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Ann. Geophys., 23, 2519-2529, 2005
www.ann-geophys.net/23/2519/2005/
© European Geosciences Union 2005
Superposed epoch analysis of dense plasma access to geosynchronous orbit
B. Lavraud, M. H. Denton, M. F. Thomsen, J. E. Borovsky, and R. H. W. Friedel
Space Science and Applications, Los Alamos National Laboratory, P.O. Box 1663, MS D466, Los Alamos, 87545 New Mexico, USA
Abstract. We report on the occurrence of dense plasma access to
geosynchronous orbit. We performed a superposed epoch analysis of
1464 events of dense (>2 cm–3 at onset) plasma observed by the MPA
instruments on board the Los Alamos satellites, for the period 1990–2002. The
results allow us to study the temporal evolution of various plasma
parameters as a function of local time. We show that dense plasma access to
geosynchronous orbit mostly occurs near local midnight. This dense plasma
population is shown to be freshly injected from the mid-tail region, colder
than the typical plasma sheet and composed of a relatively small O+
component. This population is thus probably the result of a cold, dense plasma
sheet (CDPS) injection from the mid-tail region. Cold and dense ion
populations are also observed on the dawnside of geosynchronous orbit at a
similar epoch time. However, we demonstrate that this latter population is
not the result of the dawnward transport of the population detected near
midnight. The properties of this ion population may arise from the
contribution of both ionospheric upflows and precipitating plasma sheet
material. The correlation of an enhanced Kp index with the arrival of the
CDPS at geosynchronous orbit shows that the inward transport of this
population is allowed by an enhanced magnetospheric convection.
Surprisingly, this dense plasma does not, in general, lead to a stronger Dst
(ring current strength) within the 12 h following the CDPS injection. It is
noted, however, that the superposed Kp index returns to relatively low
values soon after the arrival of the CDPS. This may suggest that the dense
plasma is, given the average of the 1464 events of this study, only transiting
through geosynchronous orbit without accessing the inner regions and,
therefore, does not contribute to the ring current.
Keywords. Magnetospheric physics (Plasma convection;
Plasma sheet) – Space plasma physics (Transport processes)
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