The low‐latitude boundary layer at mid‐altitudes: Relation to large‐scale Birkeland currents

Data from the hot plasma instruments and the magnetometer on board Viking (apogee at 13,530 km) acquired in the high latitude pre‐dawn to pre‐noon region are combined to study the relations between the low‐latitude boundary layer (LLBL) projection and large‐scale field‐aligned currents (FAC). The projection of the LLBL is defined as the region with precipitation of ions with magnetosheath origin [Woch and Lundin, 1993]. The cusp and plasma mantle regions are excluded. The study focuses on magnetically quiet to moderately disturbed conditions. On the vast majority of passes the R1 FAC is found to be totally confined within the LLBL projection. This can be regarded as direct evidence of a LLBL source for the dayside R1 FAC. The latitudinal width of the LLBL (and consequently also that of the R1 FAC) decreases considerably going from pre‐dawn to pre‐noon MLT sectors. The current intensity increases only slightly towards pre‐noon. The R1 current density maximum known to exist in the pre‐noon region is thus primarily due to the LLBL projection converging towards local noon.