Cusp observations during a sequence of fast IMF BZ reversals
Abstract. In recent years, a large number of papers have reported the response of the cusp to solar wind variations under conditions of northward or southward Interplanetary Magnetic Field (IMF) Z-component (BZ). These studies have shown the importance of both temporal and spatial factors in determining the extent and morphology of the cusp and the changes in its location, connected to variations in the reconnection geometry. Here we present a comparative study of the cusp, focusing on an interval characterised by a series of rapid reversals in the BZ-dominated IMF, based on observations from space-borne and ground-based instrumentation. During this interval, from 08:00 to 12:00 UT on 12 February 2003, the IMF BZ component underwent four reversals, remaining for around 30 min in each orientation. The Cluster spacecraft were, at the time, on an outbound trajectory through the Northern Hemisphere magnetosphere, whilst the mainland VHF and Svalbard (ESR) radars of the EISCAT facility were operating in support of the Cluster mission. Both Cluster and the EISCAT were, on occasion during the interval, observing the cusp region. The series of IMF reversals resulted in a sequence of poleward and equatorward motions of the cusp; consequently Cluster crossed the high-altitude cusp twice before finally exiting the dayside magnetopause, both times under conditions of northward IMF BZ. The first magnetospheric cusp encounter, by all four Cluster spacecraft, showed reverse ion dispersion typical of lobe reconnection; subsequently, Cluster spacecraft 1 and 3 (only) crossed the cusp for a second time. We suggest that, during this second cusp crossing, these two spacecraft were likely to have been on newly closed field lines, which were first reconnected (opened) at low latitudes and later reconnected again (re-closed) poleward of the northern cusp.
At ionospheric altitudes, the latitudinal excursions of the cusp/cleft region in response to the series of the IMF polarity changes were clearly captured by both the ESR and the Pykkvibaer radar of the SuperDARN HF network. The Open-Closed field-line Boundary (OCB) inferred from the HF radar observations underwent latitudinal variations in response to the IMF polarity changes that are in accordance with those predicted by Newell et al. (1989). Furthermore, variations in the ionospheric parameters yielded by the EISCAT VHF and ESR radars are basically consistent with inferences drawn from the HF radar observations. We conclude that Cluster spacecraft 1 and 3 crossed the cusp for a second time as a result of the latitudinal migration of the cusp in response to the IMF polarity reversals; at that time, however, the cusp lay poleward of spacecraft 4. Snapshots of the cusp from two DMSP satellite passes provide further support for this interpretation.