A Statistical Study of Plasmaspheric Plumes and Ionospheric Outflows Observed at the Dayside Magnetopause

Abstract:

The low-energy and dense plasma originated from the ionosphere has often been observed in the dayside magnetosphere and may play an important role in the dynamics of the dayside magnetopause. We present a statistical study of the plasmaspheric plumes and the ionospheric outflows observed by the Cluster spacecraft near the dayside magnetopause from 2007 to 2009. The plasmaspheric plumes were identified by the low-energy ions (< 1 keV) which have 90° pitch angle distributions from the CIS/HIA instrument onboard Cluster in the magnetosphere. The ionospheric outflows were characterized by uni- or bi-directional field-aligned pitch angle distributions with low-energy ions observed in the dayside magnetosphere. 43 (10%) plasmaspheric plume events and 32 (7%) ionospheric outflow events were detected during 442 dayside magnetopause crossings. The occurrence rate of the plume at the duskside is significantly higher than that at the dawnside, indicating that the plasmaspheric plumes may lead to a dawn-dusk asymmetry of the dayside reconnection. The occurrence rate of the outflow shows a weak dawn-dusk asymmetry. We investigated how the plasmaspheric plume and ionospheric outflow’s occurrence rates depend on geomagnetic activity and solar wind/IMF conditions. It is found that the plume events prefer to occur during moderate geomagnetic activity (Kp = 3, -30 ≤ Dst < -10 nT), while the ionospheric outflow events do not occur when Kp = 0 and their occurrence rate does not have a clear Dst dependence. It is also found that the plume events tend to occur during southward IMF (duskward solar wind electric field). In contrast, the ionospheric outflows prefer to occur during northward IMF (dawnward solar wind electric field). Finally, the occurrence rates of both plumes and outflows increase with increasing solar wind dynamic pressure.