MESSENGER observations of the energization and heating of protons in the near‐Mercury magnetotail

Abstract

The energization and heating processes for protons in the near‐Mercury tail are examined with MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) observations. In a case study, suprathermal proton particle flux (STPF) and proton temperature are observed to be clearly enhanced during near‐Mercury substorm dipolarizations, indicating the proton energization and heating processes. STPF and proton temperature distributions in near‐Mercury central plasma sheets display dawn‐dusk asymmetries, with higher values in the dawnside plasma sheet, i.e., postmidnight, than in the duskside, i.e., premidnight. Further investigations reveal that these asymmetries are more prominent during active periods in Mercury's magnetosphere, as compared to quiet periods. Magnetic field variations in the ZMSM component display a similar feature, with variations being more prominent on the dawnside than the duskside during active periods. We propose that the dawn‐dusk asymmetry in the distributions of protons could be due to the fact that more substorm dipolarizations were initiated on the dawnside of Mercury's magnetotail.

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

“…The protons around the dipolarization front are moving duskward in the current sheet (see the proton streamlines in Figure ). However, the high‐speed proton jets are more frequently observed in the dawn sector near Mercury, which is consistent with MESSENGER observations (Sun et al, ). Since both the dipolarization fronts and the high‐speed protons prefer the dawnside, it is possible that the protons are accelerated by the dipolarization fronts (Zhou et al, ).…”

“…In previous studies at Mercury, Sun et al () has shown clear dawn‐dusk asymmetry of dipolarization fronts in the near‐Mercury‐neutral‐line region with more dipolarization fronts on the dawnside plasma sheet than on the duskside plasma sheet. The following studies on the dipolarization fronts in the near‐Mercury plasma sheet, proton energization and heating, energetic electrons and proton bulk flows have shown the similar dawn‐dusk asymmetries (Dewey et al, , ; Sun et al, ).…”

“…The dawnward drifting of the electrons may explain the energetic electrons dawn‐dusk asymmetry (Lindsay et al, ). However, the study of magnetic reconnection related magnetic structures, which are flux ropes and dipolarization fronts, in the near‐Mercury‐neutral‐line region showed both structures are also more frequently observed on the dawnside than on the duskside, which suggests the magnetic reconnection may prefer to happen on the dawnside and therefore created more energetic electrons in the postmidnight region than in the premidnight region (Sun et al, ; see also, Smith et al, ; Sun et al, ). The dawnside magnetic reconnection preferential occurrence in Mercury's plasma sheet is different from the observations in Earth's magnetosphere, where the magnetic reconnection‐related dynamic processes, such as the flux ropes (Imber et al, ) and dipolarization fronts (Liu et al, ), prefer the duskside plasma sheet.…”

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).…”

“…Particles energized and accelerated by magnetic reconnection and dipolarization fronts (Sun et al, 2017) execute a larger, more chaotic gyromotion and are more likely to be lost from the flux tubes into other magnetospheric regions or shadowing of the nightside magnetopause. During active magnetic reconnection, particle scattering effects play an even more important role in the transport of particles from one magnetospheric region to another.…”