Equatorial spread-F (ESF) and vertical winds

The Equatorial Spread-F (ESF) phenomenon is recorded in ionograms as a hier archy of plasma instabilities in the F-layer of the equatorial ionosphere. The ESF is characterized by irregularities in the plasma (electron and ion) density and electric field distributions perpendicular to the Earth's magn etic field. Large scale irregularities are generated by a primary plasma in stability that develops in electric fields and plasma densities. Other seco ndary instabilities then develop and generate irregularities at several sca le sizes that often produce a plasma 'hole' or 'bubble' that rises up with high E x B velocities. The ESF/plasma bubble phenomenon has been studied ex tensively with experimental techniques and modeling, which revealed importa nt features. In the bottom side F-layer, near sunset, when the vertical den sity gradient steepens as the layer is supported by the horizontal (North-S outh) Earth's magnetic field lines against the omnipresent Earth's gravitat ional acceleration (g), the plasma conditions can give rise to Rayleigh-Tay lor (RT) type instability. But the observed day to day variability of the E SF occurrence suggested that other agencies may also be involved in generat ing the instability. Sekar and Raghavarao (1987) with linear theory, and Ra ghavarao, Sekar and Suhasini (1992), with non-linear numerical modeling, su ggested that vertical downward (upward) winds in the ambient gas have the p otential to cause (inhibit) the ESF/bubble phenomenon. The presence of down ward winds near the equator was reported earlier. In this paper, we show ev idence for the presence of downward winds collocated with irregularities in electric fields and plasma densities as revealed by an unique combination of highly accurate measurements with instruments onboard the DE-2 satellite . The observations reported here are also consistent with the notion that t he build-up of the equatorial ionization anomaly (EIA) prior to local sunse t is important for the ESF instability. (C) 1999 Elsevier Science Ltd. All rights reserved.