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Ann. Geophys., 22, 2053-2066, 2004
www.ann-geophys.net/22/2053/2004/
© European Geosciences Union 2004
Some aspects of the geomagnetic response to solar wind pressure variations: a case study at low and middle latitudes
U. Villante and P. Di Giuseppe
Dipartimento di Fisica, Università and Area di Ricerca in Astrogeofisica, L’Aquila, Italy
Abstract. We examined geomagnetic field observations at low and middle latitudes in
the Northern Hemisphere during a 50-min interval (12 May 1999),
characterized by a complex behaviour of the solar wind dynamic pressure. For
the entire interval, the aspects of the geomagnetic response can be
organized into four groups of events which show common characteristics for the
H and D components, respectively. The correspondence between the
magnetospheric field and the ground components reveals different aspects of
the geomagnetic response in different magnetic local time (MLT) sectors. For
the H component, the correspondence is highly significant in the dusk and
night sectors; in the dawn and prenoon sectors it shows a dramatic change
across a separation line that extends approximately between (6 MLT, 35°)
and (13 MLT, 60°). For the D component, the correspondence has
significant values in the dawn and prenoon regions. We propose a new
approach to the experimental data analysis which reveals that, at each
station, the magnetospheric field has a close correspondence with the
geomagnetic field projection along an axis (M1) that progressively rotates
from north/south (night events) to east/west orientation (dawn events). When
projected along M1, the geomagnetic signals can be interpreted in terms of a
one-dimensional pattern that mostly reflects the field behaviour observed at
geostationary orbit. Several features appear more evident in this
perspective, and the global geomagnetic response to the SW pressure
variations appears much clearer than in other representations. In
particular, the MLT dependence of the geomagnetic response is much smaller
than that one estimated by previous investigations. A clear latitudinal
dependence emerges in the dusk sector. The occurrence of low frequency waves
at ~2.8mHz can be interpreted in terms of global magnetospheric modes
driven by the SW pulse. This event occurred in the recovery phase after the
day the SW almost disappeared (11 May 1999): in this sense our results
suggest a rapid recovery of almost typical magnetospheric conditions soon
after a huge expansion. Overshoot amplitudes, greater than in other cases,
are consistent with a significant reduction of the ring current.
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