Spatial distribution and spectral characteristics of energetic electrons in Mercury's magnetosphere

Abstract

[1] The Energetic Particle Spectrometer (EPS) on the MESSENGER spacecraft, in orbit about Mercury since March 2011, has detected bursts of low-and moderate-energy (tens to hundreds of keV) electrons during portions of most orbits. There have been periods when such bursts were observed regularly on every orbit over a span of several weeks, and other periods when electrons were not observed for several days at a time. We have systematically characterized these energetic events on the basis of particle intensity over the 12-month period since MESSENGER began orbital operations. Now that MESSENGER has sampled most Mercury longitudes and local times, it is evident that the largest burst events were either at high northern latitudes or near local midnight. Lower-energy events were also seen near the equator but were mostly absent in both the dawn and dusk local time sectors. The high-latitude and nightside events are similar in particle intensity, spectra, and pitch angle and are interpreted to be the result of acceleration by the same mechanism. Another group of events occurred upstream of Mercury's bow shock. For two examples of this group of upstream events with good pitch angle coverage, the particles were field-aligned and traveling away from the bow shock. This group of events is interpreted to be similar to upstream events found at Earth during which particles are accelerated at the bow shock and subsequently travel upstream into the solar wind.

Energetic particle dynamics in Mercury's magnetosphere

“…In surveying the electrons, Ho et al [] observed enhancements of particles within a radial distance of 1.5 R M from the planet. On the nightside near midnight, the electrons are observed near the magnetic equator, while on the dayside they are observed off‐equator at higher latitudes and sometimes even on open field lines in the cusp [ Ho et al , ]. In the dawn and dusk sectors, a small enhancement is observed near the equator.…”

“…These enhancements however are of lower intensity than the ones observed in the noon or midnight sector. When pitch angle distributions were obtainable within the magnetosphere, they were peaked near 90 ° [ Ho et al , ].…”

Planetary space weather: scientific aspects and future perspectives

“…Electrons within the 200-680 eV energy range were already detected by Mariner 10 and interpreted as coming from a hot plasma sheet (Ogilvie & Desch 1977). Furthermore, bursts of low-and moderate energy electrons (tens to hundreds of keVs, Ho et al 2012) have been often recorded by the MESSENGER Energetic Particle Spectrometer (EPS) and X-Ray Spectrometer (XRS). A systematic monitoring permitted to map the burst occurrence.…”

Statistical Study of Mercury's Energetic Electron Events as Observed by the Gamma‐Ray and Neutron Spectrometer Instrument Onboard MESSENGER

“…This issue was not resolved until shortly after the insertion of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft into its Mercury orbit, when data from the energetic particle spectrometer (EPS) demonstrated that the primary energetic particles within Mercury's magnetosphere are electrons and not protons (Ho, Krimigis, et al, 2011; Ho, Starr, et al, 2011). In a subsequent study, Ho et al (2012) reported the spatial distribution of 51 energetic electron (EE) events based on the observations from a full year of MESSENGER's primary orbital mission phase. They found that most of the intense, moderate‐energy (tens to hundreds of keV) electron events occured either at local midnight or at high northern latitudes.…”