The time-scale for the formation of the cold-dense plasma sheet: A case study

The time-scale for the formation of cold-dense plasma sheet ions was investigated with an event in which the interplanetary magnetic field (IMF) was northward for almost one day. The plasma sheet dawn and dusk flanks appear to reach cold dense states (n > 1 cm-3; T < 2 keV) within a few hours after IMF northward turning. Closer to the center (midnight meridian), the ion temperatures reach < 2 keV within a few hours of IMF northward turning, but the ion densities do not reach above 1 cm-3 for at least ~8 hours after IMF northward turning. The plasma sheet flanks appear to reach their coldest and densest state ~15 hours after IMF northward turning. The connection between solar wind ions and plasma sheet cold-component ions are demonstrated. This study confirms the previous statistical results: (a) the densification of the plasma sheet can be attributed to the influx of the cold-component (magnetosheath/solar wind origin) ions; and (b) the cooling of the plasma sheet can be attributed not only to the influx of the solar wind ions, but also the cooling of the hot-components. Order of magnitude calculations of plasma sheet filling rate from reconnection and diffusion suggest that both entry mechanisms could result in roughly comparable filling rates. Hence, the dawn-dusk asymmetries would be key in distinguishing the roles of the various proposed entry mechanisms.