Abstract
Abstract. First observational evidence, from the Indian longitudes, for the presence of gravity wavelike perturbations with periods of 20–30 min, acting as probable seeds for Equatorial Spread F (ESF) irregularities is described. The study is based on the daytime optical measurements of the mesopause temperature and the intensity of the thermospheric O(1D) 630.0 nm dayglow emissions using the unique MultiWavelength Dayglow PhotoMeter from Trivandrum (8.5° N; 77° E; dip lat ~0.5° N), a dip equatorial station. Measurements during the equinoctial months of a solar maximum (2001) and a solar minimum year (2006) have been used in this study. It is shown that under identical background ionospheric conditions within a solar epoch, the power of the gravity waves have a deterministic role in the generation of ESF. The mesopause temperature simultaneously observed, indicate that possible source regions for these perturbations lie in the lower atmosphere.
Highlights
The nighttime phenomenon of Equatorial Spread F (ESF) is characterized by plasma density irregularities spanning a wide range of scale sizes
We report the first observations on the role of gravity wave like perturbation of 20–30 min periodicity in the generation of ESF
The first observational evidence for the presence of gravity wave like perturbation acting as seed for ESF irregularities is presented through 630.0 nm dayglow variability and the daytime mesopause temperature in the evening hours
Summary
The nighttime phenomenon of Equatorial Spread F (ESF) is characterized by plasma density irregularities spanning a wide range of scale sizes. The background conditions on the three days during the solar maximum year and on the two days during the solar minimum year are nearly the same, as indicated by the h F Having ascertained this aspect, the other two parameters, which could play a role in the day-to-day variability of ESF occurrence, are the polarity of the meridional winds and the presence/absence of the seed perturbation. The most important outcome of the present case study, even with the limited database, is that, given identical ionospheric conditions within a solar epoch, the power of the gravity wave like perturbations seems to control the triggering of ESF It provides the first observational evidence for the presence 1o9f gravity wave like perturbations, serving as seeds for ESF. The white solid contours denote the regions of the wavelet spectrum above the 95% confidence level
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