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

A W Smith,J A Slavin,G.‐K Poh,R C Fear,C M Jackman

doi:10.1002/2017ja024295

Abstract

AbstractAn 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.

Highlights

The flybys of Mercury by the Mariner 10 and MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft demonstrated that Mercury possesses an internal magnetic field, one with the same polarity as Earth’s field and a magnetic dipole moment of 195±10 nT R3M [Ness et al, 1974; Anderson et al, 2012]
A total of 248 Minimum Variance Analysis (MVA) confirmed flux ropes were located within the 319 plasma sheet crossings, 74 of which (∼ 30%) satisfactorily fit the force-free model
It is likely that this fraction is an overestimate; if the flux ropes deviate far from cylindrical and force-free they are unlikely to be selected based on the results of their MVA properties

Summary

Introduction

The flybys of Mercury by the Mariner 10 and MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft demonstrated that Mercury possesses an internal magnetic field, one with the same polarity as Earth’s field and a magnetic dipole moment of 195±10 nT R3M (where RM is the radius of Mercury: 2440 km) [Ness et al, 1974; Anderson et al, 2012]. The combination of its weak internal field (approximately 1 % the strength of Earth’s field) and an average distance to the Sun of only ∼ 0.38 AU results in one of the most extreme magnetospheres found in the solar system; one that can vary on the timescale of minutes [Slavin et al, 2010; Sun et al, 2015]. The strong solar wind conditions and relatively small internal magnetic field contribute to create a very compact magnetosphere, with an average magnetopause standoff distance of only ∼ 1.45 RM [Ness et al, 1976; Winslow et al, 2013; Zhong et al, 2015]. During extreme solar wind dynamic events the magnetopause standoff distance has been observed to decrease to 1.03 − 1.12 RM , just above the planetary surface [Slavin et al, 2014]

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