Electric structure of dipolarization front at sub‐proton scale

Abstract

[1] Using Cluster data, we investigate the electric structure of a dipolarization front (DF) -the ion inertial length (c/w pi ) scale boundary in the Earth's magnetotail formed at the front edge of an earthward propagating flow with reconnected magnetic flux. We estimate the current density and the electron pressure gradient throughout the DF by both single-spacecraft and multi-spacecraft methods. Comparison of the results from the two methods shows that the single-spacecraft analysis, which is capable of resolving the detailed structure of the boundary, can be applied for the DF we study. Based on this, we use the current density and the electron pressure gradient from the single-spacecraft method to investigate which terms in the generalized Ohm's law balance the electric field throughout the DF. We find that there is an electric field at ion inertia scale directed normal to the DF; it has a duskward component at the dusk flank of DF but a dawnward component at the dawn flank of DF. This electric field is balanced by the Hall (j  B/ne) and electron pressure gradient (r P e /ne) terms at the DF, with the Hall term being dominant. Outside the narrow DF region, however, the electric field is balanced by the convection (V i  B) term, meaning the frozen-in condition for ions is broken only at the DF itself. In the reference frame moving with the DF the tangential electric field is almost zero, indicating there is no flow of plasma across the DF and that the DF is a tangential discontinuity. The normal electric field at the DF constitutes a potential drop of $1 keV, which may reflect and accelerate the surrounding ions. Citation: Fu, H. S., Y. V. Khotyaintsev, A. Vaivads, M. André, and S. Y. Huang (2012), Electric structure of dipolarization front at sub-proton scale, Geophys. Res. Lett., 39, L06105,

Ion flux dropout observed near dipolarization front

“…Minimum variance analysis of the magnetic field shows that all three probes were at the dusk side of the DF [10]. Although THE was the most tailward probe, it was the last one to detect the DF because of its more duskward and southward location, conforming to the saddle structure of the DF: a sandal shape in the equator plane and a bow shape (concave when viewed from Earth) in the meridian plane [8,12].…”

“…The term dipolarization front was first proposed by Nakamura et al [6] when studying the motion of a DF during a flow burst event and has since been studied extensively, both statistically [7,8] and in multicase analysis [9,10]. Observations have shown that DFs are earthward-propagating structures with thicknesses comparable to ion gyroradii and are usually associated with large amplitude transient electric fields [8,11,12], waves [13][14][15], and electric current systems [8,12,16].…”

Jets Downstream of Collisionless Shocks

“…Particularly, using Cluster observations it was argued that the tangential electric field is small in the plasma frame moving approximately with the DF velocity, whereas a fairly large normal electric component E n (Hall electric field of tens of mV/m) is typically observed in the DF's close vicinity, where the ion and electron motions are decoupled (Fu et al 2012). Knowledge of the geometry is important in case studies of the force balance Hamrin et al 2015;Karlsson et al 2015a).…”

Whistler mode waves at magnetotail dipolarization fronts

“…Dipolarization fronts, DFs, are tangential discontinuities in the magnetotail, separating the plasma sheet and fast plasma flows, which can be created by various mechanisms such as magnetic reconnection [Sitnov et al, 2009;Fu et al, 2012c] and kinetic interchange instability [Pritchett and Coroniti, 2011]. DFs are identified by a sharp increase of B z GSM and are associated with electron and ion acceleration [Asano et al, 2010;Zhou et al, 2010;Fu et al, 2011] as well as various wave activities, e.g., electron holes, whistler, lower hybrid, and electron cyclotron waves [Le Contel et al, 2009;Sergeev et al, 2009;Zhou et al, 2009;Deng et al, 2010;Khotyaintsev et al, 2011;Hwang et al, 2011].…”