Stability of the Bow shock and Formation of Hot Flow Anomalies

Spacecraft observations at the terrestrial and Martian bow shocks have revealed the presence of structures referred to as Hot Flow Anomalies (HFAs). These structures are bounded by thin shock waves behind which plasma is highly deflected and heated but the density and magnetic field undergo little or no jump in their average values compared to their respective values in the solar wind. In addition, large amplitude density and magnetic field oscillations which are well correlated are observed in this plasma. The origin of HFAs has been a topic of investigation for a number of years and it is believed that the interaction of tangential discontinuities (TD) with certain properties with the bow shock leads to their formation. Using global hybrid simulations in which ions are treated kinetically while electrons are assumed to form a massless fluid, we have recently demonstrated that the interaction of a TD with the bow shock does indeed form structures with properties very similar to HFAs. In this talk, we examine the formation of HFAs in more detail and demonstrate that the major cause of shock disruption and formation of HFA is tied to the new upstream conditions behind the TD and that it is possible to form HFAs in global and localized planar simulations through the choice of upstream conditions without the use of TDs. Specifically, we show that under certain upstream conditions, the expected flow and magnetic field properties behind a part of the bow shock lead to electric fields which are inconsistent with that required for dissipation at the shock leading to the formation of HFA instead. Implications of these results for the bow shock will be discussed as well.