The Occurrence Variability of Severe Scintillation and Range Spread F From the Varying Nature of Large-to-Meso-Scale-Wave-Structures: Observations and Simulation

Abstract

The occurrence variability on a day-to-day basis of severe S4 scintillation and range-spread F (RSF) which are the manifestations of the most robust dynamical spread F phenomenon in the nighttime equatorial-low-latitude ionosphere, remains intriguing to date. The complex nature of large-to-meso-scale-wave-structure (LSWS) that results from the two most important determining factors, large-scale pre-reversal electric field (PREF) and Meso-Scale wave electric field, poses a severe obstacle to the short-term forecasting of S4-RSF. The present study aims to investigate the competing role of the two factors by presenting S4-RSF events that occur during the summer months of 2021–2022 over the Equatorial region of Brazil. The scintillation index (S4) and total-electron-content (TEC) from the GNSS network and ionospheric drift measurements from digisonde found more frequent occurrences of severe S4-RSF during December 2021 than in January 2022. The measurements detect LSWS in both months, though December reveals phase propagation of TEC and drift oscillations for longer horizontal distances and altitudes. The strength variability of S4-RSF is understood by conducting the numerical simulation of collisional-interchange instability. In line with the observations, the simulation shows the stronger and faster equatorial plasma bubble formations from the combined action of PREF and phase-coherent mesoscale electric field. Despite the comparatively weak PREF, the stronger S4-RSF activities highlight the role of mesoscale wave electric field in defining the strength of S4-RSF.