A study of particle flows in hot flow anomalies

Abstract

Hot flow anomalies (HFAs) are distinct disturbances of the solar wind flow observed often in the bow shock vicinity. Both experiment and theory suggest that these disturbances are created at the intersection of the bow shock and tangential discontinuity (TD) of the interplanetary magnetic field (IMF). The discontinuity is swept along the bow shock by the solar wind flow and thus the resulting HFA containing hot, tenuous, and deflected plasma can be found not only in the subsolar region but also on the flanks. We have identified three HFAs in measurements of the INTERBALL-1 and MAGION-4 spacecraft. The first one was observed in the solar wind and two others created an interface between the solar wind and magnetosheath. We have analyzed the flow directions inside these HFAs in order to find a relationship between particle flows and orientations of original IMF discontinuities. Our results show that, beside the dominant plasma motion connected with the sweeping of the discontinuity, a significant velocity component directed along the discontinuity can be found. This suggests a highly elongated shape of the HFA cavity. The correlation length of particular HFA features (amplitude of bounding density enhancements, their duration, etc.) may be rather short; it is of the order of 1 R E in our case. We also suggest that HFA, which extends into the magnetosheath, modifies locally downstream the Mach number and causes a local displacement of the bow shock.