Abstract
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.
Highlights
Magnetic reconnection is an important process that occurs in different plasma environments: in the Earth’s magnetosphere, at the Sun, in astrophysical plasmas as well as in laboratory plasmas (Biskamp, 2000)
We have analyzed in detail eight crossings of the magnetopause reconnection layer which are consistent with the spacecraft crossing separatrix regions of reconnection sites during ongoing reconnection
We have studied Cluster spacecraft observations in the vicinity of the dayside magnetopause on 4 January 2004, between 12:30 and 14:30 UT. During this time the interplanetary magnetic field (IMF) was varying but mainly directed southward and we find signatures of ongoing reconnection
Summary
Magnetic reconnection is an important process that occurs in different plasma environments: in the Earth’s magnetosphere, at the Sun, in astrophysical plasmas as well as in laboratory plasmas (Biskamp, 2000). The regions between outflow and inflow regions can have complicated structures that depends on the properties of the reconnecting plasmas. These regions can be described in different ways. In the MHD description of steady reconnection of two similar plasmas, outflow and inflow regions are separated by a pair of slow shocks originating in the diffusion region (Petschek, 1964). More complicated MHD discontinuities develop when plasmas are different on either side of the current sheet and/or the reconnection is time varying (Semenov et al, 1992; Biernat et al, 1998). Lindstedt et al.: Separatrix regions of magnetic reconnection at the magnetopause
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