Leakage ions from the LLBL to MSBL: Confirmation of reconnection events at the dayside magnetopause

This letter reports on leakage ions from the low latitude boundary layer (LLBL) to the magnetosheath boundary layer (MSBL) as observed by Geotail at the dayside magnetopause. A reconnection of the interplanetary magnetic field (IMF) with magnetospheric field lines is shown to be responsible for the leakage. Based on analysis of a variation in lower cut-off levels occurring in the velocity distribution function of the leakage ions, we propose a "velocity filter effect" model with a finite source region to explain this variation. The location of the reconnection region is subsequently calculated to be 2.2R_E north of the geomagnetic equatorial plane under the proposed velocity filter effect model.