Formation of hot flow anomalies and solitary shocks

Abstract

Interaction of a tangential discontinuity (TD) with the bow shock is investigated by using electromagnetic, global hybrid simulations in which ions are treated kinetically via particle‐in‐cell methods and electrons form a massless fluid. On the basis of previous studies, it was expected that the interaction would result in the formation of a hot flow anomaly (HFA) propagating along the curved bow shock surface. The results are unexpected in two major ways. First, the hot flow anomaly is only formed during the interaction of the TD with the quasi‐parallel side of the bow shock. The lack of a HFA on the perpendicular side is due to the inability of a large fraction of ions to escape into the solar wind, as is required for an HFA to form. Second, the interaction of the TD with the quasi‐perpendicular portion of the bow shock results in a previously unknown, shock structure which we name the “solitary shock.” The solitary shock consists of a finite width (a few ion inertial length), fast magnetosonic shock‐like structure followed by a relatively less compressed, more turbulent plasma with complex and spatially varying properties in the downstream region. We have determined that the formation of the solitary shock after the passage of the TD is due to the new direction of the interplanetary magnetic field. Further, this is not a transitory phenomena and when the interplanetary magnetic field cone angle is large (∼>50°) a significant portion of the bow shock surface is affected. Solitary shocks form in the regions where the motional electric field in the magnetosheath points away from the shock. We demonstrate that solitary shocks differ from regular quasi‐perpendicular shocks due to differences in ion dissipation processes. We also present the results of a detailed survey of the effects of simulation parameters such as cell size, resistivity, system size, and 2.5‐dimensional versus three‐dimensional domains on the solitary shock solutions.