Observational test of a hot flow anomaly formation mechanism

Abstract

Hot flow anomalies (HFAs) are unique plasma structures observed in the vicinity of the Earth's bow shock. They are typically characterized by high temperature plasma which is strongly deflected relative to the solar wind and flanked by shock‐like density and magnetic field enhancements. One mechanism proposed to explain these structures involves the coupling of ions reflected off the bow shock with the solar wind. This coupling would convert the relative streaming energy between the reflected and solar wind ions into thermal energy. The hot plasma would then expand to form the observed HFA. This model predicts that the initial temperature of the HFA is roughly limited by the available energy in the relative streaming between the two ion beams, and that the observed temperature and density correspond to the coupled plasma after an expansion has occurred. We present a simple test of this mechanism by comparing the measured temperature and density with the values expected immediately following the assumed coupling and after expansion. We find that the relative streaming energy is in most cases sufficient to account for the measured temperature. However, the predicted HFA temperature after adiabatic expansion to the measured density is in many cases well below the measured temperature. This analysis indicates that if the reflected ion coupling mechanism is correct, the expansion is probably not adiabatic, and in some cases an additional process may be needed to account for the observed temperature. The observed anti‐correlation between the ion temperature and density both within individual HFAs and among the different HFAs provides further evidence that processes other than adiabatic expansion are occurring.