73 research outputs found
Global reconnection topology as inferred from plasma observations inside Kelvin-Helmholtz vortices
During a long lasting period of northward interplanetary magnetic field and
high solar wind speed (above 700 km/s), the Cluster spacecraft go across a
number of very large rolled-up Kelvin-Helmholtz (KH) vortices at the dusk
magnetopause, close to the terminator. The peculiarity of the present event
is a particular sequence of ions and electrons distribution functions observed
repeatedly inside each vortex. In particular, whenever Cluster crosses the
current layer inside the vortices, multiple field-aligned ion populations
appear, suggesting the occurrence of reconnection. In addition, the ion data
display a clear velocity filter effect both at the leading and at the
trailing edge of each vortex. This effect is not present in the simultaneous
electron data. Unlike other KH studies reported in the literature in which
reconnection occurs within the vortices, in the present event the
observations are not compatible with local reconnection, but are accounted
for by lobe reconnection occurring along an extended X-line at the terminator
in the Southern Hemisphere. The reconnected field lines "sink" across the
magnetopause and then convect tailward-duskward where they become embedded in
the vortices. Another observational evidence is the detected presence of
solar wind plasma on the magnetospheric side of the vortices, which confirms
unambiguously the occurrence of mass transport across the magnetopause
already reported in the literature. The proposed reconnection scenario
accounts for all the observational aspects, regarding both the transport
process and the kinetic signatures
Effects on magnetic reconnection of a density asymmetry across the current sheet
Abstract. The magnetopause (MP) reconnection is characterized by a density asymmetry across the current sheet. The asymmetry is expected to produce characteristic features in the reconnection layer. Here we present a comparison between the Cluster MP crossing reported by Retinò et al. (2006) and virtual observations in two-dimensional particle-in-cell simulation results. The simulation, which includes the density asymmetry but has zero guide field in the initial condition, has reproduced well the observed features as follows: (1) The prominent density dip region is detected at the separatrix region (SR) on the magnetospheric (MSP) side of the MP. (2) The intense electric field normal to the MP is pointing to the center of the MP at the location where the density dip is detected. (3) The ion bulk outflow due to the magnetic reconnection is seen to be biased towards the MSP side. (4) The out-of-plane magnetic field (the Hall magnetic field) has bipolar rather than quadrupolar structure, the latter of which is seen for a density symmetric case. The simulation also showed rich electron dynamics (formation of field-aligned beams) in the proximity of the separatrices, which was not fully resolved in the observations. Stepping beyond the simulation-observation comparison, we have also analyzed the electron acceleration and the field line structure in the simulation results. It is found that the bipolar Hall magnetic field structure is produced by the substantial drift of the reconnected field lines at the MSP SR due to the enhanced normal electric field. The field-aligned electrons at the same MSP SR are identified as the gun smokes of the electron acceleration in the close proximity of the X-line. We have also analyzed the X-line structure obtained in the simulation to find that the density asymmetry leads to a steep density gradient in the in-flow region, which may lead to a non-stationary behavior of the X-line when three-dimensional freedom is taken into account
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