A geotail observation of low-latitude boundary layer

A case study of the low-latitude boundary layer (LLBL) on the dawnside (7-9 MLT) is reported. As in previous studies, the LLBL structure is well organized if it is taken to consist of two parts, the outer-LLBL and the inner-LLBL. The inner-LLBL is where the mixing of the magnetosheath and the magnetospheric plasma is taking place on closed field lines. The outer-LLBL is where magnetosheath-like plasma is flowing tailward. Detailed analysis of particle signatures, together with the information that IMF B _y was the dominant component directed persistently dawnward for this interval, suggests an interpretation that the outer-LLBL is on reconnected open field lines. The positions of the reconnection sites relative to the spacecraft, and the dynamics of the flux tubes subsequent to reconnection to form the observed outer-LLBL, are also discussed.