Analysis of Bow Shock Oscillations Observed by the Cluster Spacecraft

We present preliminary results of an analysis of multiple bow shock crossings lasting several hours that were observed by the four Cluster spacecraft during separation distances less than 1000 km. Using a simple timing method, we determined shock normal and velocity along this normal for a large number of events. We have calculated bow shock standoff distances assuming that the shock surface has a parabolic shape. These distances have been compared with the distances predicted by gas-dynamic models based on upstream plasma parameters measured by the ACE spacecraft. We analyze the oscillations of the standoff distance during multiple crossings in order to define a typical frequency of the bow shock motion and to find upstream origin of these fluctuations. We also compare the angles θBn (the angle between the magnetic field and the shock normal) retrieved by the timing method with the angles calculated by an iterative method based on Rankine–Hugoniot jump conditions. We have achieved a good agreement between these two techniques.