Direct determination of IMF BY-related cusp current systems, using SuperDARN radar and multiple ground magnetometer data:: A link to theory on cusp current origin

We analyze an ionospheric "enhanced convection event" in the cusp on November 13, 1996, at 1900 UT, by using data of the SuperDARN radar, and of the IMAGE, Greenland, MACCS and CANOPUS magnetometer arrays; and from other magnetometer stations. The event occurs similar to 20 minutes after a transition of the IMF B-Y component from positive to negative and an associated reconfiguration of the ionospheric electric potential pattern. The data allow an instantaneous two-dimensional view of the event in terms of the ionospheric electric and ground magnetic field. The "method of characteristics" is used to obtain distributions of ionospheric conductances, actual ionospheric currents, and field-aligned currents (FACs) for the interval under study. Our results show that the region of enhanced convection observed by the radar is associated with a region of low conductances, ranging only slightly above the UV conductance values. However, owing to the strongly enhanced electric field, it produces enhanced westward flowing Hall and southward flowing Pedersen currents. At the northern and southern borders of the enhanced convection region, sheets of downward and upward FACs are observed, respectively, with magnitudes of around 0.5 A/km(2). The geometry of the current system resembles a DPY current system [Friis-Christensen and Wilhjelm, 1975]. Using our results, we test alternative theories on cusp current origin that lead to different predictions of the relative location of the cusp current system with respect to the open-closed field line boundary. The location of this boundary is inferred from DMSP F10 satellite data. The center of our resulting current system is located clearly poleward of the open-closed field line boundary, thus favoring the idea of Lee et al. [1985] that the cusp FACs are caused by a rotational discontinuity of the magnetic field at the magnetopause. In contrast, the idea of Clauer and Banks [1986] that a mapping of the solar wind E-z component to the ionosphere is responsible for the cusp current system is not supported.