Energetic Electron Acceleration and Injection During Dipolarization Events in Mercury's Magnetotail

Abstract

Energetic particle bursts associated with dipolarization events within Mercury's magnetosphere were first observed by Mariner 10. The events appear analogous to particle injections accompanying dipolarization events at Earth. The Energetic Particle Spectrometer (3 s resolution) aboard MESSENGER determined the particle bursts are composed entirely of electrons with energies ≳ 300 keV. Here we use the Gamma‐Ray Spectrometer high‐time‐resolution (10 ms) energetic electron measurements to examine the relationship between energetic electron injections and magnetic field dipolarization in Mercury's magnetotail. Between March 2013 and April 2015, we identify 2,976 electron burst events within Mercury's magnetotail, 538 of which are closely associated with dipolarization events. These dipolarizations are detected on the basis of their rapid (~2 s) increase in the northward component of the tail magnetic field (ΔBz ~30 nT), which typically persists for ~10 s. Similar to those at Earth, we find that these dipolarizations appear to be low‐entropy, depleted flux tubes convecting planetward following the collapse of the inner magnetotail. We find that electrons experience brief, yet intense, betatron and Fermi acceleration during these dipolarizations, reaching energies ~130 keV and contributing to nightside precipitation. Thermal protons experience only modest betatron acceleration. While only ~25% of energetic electron events in Mercury's magnetotail are directly associated with dipolarization, the remaining events are consistent with the Near‐Mercury Neutral Line model of magnetotail injection and eastward drift about Mercury, finding that electrons may participate in Shabansky‐like closed drifts about the planet. Magnetotail dipolarization may be the dominant source of energetic electron acceleration in Mercury's magnetosphere.

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

“…The directionality analysis of this work supports the updated injection model described by Dewey et al (2017) and observations by Lindsay et al (2016). Dewey et al (2017) suggests electrons that drift eastward about the planet following magnetotail energization and injection may participate in Shabansky‐like orbits, in which the guiding center migrates to higher latitudes in the northern hemisphere due to compression of Mercury's dayside magnetosphere.…”

“…Dewey et al (2017) suggests electrons that drift eastward about the planet following magnetotail energization and injection may participate in Shabansky‐like orbits, in which the guiding center migrates to higher latitudes in the northern hemisphere due to compression of Mercury's dayside magnetosphere. Only gyrating electrons participate in the Shabansky‐like orbits (field‐aligned electrons are not able to execute these orbits).…”

“…In addition, a subset of the NS and GRS data sets were acquired at significantly shorter (1 s for the NS and 10 ms for the GRS) acquisition intervals than the EPS (3 s) or XRS (20 s). Such high time‐resolution capabilities are critical for characterizing charged‐particle measurements at Mercury, where the characteristic timescales are as short as a few seconds (Dewey et al, 2017; Slavin et al, 2010). Based on their sensitivity and timing characteristics, the combined NS and GRS measurements provide an optimum sensitivity and time resolution for characterizing Mercury's energetic electrons.…”

Studying Dawn‐Dusk Asymmetries of Mercury's Magnetotail Using MHD‐EPIC Simulations

“…The MESSENGER observations of current sheet thickness (Poh et al, ), flux ropes, dipolarization events (Smith et al, ; Sun et al, , ), and energetic electron events (Dewey et al, ) do not and cannot distinguish the events under different IMF conditions. For the current sheet thickness observation, the current sheet sampling is almost uniform in time.…”

Transport of Mass and Energy in Mercury's Plasma Sheet

“…Solar wind and planetary ions in Mercury's PS are known to have large gyroradius relative to the scale length of magnetic field variations due to Mercury's weak intrinsic magnetic field. At Mercury, particles were also observed to be energized and accelerated near the X-line by magnetic reconnection and reconnection-related phenomena, such as dipolarization fronts (Dewey et al, 2017;Sun et al, 2017). At Mercury, particles were also observed to be energized and accelerated near the X-line by magnetic reconnection and reconnection-related phenomena, such as dipolarization fronts (Dewey et al, 2017;Sun et al, 2017).…”