In-situ studies of equatorial spread- F over SHAR—steep gradients in the bottomside F-region and transitional wavelength results

Abstract

RH-560 rockets instrumented with Langmuir probes were launched from SHAR, India (dip 11°N) for in-situ studies of electron density irregularities associated with equatorial spread- F (ESF) when the F-region plasma was drifting down and strong range spread- F was observed with an ionosonde at SHAR. A high variability was observed in the steepness of the base of the F-region. The bases were found to be steeper during the periods when the F-region plasma was drifting down. On one of the flights irregularities were observed in the region around 280 km where the gradients in electron density were downwards, indicating that the gradient drift instability is the main mechanism for their generation. Assuming a power law of the type P k ∝ k n for irregularities of transitional scale (20–200 m), it was found that the spectral index n ranges between −1.5 and −4.6, when the mean integrated spectral power P T of the irregularities in the above scale size range varied from −45 to −12 db. A relationship between n and P T was observed and can be represented by a Gaussian function using the above expression; the altitude variation of n normalized for a P T value of −10 db showed that the nature of spectral index remains the same between 230 km and the apogee of the rocket. This is at variance with the observations of Kelley et al. [(1982), J. geophys. Res. 87, 1575] that 280 km is the threshold altitude for the steep drift wave type of spectra to a shallower spectra.