A Statistical Study of Hot Flow Anomalies at Earth's Bow Shock
Abstract:Hot flow anomalies (HFAs) at Earth’s bow shock were identified in THEMIS C satellite data for August 2007 to December 2009. The 142 events were classified as “young” or “mature” and then “spontaneous” or “regular”. HFAs were categorized as “mature” if there were strong shocks at the edges and a single hot ion population. HFAs were categorized as “young” if there were two ion populations, density and magnetic strength depletions, plasma heating, and no shocks at the edges. It is suggested that young HFAs evolve into mature HFAs. "Spontaneous" HFAs (SHFA) are HFAs not associated with a solar wind discontinuity whereas "regular" HFAs are associated with a solar wind discontinuity.
In this study, SHFAs and HFAs do not show major differences in their plasma parameters. They have a magnetic local time range from 7.3 - 16.7. The normalized HFA occurrence rate is largest for approximately radial interplanetary magnetic field, cone angles less than ~55° (less than ~35° for young regular HFAs), magnetic local times away from noon, relatively larger Mach numbers in the range observed, and high solar wind speeds. The analysis also suggests the operation of an HFA cooling mechanism.