Dipolarization fronts in the near-Earth space and substorm dynamics

I I Vogiatzis,Q.-G Zong,E T Sarris,A M Tian,A Isavnin,S W Lu

doi:10.5194/angeo-33-63-2015

Abstract

Abstract. During magnetospheric substorms and plasma transport in the Earth's magnetotail various magnetic structures can be detected. Dipolarization fronts and flux ropes are the most prominent structures characteristic for substorm dynamics. However, they are treated as separate magnetotail features independent of each other. In this paper, we analyze a number of dipolarization fronts observed by the THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft at different geocentric distances by applying the magnetohydrostatic Grad–Shafranov (GS) reconstruction technique. Our analysis shows that there is a possibility of dipolarization fronts to originate from highly dissipated flux ropes which are in the late stage of their evolution, subjected to a continuous magnetic deterioration due to the reconnection process. These results may improve our understanding of magnetoplasma processes in Earth's magnetotail.

Highlights

Dipolarization fronts are a common feature of substorm dynamics routinely observed in the Earth’s magnetotail
In this case-based study, we report on a number of earthward dipolarization fronts and flux ropes identified during THEMIS tail seasons (Table 1) and analyze two representative events registered at −9.8 RE and −15.7 RE
A first attempt to decipher the process behind near-Earth flux rope dissipation was made by Vogiatzis et al (2011), where reconnection was proposed as the responsible mechanism for flux rope deterioration

Summary

Introduction

Dipolarization fronts are a common feature of substorm dynamics routinely observed in the Earth’s magnetotail. As was stated by THEMIS principal investigator in Angelopoulos (2008), “substorms represent a fundamental mode of global magnetospheric circulation, a macroscopic instability whose phenomenological and theoretical understanding is crucial for space science, basic plasma physics and space weather” and as such many studies have tried to decipher the mechanism of substorm instability (Lui, 2001, 2003, 2004; Angelopoulos et al, 2008; Baker et al, 1996; Shiokawa et al, 1998) In this case-based study, we report on a number of earthward dipolarization fronts and flux ropes identified during THEMIS tail seasons (Table 1) and analyze two representative events registered at −9.8 RE (near-Earth location, where flow braking and diversion usually happens; e.g., Keika et al, 2009; Shiokawa et al, 1997; Yao et al, 2012) and −15.7 RE (downtail location, where usually a near-Earth neutral line is formed; Cao et al, 2012, and references therein). The data analysis results and the phenomenological model presented to interpret the near-Earth event may place a new interpretation context concerning the nature of (a) the dipolarization fronts and (b) the triggering mechanism for substorm onset initiation

Analysis and results

Downtail flux rope event

Near-Earth flux rope event

Discussion and conclusions