Hindcasting of Equatorial Spread F Using Seasonal Empirical Models

Abstract

The role of gravity waves in modulating equatorial spread F (ESF) day‐to‐day variability is investigated using ionosonde data at Trivandrum (geographic coordinates, 8.5°N, 77°E; mean geomagnetic latitude −0.3°N) a magnetic equatorial location. A novel empirical model that incorporates the combined effects of electrodynamics and gravity waves in modulating ESF occurrence during autumnal equinox season was presented by Aswathy and Manju (2017). In the present study, the height variations of the requisite gravity wave seed perturbations for ESF are examined for the vernal equinoxes, summer solstices, and winter solstices of different years. Subsequently, the empirical model, incorporating the electrodynamical effects and the gravity wave modulation, valid for each of the seasons is developed. Accordingly, for each season, the threshold curve may be demarcated provided the solar flux index (F10.7) is known. The empirical models are validated using the data for high, moderate, and low solar activity years corresponding to each season. In the next stage, this model is to be fine tuned to facilitate the prediction of ESF well before its onset.

A brief review of equatorial ionization anomaly and ionospheric irregularities

“…Recently, Anderson and Redmon () used h ’F 1930 (minimum virtual height of bottom‐side at 19:30 LT) with 80% success. Aswathy and Manju () used an empirical approach to hindcast equatorial spread F irregularities in the electrodynamical regime with ~96% success.…”

First Time Estimation of Thermospheric Neutral Density Profiles From Seed Perturbations of ESF Triggering: A Novel Evidence for Ionosphere Thermosphere Coupling

“…The exponential fits obtained for different seasons are different (Manju et al, ). Similarly, the exponential fits obtained for different levels of solar activity in the same season are also different (Aswathy & Manju, , ; Manju et al, ). Manju et al () had conjectured that the altitude variation of the required seed perturbation for ESF occurrence is due to the neutral density variations with altitude.…”

“…In the present analysis, only days when the ionospheric base height is above the threshold height (Manju et al, ) are considered. The threshold curves for ESF triggering for different seasons and solar activity levels given by Aswathy and Manju (, ) are used in this study.…”

“…They have shown that on the days when the base height of the F layer manifests at a higher level, a small‐amplitude seed perturbation is sufficient to make the plasma irregular, while the condition of low base height of F layer demands a high‐amplitude seed perturbation to generate instability. The critical value of the seed perturbation amplitude at a particular level of the base height of F layer is clearly established in terms of threshold curves, which are variable seasonally as well as solar activity wise (Aswathy & Manju, , ). The altitudinal variation of the threshold values of seed perturbations for ESF initiation was hypothesized to be due to the altitudinal variation of the ambient neutral density (Aswathy & Manju, , ).…”

“…The critical value of the seed perturbation amplitude at a particular level of the base height of F layer is clearly established in terms of threshold curves, which are variable seasonally as well as solar activity wise (Aswathy & Manju, , ). The altitudinal variation of the threshold values of seed perturbations for ESF initiation was hypothesized to be due to the altitudinal variation of the ambient neutral density (Aswathy & Manju, , ). The first part of the present study deals with the observational evidence for the neutral density modulation of the threshold curves for ESF initiation.…”