Gravity wave control on ESF day-to-day variability: An empirical approach

The gravity wave control on the daily variation in nighttime ionization irregularity occurrence is studied using ionosonde data for the period 2002-2007 at magnetic equatorial location Trivandrum. Recent studies during low solar activity period have revealed that the seed perturbations should have the threshold amplitude required to trigger equatorial spread F (ESF), at a particular altitude and that this threshold amplitude undergoes seasonal and solar cycle changes. In the present study, the altitude variation of the threshold seed perturbations is examined for autumnal equinox of different years. Thereafter, a unique empirical model, incorporating the electrodynamical effects and the gravity wave modulation, is developed. Using the model the threshold curve for autumnal equinox season of any year may be delineated if the solar flux index (F-10.7) is known. The empirical model is validated using the data for high, moderate, and low solar epochs in 2001, 2004, and 1995, respectively. This model has the potential to be developed further, to forecast ESF incidence, if the base height of ionosphere is in the altitude region where electrodynamics controls the occurrence of ESF. ESF irregularities are harmful for communication and navigation systems, and therefore, research is ongoing globally to predict them. In this context, this study is crucial for evolving a methodology to predict communication as well as navigation outages. Plain Language Summary The manifestation of nocturnal ionospheric irregularities at magnetic equatorial regions poses a major hazard for communication and navigation systems. It is therefore essential to arrive at prediction methodologies for these irregularities. The present study puts forth a novel empirical model which, using only solar flux index, successfully differentiates between days with and without nocturnal ionization irregularity occurrence. The model-derived curve is obtained such that the days with and without occurrence of irregularities lie below and above the curve. The model is validated with data from the years 2001 (high solar activity), 2004 (moderate solar activity), and 1995 (low solar activity) which have not been used in the model development. Presently, the model is developed for autumnal equinox season, but the model development will be undertaken for other seasons also in a future work so that the seasonal variability is also incorporated. This model thus holds the potential to be developed into a full-fledged model which can predict occurrence of nocturnal ionospheric irregularities. Globally, concerted efforts are underway to predict these ionospheric irregularities. Hence, this study is extremely important from the point of view of predicting communication and navigation outages.