First Results of a Statistical Study of Hot Flow Anomalies from THEMIS Measurements

Hot flow anomalies (HFAs) are diamagnetic cavities filled with hot, tenuous, and deflected plasma population occurring upstream in the solar wind from Earth's bow shock. The occurrence patterns of HFAs at the quasi-parallel versus the quasi-perpendicular shock and vice-versa has been investigated. We have also examined how HFAs evolve with time. HFAs presented here were identified in THEMIS C satellite data for June 2008 to November 2008 upstream from the bow shock. The 163 events were classified as "young" or "mature" and have a wide range of MLTs. Young HFAs have two distinct ion populations whereas mature HFAs have a single hot ion population. One-dimensional cut plots of the electron distributions show differences between young and mature HFAs.