Nonlinear Rayleigh‐Taylor instabilities, atmospheric gravity waves and equatorial spread F

Abstract

Although it is generally accepted that equatorial spread F (ESF) is due to nonlinear evolution of the Rayleigh‐Taylor instability, a number of important properties of the process remain unexplained. In particular, we investigate two as yet unexplained features of ESF: the common dominance of very large scale features (≥20 km) and their large amplitude. Although associated for years with spread F we show here for the first time that atmospheric gravity waves can initiate the Rayleigh‐Taylor instability. In agreement with other analytical theories and computer simulations we find that the initiated instability will be saturated by nonlinear coupling of unstable modes to damped modes if the amplitude of the seed gravity wave is small. However, if the amplitude of the seed gravity wave is large enough, the relative plasma density perturbations can reach 50% or more, implying essentially no saturation. The required initial amplitudes are not unreasonable. In the latter case, significant enhancements and depletions of the plasma density occur within several hundred seconds. The analysis presented here demonstrates the possibility that large‐scale spread F is triggered by gravity waves with the Rayleigh‐Taylor instability as a source of amplification. In a separate calculation we also show that large‐amplitude plasma perturbations can be produced by an explosive instability of the Rayleigh‐Taylor modes. It is found that the condition for explosive growth of the Rayleigh‐Taylor modes can be satisfied in the ionospheric F region. We propose that the explosive mode coupling of the Rayleigh‐Taylor instability is a possible mechanism for production of large‐amplitude bottomside sinusoidal structures.

Characteristics of mesospheric gravity waves near the magnetic equator, Brazil, during the SpreadFEx campaign

“…The resultant instabilities grow via the Rayleigh-Taylor mechanism which creates bubbles (or plumes) of plasma which can attain altitudes as high as 1500 km at the equator within a few hours (e.g. Woodman and La Hoz, 1976;Huang et al, 1993). The bubbles are readily observed in radar plots and appear as towering plumes extending from the lower ionosphere to the top side (e.g.…”

A numerical simulation study of the collisional‐interchange instability seeded by the pre‐reversal vertical drift

“…These plumes are identified as large-scale depletions or equatorial plasma bubbles (EPB) [Sobral et al, 1980] and are believed to be generated by CII mechanism [Sultan, 1996]. Numerous theoretical and numerical studies have been performed to assess the linear and nonlinear aspects of these complex processes [Ossakow et al, 1979;Zargham and Seyler, 1987;Raghavarao et al, 1992;Huang et al, 1993;Keskinen et al, 1998]. These studies revealed the generation of the EPB initiated by a seed perturbation at the bottomside of the rising F layer.…”

Latitudinal and temporal variation of equatorial ionospheric irregularities determined from GPS scintillation observations

“…Extensive observations of equatorial plasma bubbles by radar (e.g. Kelley et al, 1981;Hysell et al, 1990;Kelley and Hysell, 1991;Huang et al, 1993), satellite borne ion drift sensors (e.g. Singh et al, 1997), and optical observations of F region night-glow emissions (e.g.…”