Formation of the flank LLBL: A case study

A formation of the low-latitude boundary layer (LLBL) on open field lines during intervals of southward oriented interplanetary magnetic field is now understood in terms of dayside reconnection. However, observations of counterstreaming particles suggest that the LLBL can exist on closed field lines over some portion of the magnetopause. The fluctuations of plasma parameters inside a layer are attributed either to a transient nature of the processes forming a layer or to sweeping of deformations of the magnetopause or inner edge of the LLBL surfaces along the spacecraft. The INTERBALL-1 and MAGION-4 crossed the LLBL near the terminator during intervals of disturbed solar wind conditions. Two-point observations allow us to distinguish the temporal and spatial changes, to estimate the LLBL thickness and its variations and to find the sources of observed plasma populations. Observations suggest that the flank LLBL is supplied by high-latitude reconnection near the cusp and that it lies on open field lines.