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

Abstract

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 (F10.7) 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.

Hindcasting of Equatorial Spread F Using Seasonal Empirical Models

“…In the case of ss, it varies by a factor of 1.6, from 0.09 to 0.15. The figure similar to Figure was reported earlier in Aswathy and Manju () for ae. In the case of ss, it varies by a factor of 1.6, from 0.09 to 0.15 .…”

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.…”