Role of gravity wave seed perturbations in ESF day-to-day variability: A quantitative approach

The role of seed perturbations in the day-to-day variability of occurrence of Equatorial Spread F (ESF) is studied using Ionosonde data for years 2005–2007 at magnetic equatorial location Trivandrum (8.5°N, 77°E, and dip 1.3°N). In this paper, we present the novel result that, the amplitude of the seed perturbations, is a very critical parameter which decides, whether or not ESF would occur on a given day and that, this threshold level of seed perturbation amplitude required on a given day decreases as the post sunset height of the F layer increases. Further, the requisite seed perturbation at a particular altitude shows solar activity dependence with progressively lower requisite seeds being observed at lower levels of solar activity. Moreover, the requisite seed is also found to be showing unique altitudinal dependence for each season. These results underline the need to evolve a new ESF prediction parameter, which takes into account the amplitude of the seed perturbation along with the layer height.