Equatorial electrojet irregularities investigations using a 50 MHz back-scatter radar and a digisonde at Sao Luis: some initial results

A new 50 MHz coherent back-scatter radar has recently become operational at the Equatorial Space Observatory of INPE at Sao Luis (2.33degreesS, 44.2degreesW, dip: -0.5degrees), in Brazil, located close to the center of the equatorial electrojet. Preliminary results from a 12-day campaign in December 1999 involving this radar and a nearby digisonde are presented in this paper. The range-time-intensity (RTI) maps of electrojet irregularities show short-period (similar to5 min or larger) fluctuations and significant day-to-day variability. A notable characteristic is the midday descent in the altitude of the RTI pattern observed on some days when the dominant Doppler spectra are of type-2. This feature is similar to the diurnal variation of the base height of the q-type sporadic-E-layer (h'Es-q) simultaneously observed by the digisonde. Quasi-periodic fluctuations in the RTI and type-1 and type-2 spectral intensities seem to indicate electric field modulation from winds associated with typical gravity wave periods. Large surges in the westward irregularity drift velocity, presenting a higher velocity tail to the Doppler spectral distribution, is observed at sunset concurrent with the evening F-layer vertical uplift as observed by the digisonde. Some implications of these results on the E- and F-region electrodynamics are discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.