Cluster observations of hot flow anomalies

Abstract

On 2 April 2002, Cluster entered the solar wind at high northern latitudes and observed a series of disturbances upstream of the bow shock. We suggest that these were signatures of a sequence of hot flow anomalies (HFAs), observed at different stages of development. We estimate the orientation and motion of the HFAs and structures within them using multispacecraft data and examine the variation of plasma flow in the context of the orientation of the underlying discontinuities. We demonstrate that the motional electric field at each disturbance was in a direction such that it would act to focus ions reflected from the bow shock toward the discontinuity, consistent with current understanding of HFA formation. The first disturbance consisted of hot plasma flowing along the estimated discontinuity plane during the core of the event, together with a weakened solar wind beam. Toward the edge of the event, compressed solar wind plasma flowed approximately perpendicular to the discontinuity plane, consistent with expansion of the hot plasma. We suggest that the presence of multiple ion distributions signifies that the HFA was at an early stage of its evolution. A second HFA, which had a more complex signature, contained a single ion distribution within the event core, which is the more typical signature of an HFA. It also contained a large velocity deflection, consistent with expansion of the plasma perpendicular to the discontinuity plane. The final HFA event we present is associated with a shock encounter, demonstrating the level of complexity that can be associated with the bow shock response to changing upstream conditions. Copyright 2004 by the American Geophysical Union.