Abstract
Equatorial spread F (ESF) irregularities are caused by plasma instability processes in the equatorial ionosphere and especially related to plasma bubble phenomena in the topside F region. Plasma bubbles result from nonlinear evolution of the Rayleigh-Taylor instability. The prereversal enhancement (PRE) of the postsunset vertical plasma drift increases the height of the equatorial F layer, creating conditions conducive to the growth of the Rayleigh-Taylor instability, and is an important factor that controls the generation of ESF. Three different relationships of the quantitative connection between the PRE and ESF occurrence have been derived. (1) A threshold PRE must be reached for the occurrence of ESF. ESF will be generated when the PRE is larger than the threshold and will not be generated when the PRE is smaller than the threshold. (2) The occurrence probability of ESF increases approximately linearly with the PRE. (3) The occurrence probability of ESF is characterized by a continuous probability distribution as a function of the PRE magnitude. The second and third relationships imply that the PRE can be used to specify the occurrence probability of ESF. This paper will review these relationships and discuss how these relationships are connected to each other. The effects of seeding perturbations on the generation and global distribution of ESF will be briefly discussed.
Highlights
The generation of ESF Equatorial spread F (ESF) stems from observations of the “spread” of the traces in equatorial ionograms and is used to describe plasma instability phenomena that occur in the equatorial F region ionosphere (Kelley 1989)
The prereversal enhancement (PRE) of the postsunset vertical plasma drift is an important factor that controls the generation of ESF
A lot of efforts have been made to determine the quantitative dependence of ESF occurrence on the PRE
Summary
The generation of ESF Equatorial spread F (ESF) stems from observations of the “spread” of the traces in equatorial ionograms and is used to describe plasma instability phenomena that occur in the equatorial F region ionosphere (Kelley 1989). Plasma bubbles result from nonlinear evolution of the RayleighTaylor instability that is excited in the bottom side of the equatorial F region. During the process of nonlinear growth of the instability, high-plasma density magnetic flux tubes at the bottomside of the ionospheric F region change places with lower density flux tubes from below, and the depleted magnetic fluxes rise to the topside, forming plasma bubbles (Woodman and La Hoz 1976; Scannapieco and Ossakow 1976). Plasma density irregularities associated with bubbles (ESF irregularities) exist in both the bottomside and topside F regions. The linear growth rate of the Rayleigh-Taylor instability is inversely proportional to the ion-neutral collision frequency. The growth rate of the Rayleigh-Taylor instability becomes larger at higher altitudes because of smaller ion-neutral collision frequency there.
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