Cluster observations of hot flow anomalies with large flow deflections: 2. Bow shock geometry at HFA edges

Abstract

Case and statistical studies of the bow shock geometry at hot flow anomaly (HFA) edges have been performed based on 87 HFAs with large flow deflections observed by the Cluster C1 spacecraft from 2003 to 2009. The results suggest that HFAs can be formed at both quasi‐parallel and quasi‐perpendicular shocks. In an accompanying paper, we show that the ions might be near‐specularly reflected at the bow shock and interact with the solar wind to form HFAs. The guiding center of specularly reflected ions will typically be swept downstream to the bow shock at quasi‐perpendicular shocks. However, this study shows that in at least 13 of these 87 (15%) HFAs, both the leading and trailing edges are at quasi‐perpendicular shocks. These HFAs show a high gyration velocity and a high fast magneto‐sonic Mach number, increasing the gyro‐radius and the possibility of pitch angle scattering, which might help the ions escape from the bow shock and move upstream. In addition, HFAs with both edges at quasi‐perpendicular shocks are closer to the bow shock than those with both edges at quasi‐parallel shocks. This might help the reflected ions at a quasi‐perpendicular shock interact with the incident solar wind immediately after the reflection and increase the possibility of HFA formation.