Separatrix regions of magnetic reconnection at the magnetopause
ABSTRACT Using data from the four Cluster spacecraft we study the separatrix regions of magnetic reconnection sites at the dayside magnetopause under conditions when reconnection is occurring in the magnetopause current layer which separates magnetosheath plasma from the hot magnetospheric plasma sheet. We define the separatrix region as the region between the separatrix – the first field line opened by reconnection – and the reconnection jet (outflow region). We analyze eight separatrix region crossings on the magnetospheric side of the magnetopause and present detailed data for two of the events. We show that characteristic widths of the separatrix regions are of the order of ten ion inertial lengths at the magnetopause. Narrow separatrix regions with widths comparable to a few ion inertial lengths are rare. We show that inside the separatrix region there is a density cavity which sometimes has complex internal structure with multiple density dips. Strong electric fields exist inside the separatrix regions and the electric potential drop across the regions can be up to several kV. On the magnetosheath side of the region there is a density gradient with strong field aligned currents. The observed strong electric fields and currents inside the separatrix region can be important for a local energization of ions and electrons, particularly of ionospheric origin, as well as for magnetosphere-ionosphere coupling.
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ABSTRACT:  The factors controlling asymmetric reconnection and the role of the cold plasma population in the reconnection process are two outstanding questions. We present a case study of multipoint Cluster observations demonstrating that the separatrix and flow boundary angles are greater on the magnetosheath than the magnetospheric side of the magnetopause, probably due to the stronger density than magnetic field asymmetry at this boundary. The motion of cold plasmaspheric ions entering the reconnection region differs from that of warmer magnetosheath and magnetospheric ions. In contrast to the warmer ions, which are probably accelerated by reconnection in the diffusion region near the subsolar magnetopause, the colder ions are simply entrained by E × B drifts at high latitudes on the recently reconnected magnetic field lines. This indicates that plasmaspheric ions can sometimes play only a very limited role in asymmetric reconnection, in contrast to previous simulation studies. Three cold ion populations (probably H+, He+, and O+) appear in the energy spectrum, consistent with ion acceleration to a common velocity.Journal of Geophysical Research: Space Physics 03/2014; 119(3). DOI:10.1002/2013JA019168 · 3.44 Impact Factor
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ABSTRACT: We study in detail high-frequency (HF) plasma waves between the electron cyclotron and plasma frequencies within a reconnection diffusion region (DR) encountered by Cluster in the magnetotail using continuous electric field waveforms. We identify three wave types, all observed within the separatrix regions: Langmuir waves (LW), electrostatic solitary waves (ESWs), and electron cyclotron waves (ECWs). This is the first time the ECWs have been observed inside this region. Direct comparison between waveforms and electron distributions are made at the timescale of one energy sweep of the electron detector (125 ms). Based on the wave and electron distribution characteristics, we find that the separatrix region has a stratified spatial structure. The outer part of the region is dominated by LW emissions related to suprathermal electron beams propagating away from the X-line. Furthest in, nearest to the current sheet, we observe ESWs associated with counterstreaming electron populations. Studying HF waveforms allows for a precise mapping of kinetic boundaries in the reconnection region and helps to improve our understanding of the electron dynamics in the DR.Geophysical Research Letters 03/2013; 40(6):1032-1037. DOI:10.1002/grl.50227 · 4.46 Impact Factor
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ABSTRACT: Using Cluster data, we investigate the electric structure of a dipolarization front (DF) - the ion inertial length (c/ωpi) 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 (∇ Pe/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 (Vi × 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.Geophysical Research Letters 03/2012; 39(6):6105-. DOI:10.1029/2012GL051274 · 4.46 Impact Factor