Coherent backscatter radar imaging in Brazil: large-scale waves in the bottomside F-region at the onset of equatorial spread <I>F</I>

• Main Authors: F. S. Rodrigues, D. L. Hysell, E. R. de Paula
• Format: Article
• Language: English
• Published: Copernicus Publications 2008-10-01
• Series: Annales Geophysicae
• Online Access: https://www.ann-geophys.net/26/3355/2008/angeo-26-3355-2008.pdf

The 30 MHz coherent backscatter radar located at the equatorial observatory in São Luís, Brazil (2.59&deg; S, 44.21&deg; W, &minus;2.35&deg; dip lat) has been upgraded to perform coherent backscatter radar imaging. The wide field-of-view of this radar makes it well suited for radar imaging studies of ionospheric irregularities. Radar imaging observations were made in support to the spread <I>F</I> Experiment (SpreadFEx) campaign. This paper describes the system and imaging technique and presents results from a bottom-type layer that preceded fully-developed radar plumes on 25 October 2005. The radar imaging technique was able to resolve decakilometric structures within the bottom-type layer. These structures indicate the presence of large-scale waves (~35 km) in the bottomside F-region with phases that are alternately stable and unstable to wind-driven gradient drift instabilities. The observations suggest that these waves can also cause the initial perturbation necessary to initiate the Generalized Rayleigh-Taylor instability leading to spread <I>F</I>. The electrodynamic conditions and the scale length of the bottom-type layer structures suggest that the waves were generated by the collisional shear instability. These results indicate that monitoring bottom-type layers may provide helpful diagnostics for spread <I>F</I> forecasting.