Propagation characteristics of young hot flow anomalies near the bow shock: Cluster observations

摘要: Based on Cluster observations, the propagation velocities and normal directions of hot flow anomaly (HFA) boundaries upstream Earth's bow shock are calculated. Twenty-one young HFAs, which have clear leading trailing boundaries, were selected, multispacecraft timing method considering errors was employed for investigation. According to difference in velocity edges, we categorized these events into three groups, namely, contracting, expanding, stable events. The contraction speed is a few tens kilometers per second contracting expansion more than hundred expanding For events, edges propagate at almost same within error range. We further investigated what causes them contract, expand, or stay by carefully calculating thermal pressure HFAs two distinct ion populations (solar wind beam reflected flow). It found that extreme value sum magnetic inside compared with nearest point outside higher lower there no significant total (sum thermal, magnetic, dynamic pressure) variation has effect evolution most (70%) implies plays an important role HFAs.