The dynamic cusp

Abstract

A unique alignment of the Viking satellite with respect to a network of magnetometers in Greenland has provided the opportunity to study the relationship of pulsations and plasma characteristics in the dayside cusp. Observations in the interplanetary medium were not available during the event studied here, but particle data from the DMSP satellite and hot plasma observations from Viking provide strong evidence that the IMF had a strong northward component. The presence of Pc 1 bursts, Pc 4–5 pulsations, and a tailward traveling twin vortex pattern of ionospheric convection suggests that the magnetosphere may have been temporarily compressed. Magnetic field data acquired at synchronous altitude from GOES 5 and on the ground from Huancayo support this suggestion. Plasma with ion dispersion characteristics associated with a cusp during southward IMF was detected by Viking over a 3.5° range of latitude. The presence of standing Alfvén waves and ring current ions suggests that this “cusplike” plasma was observed on closed geomagnetic field lines. As Viking moved further poleward, it detected a different region of plasma with characteristics associated with a cusp during northward IMF. The presence of plasma on closed field lines with “southward IMF” ion dispersion characteristics can be explained with a poleward moving plasma source. We suggest that the magnetosphere, during a northward IMF, is temporarily compressed by a solar wind pressure enhancement that produces the Pc 1 bursts, Pc 4–5 pulsations, and ionospheric vortices. As the magnetosphere recovers to its “precompressed” shape, the source of cusp plasma will move poleward until it reaches an equilibrium position for northward IMF. The Viking satellite, following in the wake of this source, will detect plasma with “southward IMF” characteristics until it reaches the latitude of the actual “northward IMF” cusp. These observations support the view that the shape of the magnetosphere may rarely be static but is often changing as a result of the delicate and variable balance between the solar wind and geomagnetic field.