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

Plasma Depletion and Mirror Waves Ahead of Interplanetary Coronal Mass Ejections

We find that the sheath regions between fast interplanetary coronal mass ejections (ICMEs) and their preceding shocks are often characterized by plasma depletion and mirror wave structures, analogous to planetary magnetosheaths. A case study of these signatures in the sheath of a magnetic cloud (MC) shows that a plasma depletion layer (PDL) coincides with magnetic field draping around the MC. In the same event, we observe an enhanced thermal anisotropy and plasma beta as well as anti-correlated density and magnetic fluctuations which are signatures of mirror mode waves. We perform a superposed epoch analysis of ACE and Wind plasma and magnetic field data from different classes of ICMEs to illuminate the general properties of these regions. For MCs preceded by shocks, the sheaths have a PDL with an average duration of 6 hours (corresponding to a spatial span of about 0.07 AU) and a proton temperature anisotropy ${T_{\perp p}\over T_{\parallel p}}\simeq 1.2$ -1.3, and are marginally unstable to the mirror instability. For ICMEs with preceding shocks which are not MCs, plasma depletion and mirror waves are also present but at a reduced level. ICMEs without shocks are not associated with these features. The differences between the three ICME categories imply that these features depend on the ICME geometry and the extent of upstream solar wind compression by the ICMEs. We discuss the implications of these features for a variety of crucial physical processes including magnetic reconnection, formation of magnetic holes and energetic particle modulation in the solar wind.Comment: fully refereed, accepted for publication in J. Geophys. Re

Pickup water group ions at comet Grigg‐Skjellerup

The density and velocity distribution of cometary water group ions was measured by the Giotto spacecraft in the regions upstream and downstream of the 'bow shock' at comet Grigg-Skjellerup. The results show that the distributions of ions are ring-like until quite close to the shock, the timescales for pitch angle and energy diffusion appear similar and the ion density follows a r-2 dependence