MESSENGER Observations of Magnetotail Loading and Unloading: Implications for Substorms at Mercury

S M Imber,J A Slavin

doi:10.1002/2017ja024332

Abstract

AbstractWe present the first statistical study of loading and unloading of magnetic flux in Mercury's magnetotail. These events describe the global circulation of magnetic flux through the magnetosphere and provide strong evidence that terrestrial‐type substorms take place at Mercury. 438 events were identified over the 4 years of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission by a gradual, short‐lived increase in the magnetotail lobe magnetic field strength, coincident with an outward flaring of the magnetotail. Substorm duration ranged from tens of seconds to several minutes, with a median of 195 s and a mean of 212 s. The median amplitude of lobe magnetic field increase was ~11.5 nT, which represents an increase of 23.4% on the background lobe field strength, compared with ~10% for terrestrial substorms. The magnetotail lobes were found to contain ~2–3 MWb of magnetic flux based on 1031 tail passes, with a mean of 2.52 MWb and a standard deviation of 0.48 MWb. An estimate of the change in open flux content during the loading phase of each substorm ranged from 0.08 to 3.7 MWb with a mean value of 0.69 MWb and a standard deviation of 0.38 MWb. These changes in open flux content are an underestimate as the change in magnetotail radius during the events was not accounted for. The maximum lobe flux content during each substorm (~3 MWb) represented ~40% of the total available magnetic flux in the system (~7.5 MWb). During terrestrial substorms, the maximum lobe magnetic flux content is ~10–12% of the total flux from the dipole. A typical substorm at Mercury therefore cycles through a significantly larger fraction of the available magnetic flux than all but the largest substorms at the Earth.

Highlights

The extreme solar wind in the inner heliosphere drives Mercury’s highly dynamic magnetosphere
These events describe the global circulation of magnetic flux through the magnetosphere and provide strong evidence that terrestrial-type substorms take place at Mercury. 438 events were identified over the 4 years of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission by a gradual, short-lived increase in the magnetotail lobe magnetic field strength, coincident with an outward flaring of the magnetotail
The median amplitude of lobe magnetic field increase was ~11.5 nT, which represents an increase of 23.4% on the background lobe field strength, compared with ~10% for terrestrial substorms

Summary

Introduction

The extreme solar wind in the inner heliosphere drives Mercury’s highly dynamic magnetosphere. The range of dayside values observed by Gershman et al corresponds to extremes in magnetospheric open flux content of a factor of two These observations are further supported by observations of short-lived (~3 s) depressions in the cusp magnetic field strength, termed cusp plasma filaments, thought to be related to plasma injection into the cusp. A study of both the Mariner 10 data and the MESSENGER flybys reported further observations of tail reconnection signatures observed near the peak of tail loading-unloading events, analogous to observations of the terrestrial magnetosphere (Slavin, Imber, et al, 2012) Both flux ropes and the signature of their compressional effect on the lobes have been extensively studied at the Earth and shown to be related to tail reconnection (e.g., Slavin et al, 2003; Imber et al, 2011).

Observations

Substorm Statistics

Results and Discussion

Conclusions