Cusp observations during a sequence of fast IMF BZ reversals
Other literature type
Cai, H. T.
McCrea, I. W.
Dunlop, M. W.
Davies, J. A.
Bogdanova, Y. V.
Milan, S. E.
Ma, S. Y.
arxiv: Physics::Space Physics
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 (<I>B<sub>Z</sub></I>). 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 <I>B<sub>Z</sub></I>-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 <I>B<sub>Z</sub></I> 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
<I>B<sub>Z</sub></I>. 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.