Transequatorial F-region ionospheric plasma bubbles: solar cycle effects

During the recent past, wide-angle optical imaging observations of F-region nightglow emissions (e.g. OI 630 nm) have provided excellent results related to the occurrence, evolution and dynamics of large-scale range spread-F irregularities, as they are characterized by large-scale ionospheric plasma depletions, generally known as transequatorial plasma bubbles, which result in quasi north-south aligned intensity depleted bands. The intensity depletions seen in the airglow images are the optical signature, at the height range of the emitting layer of transequatorial magnetic field-aligned plasma bubbles. An all-sky imaging system, observing the OI 630 nm emission, was operational at Cachoeira Paulista (22.7 degreesS, 45.0 degreesW; similar to 16 degreesS dip latitude), Brazil, during the period March 1987 to October 1991. It was put back in operation again in September 1994 and observations are continuing. These observations have provided an extensive data-base of OI 630 nm images which permitted us to address several aspects related to the formation and development of large-scale spread-F plasma irregularities during both high- and low solar activity periods. An analysis of about 11,000 images from these investigations are presented and discussed in this paper. The seasonal occurrence characteristics are fairly similar for both low and high solar activities. However, the occurrences of intensity depleted bands are much less during low solar activity (33%) as compared with high solar activity (55%). Also, some of the intensity depleted bands in the images (which show the optical signatures at the height of the emitting layer around 250-300 km) indicating that plasma bubbles attaining very high altitudes (>1500 km) at the magnetic equator (by mapping the depletion bands along geomagnetic field lines to the equatorial plane (e.g., Mendillo and Tyler, J. Geophys. Res. 88 (1983) 5778), are much less during low solar activity (34% of the images with intensity depleted bands) as compared with high solar activity (66% of the images with intensity depleted bands). The average nocturnal variations of intensity depleted regions show different characteristics during the high and low solar activity periods. (C) 2000 Elsevier Science Ltd. All rights reserved.