Properties of earth’s bow shock at large geocentric distances: A case study

In this study we report our analysis of rare observations of Earth’s bow shock motion at sunward geocentric distances as far as 24 Earth radii (RE). We use observations from solar wind monitors and two spacecraft constellations on the dayside geospace made during a period of low Mach number and low β solar wind conditions associated with a magnetic cloud flux rope of an interplanetary coronal mass ejection (ICME). We use data from solar wind monitors and two spacecraft constellations on the dayside geospace. It is shown that during this period the bow shock standoff distance under these conditions is underestimated by empirical model predictions. The bow shock motion is rather asymmetric, in that the bow shock expansion in the solar wind at 120 km/s is much faster than when it recedes at 14 km/s. It is shown that such an asymmetric motion is driven in part by magnetosheath conditions immediately downstream of the bow shock. As the bow shock expands in the upstream solar wind, heating and deceleration of the solar wind plasma is much more effective due to a higher cross shock electrostatic potential and gyrokinetic effects. These observations are also supported by a numerical model. Further downstream of the bow shock in the magnetosheath, the plasma flow exhibits significant slowdown as a result of the fast outward propagation of the bow shock and further plasma compression.