Two-Stage Oscillatory Response of the Magnetopause to a Current/Vortex Sheet Followed by Northward IMF: Cluster Observations

FARRUGIA, C. J.; GRATTON, F. T.; LUND, E. J.; TORBERT, R.; SANDHOLT, P.; COWLEY, S.; GNAVI, G.; BILBAO, L. E.; MOUIKIS, C.; KISTLER, L.; MANN, I.; WATERMANN, J. F.; SINGER, H. J.

Durham

Workshop; Cluster - Themis Workshop (16th Cluster meeting); 2008

University of New Hampshire

ABSTRACT. We discuss the motion and structure of the magnetopause/boundary layer observed by Cluster in response to a joint tangential discontinuity/vortex sheet (TD/VS) observed by the ACE spacecraft on December 7, 2000. The observations are then supplemented by theory. Sharp polarity reversals in the east-west components of the field and flow occurred at the discontinuity. These rotations were followed by a period of strongly northward IMF. These two factors elicited a two-stage response at the magnetopause, as observed by Cluster situated in the boundary layer at the duskside terminator. First, the magnetopause suffered a large deformation from its equilibrium position, with large-amplitude oscillations of about 3 min period being set up. These are argued to be mainly the result of tangential stresses associated with change in the east-west component of the flow, the contribution of dynamic pressure changes being small in comparison. This strengthens recent evidence of the importance to magnetospheric dynamics of changes in azimuthal solar wind flow. The TD/VS impact caused a global response seen by ground magnetometers in an MLT range spanning at least 12 hours. The response monitored on ground magnetometers is similar to that brought about by magnetopause motions driven by dynamic pressure changes. Second, Cluster recorded higher frequency waves (about 79 s). Two clear phases could be distinguished from the spectral power density, which decreased by a factor of three in the second phase. Applying compressible, linearized MHD theory, we show that these perturbations are consistent with surface waves originating from the Kelvin-Helmholtz instability. Varying the local magnetic shear at the Cluster locale, as suggested by the temporal profile of the IMF clock angle, we find that locally stability was reinstated, so that the reduced power in the second phase is argued to be due residual KH activity arriving from locations farther to the dayside. This work is supported by NASA grant NNX08AD11G, NASA Cluster grant to UNH and by Argentine CONICET, grant PIP 5291/05.