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Ann. Geophys., 21, 947-953, 2003
www.ann-geophys.net/21/947/2003/
© European Geosciences Union 2003
The dependence of cusp ion signatures on the reconnection rate
S. K. Morley1 and M. Lockwood1,2
1Department of Physics and Astronomy, University of Southampton, UK
2Rutherford Appleton Laboratory, Chilton, Oxfordshire, UK
Abstract. The interpretation of
structure in cusp ion dispersions is important for helping to understand the
temporal and spatial structure of magnetopause reconnection.
"Stepped" and "sawtooth" signatures have been shown to be
caused by temporal variations in the reconnection rate under the same physical
conditions for different satellite trajectories. The present paper shows that
even for a single satellite path, a change in the amplitude of any reconnection
pulses can alter the observed signature and even turn sawtooth into stepped
forms and vice versa. On 20 August 1998, the Defense Meteorological Satellite
Program (DMSP) craft F-14 crossed the cusp just to the south of Longyearbyen,
returning on the following orbit. The two passes by the DMSP F-14 satellites
have very similar trajectories and the open-closed field line boundary (OCB)
crossings, as estimated from the SSJ/4 precipitating particle data and Polar
UVI images, imply a similarly-shaped polar cap, yet the cusp ion dispersion
signatures differ substantially. The cusp crossing at 08:54 UT displays a
stepped ion dispersion previously considered to be typical of a meridional
pass, whereas the crossing at 10:38 UT is a sawtooth form ion dispersion,
previously considered typical of a satellite travelling longitudinally with
respect to the OCB. It is shown that this change in dispersed ion signature is
likely to be due to a change in the amplitude of the pulses in the reconnection
rate, causing the stepped signature. Modelling of the low-energy ion cutoff
under different conditions has reproduced the forms of signature observed.
Key words. Ionosphere (particle
precipitation) Magnetospheric physics (energetic particles, precipitating,
magnetopause, cusp and boundary layers)
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