Direct determination of IMF B-Y-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 Novemb er 13, 1996, at 1900 UT, by using data of the SuperDARN radar, and of the I MAGE, Greenland, MACCS and CANOPUS magnetometer arrays; and from other magn etometer stations. The event occurs similar to 20 minutes after a transitio n of the IMF B-Y component from positive to negative and an associated reco nfiguration of the ionospheric electric potential pattern. The data allow a n instantaneous two-dimensional view of the event in terms of the ionospher ic electric and ground magnetic field. The "method of characteristics" is u sed 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 rad ar is associated with a region of low conductances, ranging only slightly a bove the UV conductance values. However, owing to the strongly enhanced ele ctric field, it produces enhanced westward flowing Hall and southward flowi ng Pedersen currents. At the northern and southern borders of the enhanced convection region, sheets of downward and upward FACs are observed, respect ively, with magnitudes of around 0.5 A/km(2). The geometry of the current s ystem resembles a DPY current system [Friis-Christensen and Wilhjelm, 1975] . Using our results, we test alternative theories on cusp current origin th at lead to different predictions of the relative location of the cusp curre nt 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 o ur 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 c usp 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 m apping of the solar wind E-z component to the ionosphere is responsible for the cusp current system is not supported.