Comprehensive survey of energetic electron events in Mercury's magnetosphere with data from the MESSENGER Gamma‐Ray and Neutron Spectrometer

Abstract

Data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Gamma-Ray and Neutron Spectrometer have been used to detect and characterize energetic electron (EE) events in Mercury's magnetosphere. This instrument detects EE events indirectly via bremsstrahlung photons that are emitted when instrument and spacecraft materials stop electrons having energies of tens to hundreds of keV. From Neutron Spectrometer data taken between 18 March 2011 and 31 December 2013 we have identified 2711 EE events. EE event amplitudes versus energy are distributed as a power law and have a dynamic range of a factor of 400. The duration of the EE events ranges from tens of seconds to nearly 20 min. EE events may be classified as bursty (large variation with time over an event) or smooth (small variation). Almost all EE events are detected inside Mercury's magnetosphere on closed field lines. The precise occurrence times of EE events are stochastic, but the events are located in well-defined regions with clear boundaries that persist in time and form what we call "quasi-permanent structures." Bursty events occur closer to dawn and at higher latitudes than smooth events, which are seen near noon-to-dusk local times at lower latitudes. A subset of EE events shows strong periodicities that range from hundreds of seconds to tens of milliseconds. The few-minute periodicities are consistent with the Dungey cycle timescale for the magnetosphere and the occurrence of substorm events in Mercury's magnetotail region. Shorter periods may be related to phenomena such as north-south bounce processes for the energetic electrons.

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

“…The NS operates in three modes: near‐planet (altitude less than 7,000 km), far‐planet (altitude greater than 7,000 km), and burst. The NS was operated with accumulation times of 20, 300, and 1 s for the near‐planet, far‐planet, and burst modes, respectively (Lawrence et al, 2015). The near‐planet mode accumulation time (20 s) is short enough to study the energetic events, and hence, the data from this mode are used in this study.…”

“…Lawrence et al (2015) carried out a comprehensive survey of EE events using 30 months of continuously recorded NS data. This study showed that while Mercury does not have a trapped population of energetic particles, the stochastically timed occurrences of EE events are located in well‐defined regions and form “quasi‐permanent structures” within Mercury's magnetosphere.…”

“…This study showed that while Mercury does not have a trapped population of energetic particles, the stochastically timed occurrences of EE events are located in well‐defined regions and form “quasi‐permanent structures” within Mercury's magnetosphere. The Lawrence et al (2015) study identified over 2,700 electron burst events and showed their temporal, spatial, and spectral behavior. In a subsequent study, Baker et al (2016) investigated the most intense EE events among the GRS high‐resolution measurements and found signatures of accelerated electrons being injected from the near‐tail region and forming quasi‐trapped populations.…”

“…In this study, we carry out an analysis of EE events measured with the GRS ACS (abbreviated hereafter as ACS) that is analogous to the analysis of NS data carried out by Lawrence et al (2015). Based on this analysis, we then characterize EE events observed by both the NS and ACS to investigate possible anisotropic behavior of EE events.…”

“…Based on this analysis, we then characterize EE events observed by both the NS and ACS to investigate possible anisotropic behavior of EE events. While both NS and ACS are sensitive to bremsstrahlung photons produced when energetic electrons impact materials located near the respective sensors (Lawrence et al, 2015), they were located on different sides of the MESSENGER spacecraft and thus may have different sensitivities to directional EE events. In section 2, we present a brief description of the two sensors, the data used in this study, and the EE event detection and classification.…”