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Ann. Geophys., 24, 3451-3465, 2006
www.ann-geophys.net/24/3451/2006/
© European Geosciences Union 2006
Observation of two distinct cold, dense ion populations at geosynchronous orbit: local time asymmetry, solar wind dependence and origin
B. Lavraud1, M. F. Thomsen1, S. Wing2, M. Fujimoto3, M. H. Denton4, J. E. Borovsky1, A. Aasnes1, K. Seki5, and J. M. Weygand6
1Space Science and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico, USA
2The Johns Hopkins University, Applied Physics Laboratory, Laurel, Maryland, USA
3Tokyo Institute of Technology, Meguro, Tokyo, Japan
4Department of Communications Systems, Lancaster University, Lancaster, UK
5Solar-Terrestrial Environment Laboratory, University of Nagoya, Aichi, Japan
6Institute of Geophysics and Planetary Physics, UC Los Angeles, California, USA
Abstract. We report on the observation of two distinct cold
(Ti<5 keV), dense (Ni>2 cm−3) ion populations at
geosynchronous orbit. A statistical study was performed on measurements from
the geosynchronous Los Alamos plasma instruments, for the period 1990–2004,
and complemented by corresponding large-scale plasma sheet data obtained by
mapping DMSP observations in the tail. The first population, which has
previously been reported in several studies, is observed in the midnight
region of geosynchronous orbit. The second population, which has drawn less
attention, is detected on the dawn side of geosynchronous orbit. No such
cold, dense population is observed on the dusk side of geosynchronous orbit
on a frequent basis. The temporal evolution of various plasma parameters as
a function of local time shows that the two populations appear at
geosynchronous orbit as distinct populations, since the appearance of a
midnight population is not usually associated with that of a dawn
population, and vice versa. The midnight ion population is typically
observed after the IMF has been northward for some time and is convected
inward toward geosynchronous orbit after an observed mild southward turning
of the average IMF. It is interpreted that the source of the midnight
population is the cold, dense plasma sheet (CDPS). The dawn-side cold and
dense ion population is associated with previously strong southward IMF and
consequently occurs during substantial geomagnetic activity. These events
are typically observed around the end of the main phase of the corresponding
Dst decrease, down to −50 nT on average. It is unlikely that this dawn
population is simply the low-latitude boundary layer (LLBL) moving closer to
Earth because (1) no symmetric dusk population is observed and (2) on
average a small sunward flow (~15 km/s) is observed for those events.
The cold, dense population at dawn is thus observed during active times
(based on Dst, Kp and AE indices) in comparison with the midnight case. However,
since the dawn population is observed only around the end of the main
Dst decrease, it is concluded that this population does not typically contribute
to the Dst decrease during the main phase. This population may rather be
transported to geosynchronous orbit by means of a compression and convection
enhancement in the magnetosphere, with a preferential access from the dawn
flank with no apparent counterpart at dusk. DMSP data suggest that a cold
and dense plasma source is mainly present at dawn.
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