Bow Shock, Magnetopause, and Cusp I
Presiding: J R Sharber, Southwest Research Institute; N Omidi, SciberNet Inc.
SM22A-01 10:30h
Stability of the Bow shock and Formation of Hot Flow Anomalies
Spacecraft observations at the terrestrial and Martian bow shocks have revealed the presence of structures referred to as Hot Flow Anomalies (HFAs). These structures are bounded by thin shock waves behind which plasma is highly deflected and heated but the density and magnetic field undergo little or no jump in their average values compared to their respective values in the solar wind. In addition, large amplitude density and magnetic field oscillations which are well correlated are observed in this plasma. The origin of HFAs has been a topic of investigation for a number of years and it is believed that the interaction of tangential discontinuities (TD) with certain properties with the bow shock leads to their formation. Using global hybrid simulations in which ions are treated kinetically while electrons are assumed to form a massless fluid, we have recently demonstrated that the interaction of a TD with the bow shock does indeed form structures with properties very similar to HFAs. In this talk, we examine the formation of HFAs in more detail and demonstrate that the major cause of shock disruption and formation of HFA is tied to the new upstream conditions behind the TD and that it is possible to form HFAs in global and localized planar simulations through the choice of upstream conditions without the use of TDs. Specifically, we show that under certain upstream conditions, the expected flow and magnetic field properties behind a part of the bow shock lead to electric fields which are inconsistent with that required for dissipation at the shock leading to the formation of HFA instead. Implications of these results for the bow shock will be discussed as well.
SM22A-02 10:45h
Geotail Observations of the Spatial Dependence of Kelvin-Helmholtz Waves on an Inbound Passage Through the Dusk Flank Boundary Layer
On August 1, 1998, the Geotail spacecraft made an inbound passage perpendicular to the dusk magnetopause at the dusk terminator when the interplanetary magnetic field had been very northward for more than 10 hours. Typical 3-minute-period Kelvin-Helmholtz waves were observed and the density in the boundary layer and magnetopause was observed to have an unusually high value near 5 /cc. Compressible MHD calculations using the measured values at Geotail yield substantial growth rates that support the idea that the magnetopause was Kelvin-Helmholtz unstable. In contrast to many previous events where a spacecraft remained in the boundary layer, this passage allowed study of how the waves varied with distance inward from the magnetopause. In a layer adjacent to the magnetosheath, rapid magnetic field fluctuations were seen with variations of at least 50 nT/s. As the spacecraft approached the magnetopause from the magnetosheath the boundary waves led to transitions between the magnetosheath and the fluctuating region with its magnetosheath-like densities and tailward velocities. As the spacecraft moved inward, the transitions were more likely to be between the fluctuating region and a hotter region with magnetosphere-like densities of 5/cc. Gradually the velocity perturbations began to exhibit 360 degree rotations. Such rotations are similar to the vortices seen earlier by the ISEE spacecraft throughout the magnetotail which were suspected of being caused by Kelvin-Helmholtz instability of the boundary.
SM22A-03 11:00h
A Mechanism for Electron Heating at the Magnetopause
A simple picture of the reconnection geometry, including the high density magnetosheath ion expansion into the low density magnetosphere, may be adequate to explain the "heated" electron distributions observed streaming away from the dayside high-shear magnetopause. Reconnection flows at the high shear magnetopause provide velocity kicks of twice the deHoffmann-Teller velocity. In addition, electrons will interact with an ambipolar field at the front of the magnetosheath ion expansion into the magnetosphere. The ambipolar field prevents the sheath electrons from running away and violating quasi-neutrality. The magnitude of the ambipolar field should be the order of Te/e and will decrease as the sheath ions fill the magnetospheric portion of the flux tube. This ambipolar field propagates with the ion expansion front, therefore electrons that reflect off the front will lose energy in the Earth frame. Higher energy electrons that penetrate the front, magnetically mirror, and pass back through the front will also lose energy. These two acceleration mechanisms may be adequate to explain electron distributions observed both inside and outside the magnetopause on newly reconnected field lines. Data from the Cluster spacecraft will be compared to the model fits.
