FG3. Foreshock, Bowshock, Magnetosheath

2008 GEM Bow Shock Session

Conveners: N. Omidi <omidi [at] solanasci.com>, H. Kucharek <harald.kucharek [at] unh.edu>, and J. Eastwood <eastwood [at] ssl.berkeley.edu>

Russell et al. : Addressed the structure and evolution of weak collisionless shocks. In regards to the evolution of the shock, STEREO A-B observations show examples of shock steepening by coalescence of two weaker shocks where the trailing shock overtakes the leading shock. Regarding shock structure, the original theory of weak shocks suggested that their structure was due to dispersive waves whose damping provide the necessary dissipation at the shock. According to this theory these waves are present upstream of the shock layer for oblique cases and downstream of the shock for perpendicular cases. STEREO observations, however, show that waves are present both upstream and downstream of the shock.

Krauss-Varban et al. : Addressed the question of whether observed energetic ion fluxes at CME-driven shocks can be understood and quantified for the purpose of space weather forecasts. Previous attempts at ordering fluxes with Mach number and shock normal angle were not successful. To address this problem 2.5-D hybrid simulations were performed to compare fluxes of energetic ions to observations. The results show that in the case of quasi-parallel shocks both fluxes and the power law index agree well with observations without the presence of seed population of ions. In the case of the oblique shocks, the simulated fluxes are 5 orders of magnitude below the observed levels and lead to a soft power law. Addition of 1% seed population with a kappa distribution function and running for much longer times enhance the fluxes by 2 orders of magnitude. This enhancement is due to generation of fast waves in the upstream and their convection back into the shock causing the undulation of the shock surface. To account for the missing 3 orders of magnitude in the fluxes the effects of particle mirroring in the sunward converging field lines was included by reflecting the energetic ions back into the simulation box leading to lager fluxes and harder spectrum. However, additional mechanisms are still needed and under consideration for future studies.

Zhang et al. : Reported on THEMIS observations of a weak interplanetary shock interacting with the bow shock and its magnetospheric consequences. The interaction results in the earthward motion of the magnetopause at the speed of ~ 40 km/s. In addition, ground stations observed compressions over a wide range of MLTs and latitudes. It was also found that the transmitted IP shock took the form of a discontinuity associated with enhancement of B and N and a decrease in T. The field rotation associated with this structure is similar to that of the IP shock prior to its encounter with the shock. However, the fast shock expected to form based on fluid theory was not present.

Eastwood et al. : Reported on THEMIS observations of an HFA and comparisons to global hybrid simulations. During this event, THEMIS A was upstream of the bow shock while B,C,D and E were in the downstream region. THEMIS E observed the signatures of the HFA which consisted of a series of fast and slow magnetosonic waves and the remnants of the discontinuity whose interaction with the bow shock resulted in the formation of the HFA. In addition, THEMIS ground based observatories tracked the progress of the associated magnetic impulse event across the magnetosphere. An interesting aspect of these observations was that while the HFA was formed on the dusk side the associated magnetic impulse was observed on the dawn side. To compare these observations with hybrid simulations two runs using different upstream conditions guessed at based on observations were performed. In the first run, an HFA was formed on the dawn side but not on the dusk side. The lack of an HFA on the dusk side is due to the quasi-perpendicular nature of the simulated bow shock. In second run, the IMF was mostly radial leading to quasi-parallel geometry at the dayside bow shock. The interaction of the backstreaming ions with the incoming discontinuity resulted in the formation of a large structure in the foreshock which was subsequently convected back towards the shock. While the results show some similarities to the THEMIS observations no true HFA were formed. These results indicate the sensitivity of the solutions to the upstream conditions and the difficulties in deciphering the “true” upstream conditions from the observations due to the turbulent nature of this region during the event. Further work is planned for future.

Omidi et al. : Reported on the properties the ion foreshock boundary using global hybrid simulations. This new boundary was first detected in a global hybrid run with radial IMF geometry which led to a new model for foreshock cavities by Sibeck et al. (2008). According to this model, foreshock cavities correspond to spacecraft going from solar wind through the foreshock boundary into the foreshock and back into the solar wind. This accounts for the isolated nature of foreshock cavities. The results of simulations show that the ion foreshock boundary is weak at low Mach numbers and is enhanced with increasing Mach number. By performing a series of local hybrid simulations it was found that this Mach number dependence is in turn tied to the properties of the backstreaming ions where both beam velocity and density lead to the strengthening of the ion foreshock boundary. The results also show that while for small IMF cone angles the ion foreshock boundary forms on both sides of the foreshock. At larger cone angles (e.g. 20o) an asymmetry forms where the boundary is present only on one side of the foreshock.

