Cluster observations of hot flow anomalies with large flow deflections: 1. Velocity deflections

Abstract

Case and statistical studies have been performed to investigate hot flow anomalies (HFAs) with large flow deflections using data from the Cluster-C1 spacecraft from 2003 to 2009. We have selected 87 events with V_y or V_z in GSE coordinates larger than 200 km s^-1. Observations of these HFAs indicate a "location-dependent deflection": V_y or V_z deflect to a positive value when the event is located in the positive Y or Z side relative to the subsolar point and to a negative value when it is located in the negative Y or Z side relative to the subsolar point. The amplitude of the deflection increases with increasing distance in Y or Z direction. The decrease in V_x at the event center is larger when the location is closer to the Sun-Earth line. The location-dependent deflection might be due to a near-specular reflection of ions at the Earth's bow shock. The HFAs studied in this paper are close to the bow shock with the distance of the event location to the bow shock ranging from 0.03 to 3.51 R_E, which might cause the reflected ions to remain as a coherent near-specular reflected beam.