Ion reflection and acceleration near magnetotail dipolarization fronts associated with magnetic reconnection

J P Eastwood,R Mistry,H Hietala,D L Newman,G Lapenta,M V Goldman

doi:10.1002/2014ja020516

Abstract

AbstractDipolarization fronts (DFs) are often associated with the leading edge of earthward bursty bulk flows in the magnetotail plasma sheet. Here multispacecraft Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations are used to show that a spatially limited region of counterpropagating ion beams, whose existence is not evident in either the plasma moments or the electric field, is observed on the low‐density side of DFs. The THEMIS magnetic field data are used to establish appropriate comparison cuts through a particle‐in‐cell simulation of reconnection, and very good agreement is found between the observed and simulated ion distributions on both sides of the DF. Self‐consistent back tracing shows that the ion beams originate from the thermal component of the preexisting high‐density plasma into which the DF is propagating; they do not originate from the inflow region in the traditional sense. Forward tracing shows that some of these ions can subsequently overtake the DF and pass back into the high‐density preexisting plasma sheet with an order‐of‐magnitude increase in energy; this process is distinct from other ion reflection processes that occur directly at the DF. The interaction of the reconnection jet with the preexisting plasma sheet therefore occurs over a macroscopic region, rather than simply being limited to the thin DF interface. A more general consequence of this study is the conclusion that reconnection jets are not simply fed by plasma inflow across the separatrices but are also fed by plasma from the region into which the jet is propagating; the implications of this finding are discussed.

Highlights

A number of recent studies have examined the properties of earthward propagating dipolarization fronts (DFs) and their role in magnetotail dynamics and substorms
Dipolarization fronts (DFs) are associated with the leading edge of plasma flow bursts lasting several minutes—so-called bursty bulk flows (BBFs) [Angelopoulos et al, 1992, 1994], and their main signature is a rapid increase in the northward component of the magnetic field with the DF separating the preexisting plasma sheet from the fast-flowing, rarified, and heated plasma that constitutes the BBF [Runov et al, 2011b]
In this paper we have presented new THEMIS observations of ion distribution functions associated with DFs in the Earth’s magnetotail

Summary

Introduction

A number of recent studies have examined the properties of earthward propagating dipolarization fronts (DFs) and their role in magnetotail dynamics and substorms (see, e.g., recent review by Sergeev et al [2012]). DFs are associated with the leading edge of plasma flow bursts lasting several minutes—so-called bursty bulk flows (BBFs) [Angelopoulos et al, 1992, 1994], and their main signature is a rapid increase in the northward component of the magnetic field with the DF separating the preexisting plasma sheet from the fast-flowing, rarified, and heated plasma that constitutes the BBF [Runov et al, 2011b]. Previous work has shown that the DF thickness is of the ion scale (and independent of DF propagation speed) [Schmid et al, 2011], that DFs propagate coherently over several Earth radii earthward within a few minutes [Runov et al, 2011b], and that there are locations where significant wave-particle interactions can occur [Le Contel et al, 2009], leading to electron acceleration and heating [e.g., Deng et al, 2010; Khotyaintsev et al, 2011]. S. Nakamura et al, 1998; Sitnov et al, 2009; Wu and Shay, 2012], and experimentally, the connection between DFs and reconnection sites has been established [Runov et al, 2012]

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Discussion

Conclusion