Cluster magnetotail observations of a tailward-travelling plasmoid at substorm expansion phase onset and field aligned currents in the plasma sheet boundary layer

N C Draper,A N Fazakerley,M G Henderson,E Donovan,J Waterman,A Grocott,C J Owen,S W H Cowley,G Provan,H J Singer,J.-M Bosqued,J P Dewhurst,J A Davies,M Lester,S E Milan,A Lahiff,J.A Wild,R Nakamura

doi:10.5194/angeo-23-3667-2005

Abstract

Abstract. We present data from both ground- and space-based instruments for a substorm event which occurred on 5 October 2002, with an expansion phase onset time of 02:50 UT determined from the ground magnetometer data. During this substorm, the Cluster spacecraft were located around 15 RE downtail, 8 RE from midnight in the pre-midnight sector and just 2 RE above the equatorial plane (in GSM coordinates). At expansion phase onset the Cluster spacecraft were located in the plasma sheet, tailward of a near-Earth neutral line and detected a significant time delay of 6 min between the tail field Bz component becoming negative and the subsequent detection of Earthward flows. This is explained by the formation of a tailward-directed travelling compression region initially Earthward of the spacecraft; 7 min later the Cluster spacecraft entered the plasma sheet boundary layer; they remained in and close to the plasma sheet boundary layer for around 15 min before exiting to the lobe. The spacecraft then re-entered the plasma sheet 30 min after onset. Earthward then tailward directed currents detected in the plasma sheet boundary layer after onset indicate that the Cluster spacecraft encountered the dawnward and duskward portions of the reconnection flow associated current system with Region 1 sense, respectively. The reconnection site and current system were initially skewed towards the pre-midnight sector, consistent with previous observations that found the majority of substorm onsets located in this sector. At later times the reconnection site and current system had moved towards dawn, to be located more centrally in the midnight sector.

Highlights

There are currently two main categories of substorm onset model which have been developed
Examination of the magnetic field data shows that these currents were associated mainly with By varying in the z direction, rather than Bz varying in the y direction
A small Jy current detected before onset is lost a few minutes prior to onset. This indicates that the spacecraft detected the rapid thinning of the plasma sheet under the spacecraft prior to onset, i.e. during the growth phase

Summary

Introduction

There are currently two main categories of substorm onset model which have been developed. The first of these is the near-Earth neutral line model (NENL; Baker et al, 1996, and references therein). The cross-field current instability model (CCI) proposed by Lui (1996), and references therein, is the second main category of substorm onset model. The NENL model suggests the spontaneous reconnection of antiparallel magnetic field lines at around 20 to 30 RE from Earth in the magnetotail (Nagai et al, 1998), as a result of the magnetotail loading that occurs due to dayside reconnection during the growth phase of the substorm. Newly-closed field lines are returned to the dayside, while tailward of the reconnection site field lines remain connected to the interplanetary

Near-Earth instruments

Cluster data overview

Cluster Magnetic Field Data

Findings

Discussion

Conclusions