Magnetotail reconnection asymmetries in an ion-scale, Earth-like magnetosphere

John C Dorelli,Christopher M Bard

doi:10.5194/angeo-39-991-2021

Abstract

Abstract. We use a newly developed global Hall magnetohydrodynamic (MHD) code to investigate how reconnection drives magnetotail asymmetries in small, ion-scale magnetospheres. Here, we consider a magnetosphere with a similar aspect ratio to Earth but with the ion inertial length (δi) artificially inflated by a factor of 70: δi is set to the length of the planetary radius. This results in a magnetotail width on the order of 30 δi, slightly smaller than Mercury's tail and much smaller than Earth's with respect to δi. At this small size, we find that the Hall effect has significant impact on the global flow pattern, changing from a symmetric, Dungey-like convection under resistive MHD to an asymmetric pattern similar to that found in previous Hall MHD simulations of Ganymede's subsonic magnetosphere as well as other simulations of Mercury's using multi-fluid or embedded kinetic physics. We demonstrate that the Hall effect is sufficient to induce a dawnward asymmetry in observed dipolarization front locations and find quasi-periodic global-scale dipolarizations under steady, southward solar wind conditions. On average, we find a thinner current sheet dawnward; however, the measured thickness oscillates with the dipolarization cycle. During the flux-pileup stage, the dawnward current sheet can be thicker than the duskward sheet. This could be an explanation for recent observations that suggest Mercury's current sheet is actually thicker on the duskside: a sampling bias due to a longer lasting “thick” state in the sheet.

Highlights

In the magnetospheres of Mercury and Earth, observations of plasmoids, flux bundles, and dipolarization fronts (DFs) demonstrate a marked asymmetry in their distribution across the magnetotail
The general existence of tail asymmetry is thought to be a result of sub-ion-scale effects (Lu et al, 2018; Liu et al, 2019), though there is still some uncertainty about the exact manifestation and causes of specific asymmetries
Some authors argue that electron-scale physics is required (Chen et al, 2019), we show in this paper that Hall effects are sufficient to cause an asymmetry in some observed features

Summary

Introduction

In the magnetospheres of Mercury and Earth, observations of plasmoids, flux bundles, and dipolarization fronts (DFs) demonstrate a marked asymmetry in their distribution across the magnetotail. The general existence of tail asymmetry is thought to be a result of sub-ion-scale effects (Lu et al, 2018; Liu et al, 2019), though there is still some uncertainty about the exact manifestation and causes of specific asymmetries It is debated whether Hall electric fields are sufficient to reproduce this or if other ion-/electron-scale physics are required. Some authors argue that electron-scale physics is required (Chen et al, 2019), we show in this paper that Hall effects are sufficient to cause an asymmetry in some observed features It is unknown exactly why Mercury and Earth observe different asymmetries; it is hypothesized that system-size effects (relative to the ion inertial length δi) play a key role (Lu et al, 2016, 2018; Liu et al, 2019)

Methods

Results

Conclusion