Abstract
Plasma structure in the nighttime equatorial F layer, often referred to as equatorial spread F (ESF), is not uniformly distributed, either in time or in space. Observations indicate that ESF in the bottomside F layer takes the form of patches; plasma structure within the F layer takes the form of localized plasma depletions, called equatorial plasma bubbles (EPBs), which tend to occur in clusters. Another observed feature is an upwelling, which has been described as a localized, upward modulation of isodensity contours in the bottomside F layer. Interestingly, zonal widths of ESF patches, EPB clusters, and upwellings are similar. Moreover, all display an east-west asymmetry. The objective of this paper is to show, for the first time, that an ESF patch is the bottomside counterpart of an EPB cluster, and that both are products of the electrodynamical process that takes place within an upwelling. The process can be described as having three phases: (1) amplification of upwelling amplitude during the post-sunset rise of the F layer, (2) launching of the first EPB of the evening, from crest of the upwelling, and (3) structuring of plasma within the upwelling. Hence, an upwelling, whose presence is responsible for the formation of ESF patches and EPB clusters, can be envisioned as a unit of disturbance that occurs in the nighttime equatorial ionosphere.
Highlights
Overview Plasma structure, found in the nighttime equatorial F layer, is often referred to as equatorial spread F (ESF)
In the remainder of this paper, we show how an upwelling provides the framework for the development of (1) each patch that is detected by a ground-based sensor, and (2) each cluster of equatorial plasma bubble (EPB) that is encountered by a satellite-borne instrument
Radio propagation: CW-DOPPLER results Indications that ESF consists of drifting patches of plasma structure first appeared in radio propagation experiments, which were conducted in Africa during the early 1960s
Summary
Overview Plasma structure, found in the nighttime equatorial F layer, is often referred to as equatorial spread F (ESF). The term was coined originally to describe the spreading of the F trace (in range) in ionograms (Cohen and Bowles, 1961), it is used generically to refer to both the structure and the conditions that are conducive for its development. Theories and model outputs cannot be properly evaluated without observations that are both comprehensive and accurate. In this regard, we believe that the measurements needed to answer the outstanding questions on ESF are not being made (e.g., see Tsunoda 2005)
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