Slow mode transition in the frontside magnetosheath

Three magnetosheath passes with density enhancements in front of the magnetopause are studied with data from ISEE 1, 2, and 3. The density structure appears to be locally generated and slow mode in nature. In one pass when ISEE 1 and 2 were well separated, the motion of the density structure can be determined. The density structure appears to stand in the magnetosheath flow. Thus, it propagates upstream in the rest frame of the flow. The flow in and near the density structure appears to be closer to isothermal than adiabatic. The flow velocity decreases from super-slow to being close to the intermediate and slow mode velocities at the outer edge of the density structure. This study provides additional evidence that the density structure in front of the magnetopause is a slow mode transition in which the flow velocity decreases to the MHD slow mode velocity. The slow mode transition may consist of two wave fronts and a region with strong slow mode waves. This slow mode transition may play an important role in establishing the flow and field pattern near the magnetopause.