Airglow-imaging observation of plasma bubble disappearance at geomagnetically conjugate points

Abstract:

We report the first observation of the disappearance of plasma bubbles over
geomagnetically conjugate points. It was observed by airglow imagers at Darwin,
Australia (magnetic latitude: -22N) and Sata, Japan (21N) on 8 August 2002. The
plasma bubble was observed in 630-nm airglow images from 1530 UT (0030 LT) to
1800 UT (0300 LT) and disappeared equatorward at 1800-1900 UT (0300-0400 LT) in
the field of view. The ionograms at Darwin and Yamagawa (20 km north of Sata)
show strong spread-F signatures at ~16-21 UT. At Darwin, the F-layer virtual
height suddenly increased from ~200 km to ~260 km at the time of bubble
disappearance. However a similar F-layer height increase was not observed over
the conjugate point at Yamagawa, indicating that this F-layer rise was caused
not by an eastward electric field but by enhancement of the equatorward
thermospheric wind over Darwin. We think that this enhancement of the
equatorward neutral wind was caused by an equatorward-propagating large-scale
traveling ionospheric disturbance, which was identified in the north-south
keogram of 630-nm airglow images. We suggest that either F-region dynamo
or polarization electric field associated with this equatorward neutral wind
drive plasma drift across the magnetic field line to cause the observed bubble
disappearance.