A Heliospheric Plasma Sheet Crossing at 1 AU that Contains an Unusually High Density Just Downstream of Shock Wave

On the 9th of September 2011 the Wind spacecraft encountered an interplanetary (IP) fast forward shock at 11:46 UT. The shock was followed almost immediately ( 5 minutes) by a short ( 35 minutes) duration, extremely large density pulse with a density peak of 94 cm-3. The density pulse induced an extremely large positive impulse (SYM-H = 74 nT and Dst = 48 nT) on the ground. A close examination of other in situ parameters from Wind, we find that the density pulse was associated with (1) a spike in the plasma beta (β, ratio of thermal to magnetic pressure), (2) multiple sign changes in the azimuthal angle of magnetic field, Bϕ, (3) a depressed magnetic field magnitude, (4) a small radial component of magnetic field, and (5) a large ( > 90º) pitch angle change in suprathermal electrons (> 200 eV) across the density pulse. We conclude that the density pulse is associated with the heliospheric plasma sheet (HPS). The thickness of the HPS is estimated to be 8.2x105 km. The HPS density peak is about five times the value of the median size density peak inside the HPS ( 18 cm-3). Our global three-dimensional magnetohydrodynamic simulation suggests that the unusually large density pulse was a result of the compression by the IP shock from behind. During this time Geotail was in the dawn sheath region and did not see the density pulse, suggesting the transient and localized nature of the density pulse. (Work of CCW is supported by the Chief of Naval Research.)