Abstract
Abstract. The 30 MHz coherent backscatter radar located at the equatorial observatory in São Luís, Brazil (2.59° S, 44.21° W, −2.35° 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 F 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 F. 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 F forecasting.
Highlights
Electron density irregularities in the equatorial ionosphere have been observed for over 70 years
Observations have shown that electron density irregulaties with scale-sizes ranging from a few cm to hundreds of km can be detected during spread F events
We have presented in-beam radar images of F-region scattering layers using interferometric observations made with the 30 MHz radar in Brazil
Summary
Electron density irregularities in the equatorial ionosphere have been observed for over 70 years. These irregularidades were first detected by vertical ionospheric radio sounders (Berkner and Wells, 1934). Studies showed that density irregularities are responsible for range and frequency spread in ionograms. Observations have shown that electron density irregulaties with scale-sizes ranging from a few cm to hundreds of km can be detected during spread F events. Large-scale ionospheric plasma depletions can reach the topside ionosphere. These large-scale spread F events are responsible for signal disruption in satellite-based communication and navigation systems These large-scale spread F events are responsible for signal disruption in satellite-based communication and navigation systems (e.g. Kintner et al, 2001)
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