Themis Observations Of The Quasi-Perpendicular Bow Shock And Magnetosheath

This paper investigates the structure features of the quasi-perpendicular bow shock and the magnetosheath and the ion distributions in vicinity of them based on the observations of THEMIS satellites. For this, we chose some bow shock crossing events occurred in different interplanetary conditions to comparison analysis. In general, two aspects of the work are presented as follows:(1) By the analysis of the magnetic field and the conditions such as Alfven Mach number (Ma) upstream of bow shock in the crossing event at about14:00UT on25October2007, we found that a reformation of quasi-perpendicular bow shock took place. In addition, the gyrotropic ions and Field-Aligned Beams (FABs) exited in the ramp of quasi-perpendicular bow shock at the same time. That was in accordance with the past observations, which showed that the two kinds of ions have the same source region. And, the observations also showed that the FABs in the magnetosheath might be the main origin of the FAB in solar wind. Based on the above observations, we think that the FABs in solar wind were produced in the combined effect of the two models(models of reflection in the ramp and leakage from the magnetosheath) under the interplanetary background conditions of high beta and Ma.(2) In the comparison analysis of the crossing events of28September2008and12October2008, we found that the event of12October2008was more complex. During the crossing bow shock on12October2008, the special structure between the magnetosheath and bow shock called Magnetosheath Compression Layer (MCL) in this paper was found. At about12:00UT on25October2007, P2probe observed the anomalous structure, which was similar to the Hot Follow Anomaly (HFA). By the analysis, we think that the anomalous structure may be cause by P2crossing the MCL. The MCL has characteristics as follows:(1) MCL is between the magnetosheath and bow shock and is one part of the magnetosheath, which may be related with high Ma. plasma beta, and the southward direction of the interplanetary magnetic field;(2) the magnetic field and ion density in it are significantly increased, even higher than those in the magnetosheath;(3) there is a decreasing velocity especially in Vx, and a deflection in the flow direction;(4) the ion temperature in the MCL is lower than that in the magnetosheath:(5) the pitch angle scatting in the MCL is still insufficiency which can be seen from the ion velocity distribution.