Conditions for the formation of hot flow anomalies at Earth's bow shock

Hot flow anomalies (HFAs) result from the interaction of an interplanetary current sheet with Earth's bow shock and were discovered over a decade and a half ago. The deflected flow and hot interior of an HFA are consequences of ions reflected at the bow shock being channeled along the current sheet. Previous studies have shown that this requires a solar wind motional elect ric field pointing toward the current sheet on at least one side and that t he current sheet must be a tangential discontinuity. Recent reports of a ra pid displacement of the magnetopause by 5 R-e as the result of an HFA have led us to explore the interplanetary conditions surrounding all reported HF As. The kinetic aspects of HFA formation suggest that current sheets should pass relatively slowly along the bow shock; that is, their normals should have large cone angles. This hypothesis is confirmed. Individual multispace craft case studies confirm that the underlying current sheets are tangentia l discontinuities, but most HFAs have relatively small jumps in field magni tude from before to after and thus would fail traditional identification te sts as definite tangential discontinuities. The combination of our results suggests that HFAs should occur at a rate of several per day, and thus they may play a significant role in the solar-terrestrial dynamics.