SM22A-04 11:15h
The dependence of flux transfer events on solar wind conditions from three years of Cluster observations
In this study, we investigate the dependence of Cluster high-latitude and low-latitude magnetopause flank flux transfer events (FTEs) on some important parameters of solar wind and interplanetary magnetic field (IMF). Based on 1222 Cluster FTEs that we identified between February 2001 and June 2003, we find a strong dependence of Cluster FTEs on IMF Bxgsm. For positive (negative) IMF Bxgsm, there are more FTEs in the northern (southern) hemisphere. There is a strong dependence of the observed Cluster FTEs on IMF Bygsm. We believe that an unexpected strong correlation between FTE corresponding IMF Bygsm and IMF Bxgsm, i.e., spiral IMF, is responsible for such a strong dependence. There are two peaks for the Cluster FTE occurrence on IMF Bzgsm with a minimum at ~3 nT and two peaks at ~-3 nT and ~9nT. There is a strong correlation between Cluster FTE occurrence rate and IMF tilt and clock angles with a strong FTE presence for northward dawnward IMF for IMF clock angle, and northward and sunward for IMF tilt angle. This is significantly different from the former results and it is likely caused by the high-latitude FTE dependence on IMF Bxgsm, the special correlation between Cluster FTE IMF Bygsm and IMF Bxgsm, and the special Cluster data collation rate. Solar wind density, speed, dynamic pressure, and Alfven Mach number are found to have different degree of control for the FTE occurrence. FTE separation time is found to be controlled strongly by IMF Bzgsm, and IMF clock and tilt angles. There is some weaker control of FTE separation time by solar wind density and Alfven Mach number, while there is no obvious control for solar wind speed and dynamic pressure. Finally, ~50% of the FTEs correspond to IMF north-south fluctuations in the 10-minute interval proceeding each FTE.
SM22A-05 11:30h
Multiple Satellite Measurements of Particles and Fields During High Solar Wind Pressure Period
The WIND spacecraft (near L1) observed a high solar wind pressure period from 4:45 UT to 6:00 UT and an upstream ion event at 5:23-5:45 UT on June 28, 1999. The POLAR satellite at 7 hours magnetic local time ovserved a cusp-like diamagnetic cavity from 5:12 UT to 6:27 UT. Most of the time, during the period, the interplanetary magnetic field (IMF) had a duskward component, suggesting that cusp diamagnetic cavities also existed in the post-noon in the Northern Hemisphere. The diamagnetic cavity observaed by POLAR is independent of the solar wind velocity. The enhancements of the magnetic field fluctuations in the ultra-low frequency range measured by the WIND were also detected by the POLAR. The He++/H+ ratio in the diamagnetic cavities was a factor of four higher than in the quasi-trapping region before the event onset, while the He+/He++ ratio in the cavities was more than one order of the magnitude lower. The INTERBALL-1 spacecraft located just upstream of the bow shock on the pre-noon measured an upstream ion event from about 5:16 UT to 6:00 UT. The onset of the energetic ions observed by INTERBALL-1 in the upstream event was the same for different energies; the ion energy spectra were independent of the solar wind velocity and their intensities were independent of the bow shock geometry and the solar wind density. The energetic ion event onset was first detected near the cusp by POLAR at 5:12 UT, then near the bow shock in the pre-noon by INTERBALL-1 at 5:16 UT, and then in the far upstream by WIND at 5:23 UT. The measured energetic ion intensity decreased with increasing distance from the cusp diamagnetic cavities. These observational facts together with the IMF directions suggest that (1) this high solar wind pressure event produced an extremely large diamagnetic cavities (> 10 RE) within the magnetosphere, (2) the bow shock was not the main source of both the cusp and upstream energetic ions, and (3) the upstream energetic ions most likely came from the cusp.
SM22A-06 11:45h
Observation of the Dayside Cusp With IMAGE, Cluster, and DMSP
The IMAGE, Cluster, and DMSP spacecraft are used to investigate the cusp during an extended interval of IMF north on March 20-21 of 2002. Continuous monitoring by the IMAGE Far Ultraviolet spectrographic imager within the ~5 hour interval shows the dayside IMF-north emission region or spot, transpolar arcs, and evidence of a small substorm. Within the study interval the Cluster and DMSP F-13 field lines map down to the dayside spot. We are thus able to make a direct comparison between the particle measurements at Cluster and the cusp FUV emissions as well as determine the convective flow patterns at low altitudes. We relate the cusp characteristics to the rest of the oval and subsequently to the magnetosphere. The appearance and fading of the transpolar arcs provide information on changes in the magnetospheric configuration, which are then related to changes in the solar wind plasma and field characteristics.