Propagation characteristics of hot flow anomalies

Hot flow anomalies (HFAs), characterized by heated plasma and flow deflection, are frequently observed near Earth’s and other planetary bow shocks. There are two kinds of HFAs, classic HFAs formed by the interaction of tangential discontinuities (TD) and the bow shock, and spontaneous HFAs (SHFAs) which are not associated with discontinuties. A statistical study of the propagation characteristics of HFA edges has been performed base on 19 classic HFAs and 23 SHFAs with one-dimensional edges observed by Cluster from 2001 to 2010. The propagation velocity and normal direction of each edge are calculated using the timing method, the minimum directional difference (MDD) method, and the spatial-temporal difference (STD) method. The angle between the leading edge normal and the corresponding TD normal is less than 30 degrees for 93% of the classic HFAs. The angle between the edge normal and background magnetic field is near 90 degrees for 74% of the SHFAs. Observations indicate that the leading edge of the classic HFAs propagates along the same direction as the driving TD and the SHFAs propagate perpendicular to the background magnetic field. Furthermore, we find that all 42 HFAs propagate toward the Earth in the spacecraft frame as expected. However, in the solar wind frame HFAs have different propagation directions (i.e., toward the Earth, the Sun or be stationary in the solar wind frame).