A statistical study on hot flow anomaly current sheets

Abstract

Abstract Hot flow anomalies (HFAs) are phenomena frequently observed near Earth's bow shock and form when the interplanetary discontinuities interact with Earth's bow shock. We perform a statistical study to determine what kind of discontinuities are more efficient to generate HFAs. We use strict criteria to identify classic HFAs, excluding similar foreshock phenomena such as spontaneous hot flow anomalies (SHFAs) and foreshock bubbles. Our results show that magnetic field on at least one side of the interplanetary discontinuities has to be connected to the bow shock in order to form HFAs. Discontinuities with large shear angles are more efficient to form HFAs. The thickness of current sheets and the thickness of HFAs are strongly correlated and current sheets with thickness from 1000 km to 3162 km are more efficient to form HFAs. Of the HFAs, 74% have the electric field pointing toward the current sheet on the leading side and 72% have the electric field pointing toward the current sheet on the trailing side. In addition, the variations of plasma parameters and the magnetic field of HFAs with E inward on both sides are more dramatic than those with E inward on only one side. An HFA is more likely to form when the reflected flow from the bow shock is along the discontinuity.