THE COLD-DENSE PLASMA SHEET - A GEOTAIL PERSPECTIVE

GEOTAIL observations of the low-latitude boundary layer (LLBL) in the tail-flanks show that they are the region where the cold-dense plasma appears with stagnant flow signatures accompanied by bi-directional th ermal electrons (< 300 eV). It is concluded from these facts that the tail-LLBL is the site of capturing the cold-dense plasma of the magnet osheath origin on to the closed field lines of the magnetosphere. Ther e are also cases that strongly suggest that the cold-dense plasma entr y from the flanks can be significant to fill a substantial part of the magnetotail. In such cases, the cold-dense plasma is not spatially re stricted to a layer attached to the magnetopause (that is, the LLBL), but continues to well inside the magnetotail, constituting the cold-de nse plasma sheet. Inspired by the fact that these remarkable cases are found for northward interplanetary magnetic field (IMF), a statistica l study on the status of the near-Earth plasma sheet is made. The resu lts show that the plasma sheet becomes significantly colder and denser when the northward IMF continues than during southward IMF periods, a nd that the cold-dense status appears most prominently near the dawn a nd dusk flanks. These are consistent with the idea that, during northw ard IMF periods, the supply of cold-dense ions to the near-Earth tail from the flanks dominates over the hot-tenuous ions transported from t he distant tail.