Challenges of predicting low-latitude ionospheric scintillations

Abstract

Two regions of the globe that are most affected by ionospheric scintillations are the auroral region and the low-latitude region encompassing the dip equator. Our present-day dependence on satellite-based communication and navigation has led to a resurgence of interest in the prediction of ionospheric scintillations, which degrade the performance of such systems. In the low-latitude ionosphere, the genesis of intermediate-scale (∼100 m to a few km) irregularities, which cause the most severe scintillations on VHF to L-band trans-ionospheric radio signals, is in the growth of the Rayleigh-Taylor (R-T) plasma instability on the bottom-side of the post-sunset equatorial F layer. Theoretical developments and observations by various ground-based instruments, as well as in-situ measurements by instruments onboard rockets and satellites, have identified several parameters of the ambient ionosphere that play key roles in the occurrence of these irregularities over the dip equator. However, the presence of basic conditions for the linear growth of the Rayleigh-Taylor instability fails to explain the large day-to-day variability in the characteristics of these irregularities, which determines the latitudinal distribution and strength of scintillations, and is a result of the nonlinear evolution of the Rayleigh-Taylor instability. In this paper, the present status of the efforts at prediction of low-latitude scintillations on the basis of a multitude of observations and numerical simulation of the development of the Rayleigh-Taylor instability in the equatorial ionosphere are reviewed. In particular, this review describes how observations of ionospheric scintillations may be used to obtain information about the nonlinear evolution of the Rayleigh-Taylor instability under different ambient conditions, which is yet to be explored through numerical simulation of the phenomenon. Such studies have pointed out important gaps in our knowledge about the evolution of equatorial ionospheric irregularities that are responsible for producing scintillations on L-band signals recorded in the equatorial and low-latitude regions around the globe.