Lockwood, M., Lanchester, B.S., Frey, H.U., Throp, K., Morley, S.K., Milan, S.E. and Lester, M.
IMF control of cusp proton emission intensity and dayside convection: implications for component and anti-parallel reconnection.
Annales Geophysicae, 21, (4), .
We study a brightening of the Lyman-? emission
in the cusp which occurred in response to a short-lived southward
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-? 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 2–2.5min after it is opened and peaks 3–5 min after it is
opened. We discuss how comparison of the Lyman-? 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
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