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Ann. Geophys., 21, 955-982, 2003
www.ann-geophys.net/21/955/2003/
© European Geosciences Union 2003
IMF control of cusp proton emission intensity and dayside convection: implications for component and anti-parallel reconnection
M. Lockwood1,2, B. S. Lanchester1, H. U. Frey3, K. Throp1, S. K. Morley1, S. E. Milan4, and M. Lester4
1Department of Physics and Astronomy, Southampton University, Southampton, Hampshire, UK
2also at Rutherford Appleton Laboratory, Chilton, Oxfordshire, UK
3Space Sciences Laboratory, University of California, Berkeley, CA., USA
4Department of Physics and Astronomy, Leicester University, Leicester, UK
Abstract. We study a brightening of
the Lyman-a emission in the cusp which occurred in
response to a short-lived south-ward turning of the interplanetary magnetic
field (IMF) during a period of strongly enhanced solar wind plasma
concentration. The cusp proton emission is detected using the SI-12 channel of
the FUV imager on the IMAGE spacecraft. Analysis of the IMF observations
recorded by the ACE and Wind spacecraft reveals that the assumption of a
constant propagation lag from the upstream spacecraft to the Earth is not
adequate for these high time-resolution studies. The variations of the
southward IMF component observed by ACE and Wind allow for the calculation of
the ACE-to-Earth lag as a function of time. Application of the derived
propagation delays reveals that the intensity of the cusp emission varied
systematically with the IMF clock angle, the relationship being particularly
striking when the intensity is normalised to allow for the variation in the
upstream solar wind proton concentration. The latitude of the cusp migrated
equatorward while the lagged IMF pointed southward, confirming the lag
calculation and indicating ongoing magnetopause reconnection. Dayside
convection, as monitored by the SuperDARN network of radars, responded rapidly
to the IMF changes but lagged behind the cusp proton emission response: this is
shown to be as predicted by the model of flow excitation by Cowley and Lockwood
(1992). We use the numerical cusp ion precipitation model of Lockwood and Davis
(1996), along with modelled Lyman-a emission
efficiency and the SI-12 instrument response, to investigate the effect of the
sheath field clock angle on the acceleration of ions on crossing the dayside
magnetopause. This modelling reveals that the emission commences on each
reconnected field line 2–2.5 min after it is opened and peaks 3–5 min after
it is opened. We discuss how comparison of the Lyman-a
intensities with oxygen emissions observed simultaneously by the SI-13 channel
of the FUV instrument offers an opportunity to test whether or not the clock
angle dependence is consistent with the "component" or the
"anti-parallel" reconnection hypothesis.
Key words. Magnetospheric physics
(magnetopause, cusp and boundary layers; solar wind-magnetosphere interactions)
– Space plasma physics (magnetic reconnection)
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