Structure of the low-latitude boundary layer: A case study with Geotail data

A case study of the structure of the low-latitude boundary layer (LLBL) on dawnside (0600-0900 MLT) is reported. It is shown that the LLBL consists of two regions, the sheath-like region and the mixing region. The sheath-like region is where cold ions from the magnetosheath are flowing tailward. Detailed study has shown that this region is produced by reconnection and subsequent draping of open field lines over the dayside magnetosphere [Fujimoto et al., 1997]. The mixing region, on the other hand, is characterized by mixing of cold ions from the magnetosheath with the magnetospheric hot ions. Thermal electrons (< 500 eV) enhanced bidirectionally along field lines are detected to accompany these mixed ions. The balanced bi-directional flux is taken to indicate the closed topology of the field lines. An interesting finding is that the flow direction in this region tends to be sunward. Flow in the mixing region and the adjacent plasma sheet is faster sunward than in the ring current region, suggesting that these two constitute a channel of sunward returning convection in the dayside outer magnetosphere for this particular case. More new information on the mixing region from the present study is the ion distribution function showing a three-component feature and anticorrelation between plasma density and the degree of electron bidirectional heating. These observational facts would offer clues for understanding of the mixing region formation, which still remains open.