A case study of a transient geomagnetic field variation event associated with a traveling magnetospheric convection twin vortex is presented. The characteristics of this event are different from those of other presented cases, as the sense of rotation of the associated ionospheric current system is reversed, that is, it exhibits upward (downward) field-aligned currents in the tailward (dayside) part of the vortex structure. Other features are as usually observed of such transients. In particular, a clear tailward motion at high latitudes can be deduced from local as well as global magnetic field observations. At low dayside latitudes, magnetic field variations similar to sudden impulse (SI) variations are recorded. Almost simultaneous onset and no westward propagation are observed, too. As possible source mechanisms of such transient events, localized magnetic field reconnection as well as pressure pulses at the dayside magnetopause are discussed. If the switch-on of magnetic reconnection can be regarded as equivalent current flowing against the existing magnetopause current and if this countercurrent is spatially localized, an Alfv¿n wave is generated with downward (upward) field-aligned current flow in the tailward (dayside) part of the wave, as observed for some of the reported transients.

However, the SI-like behavior of the event studied in this paper, its reversed sense of rotation, and the vortex associated field-aligned current density excludes such localized magnetic reconnection as a possible source mechanism and argues in favour of the existence of pressure pulses at the magnetopause. It is shown that a spatially localized compression or dilatation of the dayside magnetopause is also associated with the onset of a current flowing parallel or antiparallel to the magnetopause current, respectively. The additional current associated with compression gives rise to a system of upward and downward field-aligned currents in the tailward and dayside part of the system, much as observed for the event analysed. As the pressure pulse is moving along the magnetopause, the generated Alfv¿n wave pattern is that of an Alfv¿n wing structure, with the ground-magnetic transient representing the passing by of the first ionospheric reflection site. Such an Alfv¿n wing structure may also explain recent observations of multiple twin vortex systems. ¿ American Geophysical Union 1992