FG5. Dayside Magnetopause Reconnection

Dayside Magnetopause Reconnection Focus Group

The dayside magnetopause reconnection focus group met on Monday afternoon. The session was very well attended. Between 30 and 40 people were present. As decided during the San Francisco meeting last December, the group focused on the three following topics:

Large-scale properties of reconnection at the magnetopause. Jeremy Ouellette from Dartmouth College presented results from the LFM code. He has run a series of simulations for constant solar wind conditions and different IMF clock angles. He found that reconnection is predominantly an anti-parallel process. For 45° and 90° IMF clock angles, reconnection occurs in two small regions on the upper dusk and lower dawn sides, whereas for 135° and 180° angles it extends across the subsolar region. Reconnection rates at the magnetopause grow linearly with IMF clock angle from 45° to 135°and then saturate, increasing only slightly from 135° to 180° clock angles. Cross polar potential drops increase linearly from 50 to 225 kV, where they saturate. Subsequently, observations of polar rain aurora were presented by Yongliang Zhang from JHU/APL. Data from the FUV experiment onboard the IMAGE spacecraft and DMSP measurements reveal the occurrence of polar rain aurora across the polar cap for periods of southward IMF and strong By. The energy range of the polar rain electrons was about 1 to 2 keV. In both cases presented, the polar rain auroras were dawn-dusk aligned and drifted anti-sunward at 200 m/s. Yongliang suggested that these polar rain auroras could result from reconnection over an extended area of the dayside magnetopause.

The physics of reconnection at the dayside magnetopause. Vadim Roytershteyn from LANL gave a presentation about the influence of sheared parallel flows on the onset of reconnection. He has built several equilibrium models of a jet embedded (k || B0) in a Harris-type current sheet. Results from the kinetic studies differ significantly from those for fluid treatments. The kinetic studies show that the instability persists in super-Alfvènic flows and produces reconnection. The thickness of the sheet was found to be one of the factors determining the transition to a fluid-like behavior. For thin sheets (<ρi) the mode behavior is determined by kinetic effects (ion anisotropy in their model) whereas the qualitative features appear to be independent of the details of the equilibrium distribution for thicker sheets.

We then discussed the properties of asymmetric reconnection when the magnetic field strengths and densities on either sides of the dissipation region differ, a situation particularly relevant to reconnection at the dayside magnetopause. Paul Cassak from the University of Delaware started by presenting results from a generalized Sweet-Parker type scaling analysis of 2D anti-parallel asymmetric reconnection. He showed that the outflow speed scales like the Alfven speed based on the geometric means of upstream fields and the density of the outflow (Vout α (B1B2/ρout)1/2 , ρout = (B1ρ2+B2ρ1)/(B1+B2) ) and the reconnection rate is a product of the aspect ratio of the dissipation region, the outflow speed, and an effective magnetic field strength given by the “reduced” field (E α (2/L)Vout Br where Br = B1B2/(B1+B2)). These results are independent of dissipation mechanism and numerical simulations agree with the theory for collisional and collisionless (Hall) reconnection. The location of the x-line differs from the location of the stagnation line. Subsequently, Joachim Birn (LANL) presented some results for asymmetric reconnection in resistive MHD. He showed that the scaling was similar to that for fast reconnection (Cassak-Shay) when using the outflow density from x-line (Vout = (B1B2/ρx)1/2). Fast flows occur preferentially into the high Alfven speed region and the flow stagnation line was displaced toward the high-field side. An investigation of the energy flow and conversion in the vicinity of the reconnection site revealed the significant role of enthalpy flux generation (compressional heating) in addition to the expected conversion of Poynting flux to kinetic energy flux.

Time-dependent reconnection and impacts of transients. Masha Kuznetsova from NASA/GFSC reported results from a global MHD simulation (BATSRUS) with high grid and temporal resolution run at CCMC to explain the occurrence of the flux transfer events (FTEs) observed by THEMIS near the flank of the magnetosphere. She found that individual extended flux ropes formed by component reconnection near the subsolar region (strong core field), but antiparallel reconnection at the flanks (weak core field). The flux rope had bends and elbows reminiscent of those invoked by Russell and Elphic to explain the occurrence of FTEs at the dayside magnetopause. The simulation showed also the formation of plasma wakes (field-strength cavities) as the ropes move over the magnetopause and that different parts of the flux rope moved in different directions.

Jean Berchem, IGPP, UCLA used an actual Cluster event to discuss the effects of a rapid northward turning of the IMF on the topology of magnetic reconnection at the magnetopause. A global MHD simulation of the event was run and solar wind ions launched upstream of the shock were traced in the time-dependent electric and magnetic fields of the MHD simulation. Ion dispersions calculated from particles collected at Cluster’s location in the simulation were found to be in very good agreement with those measured by Cluster in the cusp. In particular, the simulation reproduced very well the change in the slope of the ion dispersions observed by the spacecraft. Analysis of the simulation results indicates that reconnection occurs mostly in the subsolar region as the discontinuity interacts with the magnetopause, and then moves tailward as the field completes the rotation.

Nick Omidi from Solana Scientific Inc. reported the results of a study showing the influence of magnetosheath turbulence on magnetic reconnection at the magnetopause. He presented two global hybrid simulations in which the dayside magnetosheath exhibited waves associated with dissipation at the quasi-perpendicular shock (e.g., mirror and ion cyclotron waves). Both runs had the same solar wind plasma and southward IMF conditions. However, the resistivity was increased in the second run to damp magnetosheath waves. Comparison of the results showed that the number of FTEs formed at the magnetopause was reduced from 20 to 9 in the second run, indicating that the presence of turbulence in the magnetosheath enhances considerably time-dependent reconnection.