Thermospheric dark band structures observed in all-sky OI 630 nm emission images over the Brazilian low-latitude sector

摘要

Using two ground-based all-sky imaging systems, measurements of moving dark band structures in the OI 630.0 nm nightglow emission were detected in the low-latitude region of Brazil. On the nights of 30–31 August 1995, 18–19 July 1998, and 13–14 July 1999, the all-sky imaging observations of the OI 630 nm emission, carried out at Cachoeira Paulista (22.7°S, 45°W, magnetic declination 20°W), Brazil, showed dark band structures that stretched across the entire image and propagated from southeast to northwest. The observed band structures are thus most likely formed in the midlatitude region and move into the field of view. Digisonde observations, available for two of the events, registered abrupt increases in both the F-layer peak height (hmF2) and minimum virtual height (h'F) on 19 July 1998 and 13 July 1999, when the low-intensity band passed over Cachoeira Paulista. It should be pointed out that these thermospheric events are not related to geomagnetic disturbed conditions, as the nights for which data are presented are geomagnetically quiet (Kp < 2). In this paper we present and discuss the first observations of these thermospheric dark band structures in the Brazilian sector and their effects on the nighttime ionosphere. A possible mechanism for generation of these dark band structures is presented.