Flux ropes in the Hermean magnetotail: Distribution, properties, and formation

Abstract

An automated method was applied to identify magnetotail flux rope encounters in MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) magnetometer data. The method identified significant deflections of the north‐south component of the magnetic field coincident with enhancements in the total field or dawn‐dusk component. Two hundred forty‐eight flux ropes are identified that possess well‐defined minimum variance analysis (MVA) coordinate systems, with clear rotations of the field. Approximately 30% can be well approximated by the cylindrically symmetric, linearly force‐free model. Flux ropes are most common moving planetward, in the postmidnight sector. Observations are intermittent, with the majority (61%) of plasma sheet passages yielding no flux ropes; however, the peak rate of flux ropes during a reconnection episode is ∼5 min−1. Overall, the peak postmidnight rate is ∼0.25 min−1. Only 25% of flux ropes are observed in isolation. The radius of flux ropes is comparable to the ion inertial length within Mercury's magnetotail plasma sheet. No clear statistical separation is observed between tailward and planetward moving flux ropes, suggesting the near‐Mercury neutral line (NMNL) is highly variable. Flux ropes are more likely to be observed if the preceding lobe field is enhanced over background levels. A very weak correlation is observed between the flux rope core field and the preceding lobe field orientation; a stronger relationship is found with the orientation of the field within the plasma sheet. The core field strength measured is ∼6 times stronger than the local dawn‐dusk plasma sheet magnetic field.

A Statistical Study of the Force Balance and Structure in the Flux Ropes in Mercury's Magnetotail

“…The distribution of flux ropes is skewed toward dawnside on Mercury's magnetotail as shown in Figure . This feature is similar to the previous observations (Poh et al, ; Smith et al, ; Sun et al, ) and is different from the distributions in Earth's magnetotail (e.g., Imber et al, ; Slavin et al, ). In the Earth's studies, flux ropes and TCRs were more frequently observed on the duskside than on the dawnside in the near Earth neutral line region (Imber et al, ; Slavin et al, , ).…”

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

“…But it is still rare to see tailward jets beyond y =−1.0 R M or to see planetward jets beyond y =−0.5 R M . MESSENGER observed many such events far away from the midnight direction, such as the dipolarization fronts in Figure and statistics from other papers (Dewey et al, ; Sun et al, , ). This discrepancy may be simply caused by the varying IMF in the observations.…”

“…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.…”

“…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.…”

Evaluating Single‐Spacecraft Observations of Planetary Magnetotails With Simple Monte Carlo Simulations: 1. Spatial Distributions of the Neutral Line

“…Smith et al () investigated the number of flux ropes observed during plasma sheet crossings, as well as the spacing between consecutive observations. The majority of crossings (61%) did not feature any flux ropes, while groups of up to eight were observed during periods of intense activity.…”