Motion of Classic and Spontaneous Hot Flow Anomalies Observed by Cluster

Abstract

The propagation characteristics of hot flow anomalies (HFAs) near the Earth's bow shock are investigated, using observations from the Cluster spacecraft. A data set comprised of 19 classic HFAs (associated with tangential discontinuities, TDs) and 23 spontaneous HFAs (SHFAs, formed in the absence of solar wind discontinuities) was analyzed. For each event, the propagation velocity and normal of leading and trailing HFA edges were calculated using multiple multi-point satellite analysis methods. For classic HFAs, 93% of the events have leading edge (LE) normal directions within 30° of the normal direction of the driving TDs. For SHFAs, in 74% of the events, the angle between the normal of an SHFA's edges and background magnetic field is in the range of 70°-110° and no angle is in the ranges of 0°-20° and 160°-180°. These results indicate that the LEs of the classic HFAs propagate along the same direction as the driving TDs and most SHFAs propagate quasi-perpendicular to the background magnetic field and no SHFAs propagate parallel or anti-parallel to the background magnetic field. Moreover, according to the velocity of HFAs' edges, we find that all classic HFAs and SHFAs' edges propagate toward the Earth in the spacecraft frame as expected and 5 out of 7 SHFAs are contracting and no expanding SHFAs are found. This study provides key parameters to help understand how HFAs disturb the magnetosphere.