The Cold-Dense Plasma Sheet

According to the standard theory of the magnetospheric convection, northward IMF periods are the times when the magnetosphere is supposed to be most closed to the solar wind plasma. In this sense, the fact that the plasma sheet becomes not only cooler but also denser, which has come to be recognized clearly in the recent 10 years, is puzzling. The higher density indicates higher efficiency and the lower temerature implies little heating upon this unidentified transport mechanism. In this talk I will review (1) data analysis studies that have investigated the nature of the cold-dense plasma sheet, (2) data analysis studies that have clarified the structure of the magnetotail under northward IMF, and (3) theoretical efforts towards the understanding of the solar wind plasma entry mechanism. The keys to the progress in this reserach topic during the past 10 years are (1) continuous availability of the solar wind/IMF data, (2) easy access to the data from the whole life-time of a spacecraft, (3) oppotunities of multi-point obseravations, and (4) upgraded computational capability. The limitating factors have been (1) time resolution of the plasma instrument onboard spacecraft, (2) no coordination for multi-point observations, and (3) not yet large enough computational capabilities. There is good chance that these limitations will be resolved in the future: The CrossScale mission, being planned among European and Japanese scientists, will resolve the first two issues.