A simple mechanism for Hot Flow Anomalies

Abstract:

Hot flow anomalies (HFA’s) upstream of the Earth’s bow shock show typically strong nonadiabatic heated plasma and a significant deflection of the plasma velocity compared to the unperturbed solar wind. This deflection can be sufficient that the actual plasma velocity has a sunward component. For particular electric field conditions, hybrid simulations have shown that such structures can be generated by the interaction of solar wind current sheets (tangential discontinuities) with the bow shock. However, recent observations indicate that a large fraction of HFA’s are not associated with measurable current sheets or have the wrong motional electric field compared to the proposed mechanism. This presentation demonstrates that typical properties of HFA’s including the non-adiabatic heating and the large velocity change are easily obtained by the interaction of transient strong density depletions with a fast shock such as the bow shock. A transient depletion of the solar wind density requires different shock conditions compared to the original solar wind. During the interaction, the fast shock is traveling very rapidly (several 100 km/s) sunward and conditions behind this shock are highly reminiscent to those of HFA’s including strong nonadiabatic heating and a strong decelaration and deflection of the plasma velocity. For density depletions to a quarter or less of the original solar wind density, the plasma behind the sunward moving shock has a sunward velocity component. We present analytic solutions of this interaction which are complemented by one- and two-dimensional MHD simulations to demonstrate consistency and the spatial evolution of the HFA-like structure.