Thermospheric dark band structures at low latitudes in the Southern Hemisphere under different solar activity conditions: A study using OI 630 nm emission all-sky images

Using ground-based measurements we investigate the occurrence of dark band structures in the OI 630 nm nightglow emission all-sky images in the Brazilian low latitude region during the periods from January 1990 to December 1990 (high solar activity period-HSA), May 1995 to July 1996 (low solar activity period-LSA) and from January 1997 to December 1999 (ascending solar activity period-ASA). Unfortunately we do not have all-sky images for the descending phase of solar activity period. The OI 630 nm images obtained during these periods show thermospheric Dark Band Structures (DBS) in the low latitudes region propagating from southeast to northwest. These dark patches moved with average speed of about 50 200 m/s at an altitude of 220 - 300 km, which is the typical altitude range of the OI 630.0 nm airglow emission. Only during low solar activity period (LSA) and ascending solar activity period (ASA), the DBS occurrences were observed in the OI 630 nm nightglow emission all-sky images. It should be pointed out that these thermospheric/ionospheric events are not related to geomagnetic disturbed conditions. In this paper, we present and discuss this phenomena in the Brazilian sector under different solar activity conditions. A possible mechanism for generation of these dark band structures is presented.