Blanco-Cano et al. : Reported on the properties of foreshock ULF waves in global hybrid simulations and their dependence on the IMF cone angle and comparisons with CLUSTER observations. At intermediate cone angles the simulations show the presence of sinusoidal waves which propagate at small angles to the magnetic field and shocklets which propagate at larger wave normal angles. These waves are present in different parts of the foreshock. On the other hand, during small cone angles the two types of waves generated by the backstreaming ions correspond to parallel propagating sinusoidal waves and highly oblique fast magnetosonic waves. The properties of the excited waves were compared to linear theory which showed very good agreement. In contrast to intermediate cone angles, both types of waves are generated in the same regions of the foreshock and interact strongly during their nonlinear evolution. As a result, structures associated with large (~ 50 %) drops in solar wind density and magnetic field are formed which have been named foreshock cavitons. The size of the foreshock cavitons is of the order of 1 RE and they are convected back across the shock and into the magnetosheath leading to a highly turbulent plasma. A search through the CLUSTER data was conducted where a number of foreshock cavitons were observed. Comparisons between properties of the simulated and observed foreshock cavitons show very good agreement.

Scholer et al. : Reported on Cluster observations in the quasi-parallel foreshock region of the Earth bow shock. They investigated the association of the intensity of the diffuse ion population with the intensity of the compressional and tangential magnetic field wave intensity. They found that although the intensity of the diffuse ion population increases with decreasing distance to the Earth bow shock the wave intensity stays constant. This effect cannot be explained by existing theories and there is some evidence that this is effect is associated with filed-aligned ion beams originating at the perpendicular region of the Earth’s bow shock.

Desai et al.: Reported on abrupt enhancements in the intensities of ions in the energy range of a few 10s of keV to 100s of keV upstream of the Earth’s Bow Shock – upstream ion events – are characterized by short durations (~1-2 hours), steeply falling spectra, large (>100:1) field-aligned sunward anisotropies, and positive correlations with the solar wind speed and geomagnetic activity indices. Despite the wealth of information available, it is still not clear whether these ions are accelerated at the bow shock or somewhere inside the Earth’s magnetosphere. Furthermore, such events are also often observed simultaneously at two or more spacecraft, indicating that a large source region perhaps covering the entire size of the bow shock fills large spatial structures in the upstream region. In this paper we use simultaneous measurements of >40 keV upstream ions observed at ACE, Wind, and STEREO-A between 2007, day 1 through 2007, day 181 to calculate the occurrence probability of upstream events as a function of lateral and radial separation between L1 and STEREO-A. During the end of this ~6-month period, Wind (or ACE) and STEREO-A were separated by ~1750 RE in the radial direction and laterally in YGSE by ~3800 RE. Despite this large separation, STEREO-A continued to observe upstream events right up until the end of our survey period. More surprisingly, we found that the occurrence probability for measuring simultaneous upstream events at Wind or ACE at L1 and STEREO-A was ~20-30%, i.e., far greater than that expected from accidental coincidences. We discuss the implications of these results for the size of the source region, the conditions under which upstream events occur, and the size and nature of the spatial structures in which these ions populate and propagate in the upstream region.

Kucharek et al.: Reported on intensity variations of suprathermal ions at interplanetary discontinuities such as shocks, shocks associated with CIR’s, and compression regions. Observation from ACE/SPEICA and STEREO PLASTIC in the energy range of 250 – 800keV were used to investigate the enhancements of He+, He2+, 3He2+ at CIRs. Numerical (test particle) simulations have been used to explain why 3He2+ is less enhanced the pickup helium and solar wind alpha particles. Reasonable agreement has been achieved and there is evidence that the enhancements of the various species are controlled by the local turbulence at the shock ramp as well as local shock parameters. Future observations as well as simulations are planned to investigate not only the reflection and accelerations properties of the different interplanetary discontinuities for the various species but also the impact of these structures on the (far) downstream region of the Earth’s bow shock.

Dayside Magnetopause Reconnection focus Group

Conveners: Jean Berchem and Nick Omidi

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 a 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 the 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.