Magnetospheric response to solar wind dynamic pressure variations: Interaction of interplanetary tangential discontinuities with the bow shock

Abstract

Some magnetic impulse events observed in the polar region are related to vortices associated with plasma convection in the ionosphere. Recent analyses of satellite and ground data suggest that the interaction of solar wind dynamic pressure pulses and the magnetosphere may lead to the formation of velocity vortices in the magnetopause boundary layer region. This can in turn lead to the presence of vortices in the polar ionosphere. However, before reaching the Earth's magnetopause, these interplanetary pressure pulses must interact with and pass through the bow shock. A variation of the solar wind dynamic pressure (ApV 2) may be associated with shocks, magnetic holes, or tangential discontinuities (TDs) in the interplanetary medium. We study the interaction of interplanetary TDs with the Earth's bow shock (BS) using both theoretical analysis and MHD computer simulations. It is found that as a result of the collision between a TD and the BS, the jump in the solar wind dynamic pressure associated with the TD is significantly modified, the bow shock moves, and a new fast shock or fast rarefaction wave, which propagates in the downstream direction, is excited. Our theoretical analysis shows that the change in the plasma density across the interplanetary TD plays the most important role in the collision process. In the case with an enhanced dynamic pressure behind the interplanetary TD, the bow shock is intensified in strength and moves in the earthward direction. The dynamic pressure jump associated with the transmitted TD is generally reduced from the value before the interaction. A fast compressional shock is excited ahead of the transmitted TD and propagates toward the Earth's magnetosphere. For the case in which the dynamic pressure is reduced behind the interplanetary TD, the pressure jump across the transmitted TD is substantially weakened, the bow shock moves in the sunward direction, and a rarefaction wave which propagates downstream is excited. We also simulate and discuss the interaction of a pair of tangential discontinuities, which may correspond to a magnetic hole, with the BS. to infer that solar wind dynamic pressure variations (ApV 2) may lead to the presence of velocity vortices near the magnetopause [Friis-Christensen et al., 1988; Glassmeier et al., 1989; Southwood and Kivelson, 1990; Kivelson and Southwood, 1991]. Vortices in the low latitude-boundary layer (LLBL) map into the ionosphere via a field-aligned current system and can generate the convecting Hall current loops of a single vortex or twin vortices. Therefore one expects that a traveling ionospheric vortex or an induced unipolar (bipolar) magnetic pulse will be observed during such an event [Sibeck, 1990]. This suggestion is supported by the work of Lee [1991] in which MHD simulations of the interaction between the solar wind dynamic pressure pulses and the magnetopause boundary region were studied. The simulation results always show that flow vortices are formed in the LLBL when a shear pressure pulse impinges upon the magnetopause. In fac...

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