Ionospheric structure near the dayside boundary of closed field lines

Abstract

Variations of ionospheric electron density at altitudes of ≳600 km in the vicinity of the boundary of closed field lines are described. Ionospheric densities are determined from the Isis 1 swept frequency sounder data; the locations of the boundary are determined from latitude profiles of counting rates of electrons (E > 20 kev). Ionospheric densities at all altitudes decrease around the boundary and are low in the polar ionosphere, usually during geomagnetic storms. A trough in ionospheric density at altitudes of ≲1000 km, centered on the boundary field lines, is often observed and is always associated with a peak of density at altitudes of ≳1400 km centered on the same field lines. When there are no systematic changes below 1000 km near the boundary, the density above 1400 km either changes randomly or increases at and poleward of the boundary.