Motion of the dusk flank boundary layer caused by solar wind pressure changes and the Kelvin-Helmholtz instability: 10-11 January 1997


During an interval of steady northward IMF on 10–11 January 1997, Geotail and Interball spacecraft data at X = −5 to −15 RE on the dusk flank reveal magnetopause boundary layer motions caused by both solar wind pressure discontinuities and the Kelvin-Helmholtz instability. Several large, sudden changes in solar wind density caused the magnetopause to move across both spacecraft. Relative timing of these crossings along with corresponding geosynchronous and ground magnetic field changes allow study of the propagation of pressure fronts in the magnetosheath and the associated wave propagation in the magnetosphere. The wave in the boundary layer propagates down the tail faster than the pressure front in the magnetosheath, producing boundary motion ahead of the magnetosheath front. This boundary layer wave can bulge out the magnetopause, causing a spacecraft in the magnetosheath to become immersed in the magnetosphere for as long as 2 or 3 min before the higher density magnetosheath plasma arrives and compresses the boundary. The tailward convecting magnetosheath pressure front moves more slowly near the magnetopause than it does further out in the magnetosheath, distorting the front and enhancing the likelihood that the boundary layer wave will outrun the magnetosheath front. Integrating the velocity perpendicular to the boundary shows that the boundary waves can have amplitudes of at least 1 or 2 Re. During periods of steady solar wind pressure, waves with periods of several minutes are observed in a boundary layer that are consistent with exitation of the Kelvin-Helmholtz instability. Magnetograms from five ground magnetometer chains show both pulsations initiated by the pressure discontinuities and ongoing wave trains probably related to the KH instability.

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JGR: Space Physics



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