomon7 and Thomas Zurbuchen8, (1)University of British Columbia, Vancouver, BC, Canada, (2)Johns Hopkins Univ, Laurel, MD, United States, (3)NASA Goddard Space Flight Center, Heliophysics Sci. Div., Greenbelt, MD, United States, (4)University of Michigan Ann Arbor, Ann Arbor, MI, United States, (5)UC Berkeley, Berkeley, CA, United States, (6)The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States, (7)Lamont-Doherty Earth Observatory, Palisades, NY, United States, (8)Univ Michigan, Ann Arbor, MI, United States
The application of a recently developed proton-reflection magnetometry technique to MESSENGER spacecraft observations at Mercury has yielded two significant findings. First, loss-cone observations directly confirm particle precipitation to Mercury’s surface and indicate that solar wind plasma persistently bombards the planet not only in the magnetic cusp regions but over a large fraction of the southern hemisphere. Second, the inferred surface field strengths independently confirm the north-south asymmetry in Mercury’s global magnetic field structure first documented from observations of magnetic equator crossings. Here we extend this work with 1.5 additional years of observations (i.e., to 2.5 years in all) to further probe Mercury’s surface magnetic field and better resolve proton flux precipitation to the planet's surface. We map regions where proton loss cones are observed; these maps indicate regions where protons precipitate directly onto the surface. The augmentation of our data set over that used in our original study allows us to examine the proton loss cones in cells of dimension 10° latitude by 20° longitude in Mercury body-fixed coordinates. We observe a transition from double-sided to single-sided loss cones in the pitch-angle distributions; this transition marks the boundary between open and closed field lines. At the surface this boundary lies between 60° and 70°N. Our observations allow the estimation of surface magnetic field strengths in the northern cusp region and the calculation of incident proton fluxes to both hemispheres. In the northern cusp, our regional-scale observations are consistent with an offset dipole field and a dipole moment of 190 nT RM3, where RM is Mercury’s radius, implying that any regional-scale variations in surface magnetic field strengths are either weak relative to the dipole field or occur at length scales smaller than the resolution of our observations (~300 km). From the global proton flux map (north of 40° S) derived from proton loss-cone measurements, we find an increase in proton flux near 0° and 180° planetary longitudes. This pattern is consistent with that expected from the combined effects of increased incident solar wind density at these longitudes at local noon (given the 3:2 spin-orbit resonance of Mercury) and phasing of MESSENGER’s orbit.