MAGNETOSPHERIC RESPONSE TO SOLAR-WIND DYNAMIC PRESSURE VARIATIONS - INTERACTION OF INTERPLANETARY TANGENTIAL DISCONTINUITIES WITH THE BOW SHOCK

Some magnetic impulse events observed in the polar region are related to vortices associated with plasma convection in the ionosphere. Recen t analyses of satellite and ground data suggest that the interaction o f 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 pola r ionosphere. However, before reaching the Earth's magnetopause, the i nterplanetary pressure pulses must interact with and pass through the bow shock. A variation of the solar wind dynamic pressure (DELTArhoV2) may be associated with shocks, magnetic holes, or tangential disconti nuities (TDs) in the interplanetary medium. We study the interaction o f interplanetary TDs with the Earth's bow shock (BS) using both theore tical analysis and MHD computer simulations. It is found that as a res ult of the collision between a TD and the BS, the jump in the solar wi nd dynamic pressure associated with the TD is significantly modified, the bow shock moves, and a new fast shock or fast rarefaction wave, wh ich propagates in the downstream direction, is excited. Our theoretica l analysis shows that the change in the p density across the interplan etary TD plays the most important role in the collision process. Iu th e case with an enhanced dynamic pressure behind the interplanetary TD, the bow shock is intensified in strength and moves in the earthward d irection. The dynamic pressure jump associated with the transmitted TD is generally reduced from the value before the interaction. A fast co mpressional shock is excited ahead of the transmitted TD and propagate s toward the Earth's magnetosphere. For the case in which the dynamic pressure is reduced behind the interplanetary TD, the pressure jump ac ross the transmitted TD is substantially weakened, the bow shock moves in the sunward direction, and a rarefaction wave which propagates dow nstream 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.