IONIZATION TROUGHS BELOW F2-LAYER MAXIMUM

Direction-of-arrival information is available on ionograms from Ellsworth, Antarctica because of interference effects from the Filchner Ice Shelf. This has allowed the detection of extremely large ionospheric disturbances which appear to move towards the equator with speeds of the order of 100 m/sec, in the late afternoon and early evening hours. These disturbances produce troughs in the bottomside ionosphere having widths as large as 1000 km, the electron density at the maximum of the F2-layer being reduced by an order of magnitude. Disturbance effects are also recorded in the D and E-layers. Associations with radio auroras and magnetic activity have been found. It is suggested that the troughs mark the positions of red-auroral arcs. The apparent movement of the trough as observed at Ellsworth has been explained in terms of the rotation of the Earth, the trough position being imagined as stationary with respect to the Sun. It seems likely that topside troughs are the upper F2-region extensions of bottomside troughs and that these can be identified (along the Earth's magnetic field lines) with the plasmapause, at least in the night hours after midnight. The troughs seem to define the boundary between the quiet and active regions of high latitudes. It is suggested that this boundary moves closer to the equator when the solar wind speed increases. © 1969.