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

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.

A numerical simulation study of the collisional‐interchange instability seeded by the pre‐reversal vertical drift

“…The zonal shear flow leads to the generation of confined bottomside bubble as shown by Kelley [1998] andde Paula et al [2011]. It also leads to the generation of the shear instability which then combines with the CII to generate the bottomside Rodrigues et al [2008]. The effects of the zonal shear flow is not considered in the present work, and u ox is taken to be zero.…”

Radar and satellite investigations of equatorial evening vertical drifts and spread <i>F</i>

“…The São Luis radar is a low-power radar system that has been used for studies of equatorial ionospheric irregularities in the E and F regions Rodrigues et al, 2004Rodrigues et al, , 2008Rodrigues et al, , 2011Shume et al, 2011). The radar is equipped with four independent antenna sets connected to four receivers.…”

On the characteristics of 150-km echoes observed in the Brazilian longitude sector by the 30 MHz São Luís radar

“…Jicamarca measurements made by Royrvik (1982) almost twenty years later showed that the echoes were caused by field-aligned electron density irregularities. These echoes are observed during daytime only and are commonly referred to as 150-km echoes.…”

“…The São Luís radar has being used mainly for studies of equatorial spread F (e.g. Rodrigues et al, 2004;de Paula et al, 2004;Rodrigues et al, 2008) but observations of the equatorial electroject and 150-km echoes have also been made . The São Luís radar is equipped with two 4 kW transmitters and four independent antenna sets.…”