Structure of Hot Flow Anomaly

Hot Flow Anomalies (HFAs) were first discovered in 1980s. These are active processes of hot plasma bulks formation that usually occur at planetary bow shocks. Though HFA were studied for long time it is still not clear if they are reforming structures and what defines particular internal structure of HFA. Our study is based on the Interball Tail Probe data. We used 10-sec measurements of complex plasma analyzer SCA-1 and 1-second magnetic field measurements, and ELECTRON spectrometer 2-dimensional measurements with 3,75-sec temporal resolution. Five anomalies that were observed on the basis of well resolved structure for which we obtained displacement velocity along bow shock, flow velocities within HFA, and estimated the size. We checked if main criteria of HFA formation were fulfilled for each case. The following criteria were satisfied: motional electric field direction was directed toward current sheet at least at one side of it, bow shock was quasi-perpendicular at least at one side of HFA, and angle between current sheet normal and solar wind velocity was large. Convection velocities of plasma within HFA were calculated by subtracting average velocity from measured ion convection velocities along spacecraft trajectory through anomaly. These convection velocities viewed in coordinate system of shock normal and calculated IMF current sheet normal clearly show separation of HFA region in 3 parts: leading part, narrow central part, and trailing part. Ion velocity distributions confirm this triple structure of HFA. Thomsen et al. [1986] identified the region within HFA that they called "internal recovery". It looks like central region that we call narrow central part. Vaisberg et al. [1999] discussed separation of HFA into 2 distinct parts that correspond to leading and trailing parts. Judging from plasma convection pattern within HFAs we assumed that "internal recovery" region is the source of energy and momentum around interplanetary current sheet crossing. HFA formation mechanisms presume that HFA is formed when particles are reflected on bow shock, get swept by motional electric field and are injected back into the area. We tried to calculate the balance of energy in solar wind and within HFA to estimate what amount of reflected particles is needed for "internal recovery" area to be the real energy source. These estimations suggest that this energy balance is nearly fulfilled in 4 of 5 analyzed HFAs, and does not hold for one HFA. This energy balance may be in favor of quasi-stationary nature of HFA structure. References Thomsen, M. F., J. T. Gosling, S. A. Fuselier, S. J. Bame, and C. T. Russell (1986), Hot, diamagnetic cavities upstream from the Earth's bow shock, J. Geophys. Res., 91(A3), 2961-2973, doi:10.1029/JA091iA03p02961. Vaisberg, O.L., J.H.Waite, L.Avanov, V.N.Smirnov, D.Dempsey J.L.Burch and A.A.Skalsky, HFA-like signatures observed with Interball-Tail spacecraft, in: Solar Wind Nine, ed. By S.R.Habbal, R.Esser, J.V.Hollweg, and P.A.Isenberg, AIP 1-56396-865-7, 1999, pp. 551-554.