Geosynchronous Observations of Quasi-Periodic Variations in the Dayside Magnetospheric Magnetic Field

A variety of processes have been invoked to explain isolated and quasi-periodic events seen near the magnetopause and in the ionosphere at the feet of magnetic field lines that map to the vicinity of the dayside magnetopause. Geosynchronous orbit offers an excellent vantage point to distinguish between these mechanisms. Recent work indicates a one-to-one relationship between compressional perturbations in the dayside geosynchronous magnetic field and solar wind dynamic pressure variations on time scales greater than ~10 min, obviating any need to invoke cavity mode resonances. The geosynchronous signatures of magnetic reconnection and magnetopause erosion are far weaker, and can only be identified on quiet days or via statistical methods. While compressional perturbations in the dayside geosynchronous magnetic field on time scales ranging from ~1 to 15 min generally cannot be associated with solar wind dynamic pressure variations seen by spacecraft far upstream, they can often be associated with pressure perturbations generated by kinetic processes within the foreshock. This close relationship, the fact that the events move in the direction predicted for solar wind features striking the magnetosphere, and the absence of any tendency for the events to occur during periods of high solar wind velocity or southward IMF orientation, suggests that pressure variations drive magnetopause motion with amplitudes far larger than those associated with bursty merging or the Kelvin-Helmholtz instability. Pressure pulses generated within the foreshock also offer an opportunity to explain observations of nearly constant magnetopause motion and the large scatter noted when the locations of magnetopause crossings are binned by solar wind parameters. When the fast mode waves launched by the solar wind dynamic pressure variations intersect abrupt density and pressure gradients at the inner edge of the low-latitude boundary layer (and possibly the outer edge of the plasmasphere), they generate field-aligned currents and corresponding isolated and quasi-periodic ionospheric events. Geosynchronous observations can be used to distinguish between the various models proposed to account for these events.