The Earth's bow shock velocity distribution function

Abstract

It is well established that interplanetary disturbances cause the Earth's bow shock to move sunward or earthward. Since the launch of the Cluster mission, precise determination of the shock velocity was carried out allowing for the possibility to perform sophisticated investigations about the shock dynamics. The shock normal and velocity are determined for 381 Earth's bow shock crossings using data from the Spatio Temporal Analysis of Field Fluctuations - Spectrum Analyzer experiment on board the Cluster spacecraft. We have found that the observed radial shock velocity is well fitted by a Maxwellian for speeds less than 80 km s-1. The Maxwellian fit provides a standard deviation σVobs=24 km s-1. Assuming that the shock motion is exclusively controlled by the time rate of change of solar wind ram pressure, a probability density for the shock velocity near the nose region is constructed using the basic elements of probability theory. The obtained results are in a very good agreement with the observations when typical conditions of the solar wind are considered. The present study indicates that the ram pressure is a predominant parameter in shock dynamics and the relevance of the Mach number is not a determinant.