MESSENGER Observations of Suprathermal Electrons in Mercury’s Magnetosphere

Friday, 18 December 2015
Poster Hall (Moscone South)
George C Ho1, Stamatios M Krimigis1, Richard D Starr2, Jon Duane Vandegriff1, Daniel N. Baker3, Robert E Gold1, Brian J Anderson4, Haje Korth1, David Schriver5, Ralph L McNutt Jr1 and Sean C Solomon6, (1)Applied Physics Laboratory Johns Hopkins, Laurel, MD, United States, (2)Catholic University of America, Washington, DC, United States, (3)University of Colorado at Boulder, Boulder, CO, United States, (4)Johns Hopkins University, Baltimore, MD, United States, (5)University of California Los Angeles, Los Angeles, CA, United States, (6)Columbia University of New York, Palisades, NY, United States
The X-Ray Spectrometer (XRS) on the MESSENGER spacecraft, in orbit about Mercury from March 2011 to April 2015, routinely detected fluorescent X-rays induced by low-energy (1-10 keV) electrons. These electrons are in general below the threshold energy response of the Energetic Particle Spectrometer (EPS), one of two sensors on MESSENGER’s Energetic Particle and Plasma Spectrometer (EPPS) instrument that measures electrons at energies above 35 keV. Hence, the XRS provided a measure of this lower-energy suprathermal electron population at Mercury. We devised an automated algorithm to select these events from the XRS data set from April 2011 to March 2015 on the basis of the duration, location, and spectral slope of the events. We identified 3102 events in 3900 orbits around Mercury, sampling all Mercury longitudes multiple times over the four-year period. It is evident that these suprathermal electrons were present near the planet at all local times, but the majority were on the nightside of the planet, and a dawn–dusk asymmetry is clearly seen in the data. When the event locations are plotted in simplified B versus L coordinates (where B is the magnitude of the magnetic field, L defines an axisymmetric surface of those lines of magnetic force from the dipole component of Mercury’s internal field that intersect the magnetic equator at a distance L RM from the dipole center, and RM is Mercury’s radius), several distinct clusters of events can be seen. We infer that all of these are signatures of accelerated electrons being injected from Mercury’s tail region to form a quasi-trapped electron distribution at Mercury.