Observation of two distinct cold, dense ion populations at geosynchronous orbit: local time asymmetry, solar wind dependence and origin

Abstract. We report on the observation of two distinct cold (T_i<5 keV), dense (N_i>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 D_st 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 D_st, K_p and A_E indices) in comparison with the midnight case. However, since the dawn population is observed only around the end of the main D_st decrease, it is concluded that this population does not typically contribute to the D_st 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.