Multiscale Currents Observed by MMS in the Flow Braking Region.

R Nakamura ,Takuma Nakamura,Per‐Arne Lindqvist,Ferdinand Plaschke,Yuri V Khotyaintsev,S A Fuselier ,J Birn ,J E Stawarz ,R E Ergun ,A Varsani ,В А Сергеев ,W Baumjohann ,D Fischer ,W Magnes ,T Nagai ,G Le ,G D Reeves ,O Le Contel ,B J Anderson ,B L Giles ,R J Strangeway ,I J Cohen ,А V Artemyev ,B H Mauk ,K R Bromund ,A A Petrukovich ,C T Russell ,S Apatenkov ,A N Jaynes ,J A Slavin ,L Kepko ,D J Gershman ,H J Singer ,J L Burch ,R B Torbert ,K J Genestreti ,D L Turner

doi:10.1002/2017ja024686

Abstract

We present characteristics of current layers in the off‐equatorial near‐Earth plasma sheet boundary observed with high time‐resolution measurements from the Magnetospheric Multiscale mission during an intense substorm associated with multiple dipolarizations. The four Magnetospheric Multiscale spacecraft, separated by distances of about 50 km, were located in the southern hemisphere in the dusk portion of a substorm current wedge. They observed fast flow disturbances (up to about 500 km/s), most intense in the dawn‐dusk direction. Field‐aligned currents were observed initially within the expanding plasma sheet, where the flow and field disturbances showed the distinct pattern expected in the braking region of localized flows. Subsequently, intense thin field‐aligned current layers were detected at the inner boundary of equatorward moving flux tubes together with Earthward streaming hot ions. Intense Hall current layers were found adjacent to the field‐aligned currents. In particular, we found a Hall current structure in the vicinity of the Earthward streaming ion jet that consisted of mixed ion components, that is, hot unmagnetized ions, cold E × B drifting ions, and magnetized electrons. Our observations show that both the near‐Earth plasma jet diversion and the thin Hall current layers formed around the reconnection jet boundary are the sites where diversion of the perpendicular currents take place that contribute to the observed field‐aligned current pattern as predicted by simulations of reconnection jets. Hence, multiscale structure of flow braking is preserved in the field‐aligned currents in the off‐equatorial plasma sheet and is also translated to ionosphere to become a part of the substorm field‐aligned current system.

Highlights

The most dramatic energy release in the near-Earth magnetotail is considered to be driven by the near-Earth magnetic reconnection-associated flows, called bursty bulk flows (BBFs), and electromagnetic disturbances propagating Earthward and interacting with plasmas in the near-Earth dipolar region
We present characteristics of current layers in the off-equatorial near-Earth plasma sheet boundary observed with high time-resolution measurements from the Magnetospheric Multiscale mission during an intense substorm associated with multiple dipolarizations
Our observations show that both the near-Earth plasma jet diversion and the thin Hall current layers formed around the reconnection jet boundary are the sites where diversion of the perpendicular currents take place that contribute to the observed field-aligned current pattern as predicted by simulations of reconnection jets

Summary

Introduction

The most dramatic energy release in the near-Earth magnetotail is considered to be driven by the near-Earth magnetic reconnection-associated flows, called bursty bulk flows (BBFs), and electromagnetic disturbances propagating Earthward and interacting with plasmas in the near-Earth dipolar region. The occurrence frequency of Earthward BBFs or the rapid flux transport rate (enhanced dawn-to-dusk electric field) significantly drops inward of 10–15 RE (e.g., Schödel et al, 2001), called the flow braking region, where significant energy dissipation and current disruption (unloading) processes take place (e.g., Sergeev et al, 2012, and references therein). Based on the analysis of the flows and motion of these FACs in comparison with other spacecraft and MHD simulations (Birn & Hesse, 2014), it was concluded that MMS encountered a FAC system at the high-latitude side of the near-Earth flow-braking region These observations show that the processes of Earthward flow braking as well as accumulated magnetic flux evolving tailward, which has been detected in the center of the plasma sheet (Nakamura et al, 2009), can be detected at the boundary region of the near-Earth plasma sheet. Using high-resolution ion and electron data, particle population responsible for the currents is examined to understand the physical processes of these active plasma boundaries

Overview of the Event

FAC Layers and Characteristics of Electrons

Characteristics of Perpendicular Currents

Discussion

Conclusions