Detection of energetic electrons in Mercury's magnetosphere with the MESSENGER Gamma-Ray Spectrometer

Details Detection of energetic electrons in Mercury's magnetosphere with the MESSENGER Gamma-Ray Spectrometer Detection of energetic electrons in Mercury's magnetosphere with the MESSENGER Gamma-Ray Spectrometer

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p44a..09m&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P44A-09

Physics

[2716] Magnetospheric Physics / Energetic Particles: Precipitating, [5443] Planetary Sciences: Solid Surface Planets / Magnetospheres, [6235] Planetary Sciences: Solar System Objects / Mercury

Scientific paper

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, entered orbit around Mercury on 18 March 2011. Following instrument turn-on, several instruments in the payload began detecting energetic particles with energies from less than ~30 keV to above ~300 keV. In addition to measurements with the Energetic Particle Spectrometer (EPS) component of the Energetic Particle and Plasma Spectrometer (EPPS), fewer, but related, bursts of hard X-rays have been measured in the sensors of the Gamma-Ray and Neutron Spectrometer (GRNS) on the MESSENGER spacecraft. The Gamma-Ray Spectrometer (GRS) and Neutron Spectrometer (NS) detectors are included to detect the presence of gamma rays and neutrons, respectively, associated with the elemental composition of Mercury's surface (tens of centimeters deep) material. Such remote detections occur with relatively low count rates; hence large bursts of signals in these detectors were not anticipated. Also associated with these events are high count rates for some of the events in the X-Ray Spectrometer (XRS). Similar observations were made with the XRS during MESSENGER's Mercury flybys and identified with bremsstrahlung from in situ electrons with energies ~10 keV. Although the temporal signatures of the events differ among sensors, the accumulated evidence to date corroborates the XRS data from the MESSENGER flybys, as well as the inference of bursts of energetic electrons identified during the first Mercury flyby of Mariner 10. High-count rate events in the GRS and NS sensors have been seen typically once per orbit but with large variation in magnitude. Quantitative analysis shows that both are consistent with hard X-rays produced by thick-target bremsstrahlung in the sensor housings. The shape of the high-energy portion of the gamma-ray spectrum above background, extending from ~80 keV to at least 200 keV for the highest intensity events, does not follow a power law and is not consistent with a power-law electron distribution as might be expected from a proximate electron acceleration event. These spectra are more consistent with a thermalized electron distribution with a temperature of ~30 keV as might be expected for a quasi-steady plasma sheet distribution. All events to date have been observed at moderate latitudes, mostly but not exclusively in the northern hemisphere where MESSENGER's location has been closer to the planet at the event occurrences.

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