We are most interested, herein, in one form of ESF, equatorial plasma bubbles (EPBs), which are initiated in the bottomside of the equatorial F layer, with upward development into the topside ionosphere.
In situ measurements consisted of spatial scans versus longitude in the bottomside of the equatorial F layer; TEC measurements, using ground-based radio receivers could be interpreted in terms of altitude-modulated isodensity contours in the bottomside F layer.
The blue dashed line represents an isodensity contour in the bottomside of the F layer (the sketch is centered on the end of the PSSR).
The LSWS-scale zonal E should be associated a divergent, eastward Pedersen-current in the bottomside of the equatorial F layer (see "Strength of 'seed' plasma perturbations" section).
The PRE E, which is responsible for the PSSR, also drives an eastward Pedersen current in the bottomside of the equatorial F layer, where ∇N is directed upward.
In addition to the pre-reversal enhancement (PRE) of dayside E, which is responsible for PSSR, we consider a role played by a polarization E, which accompanies an upwelling in the bottomside of equatorial F layer.
Our knowledge and understanding of ESF have evolved from views of structure in the bottomside of the F layer with ground-based sensors, and from measurements made in situ within the F layer with space-borne instruments.
An upwelling is defined as a localized, upward displacement of isodensity contours in the bottomside of the F layer, as shown in Figure 1 (an upwelling can appear in apparent isolation, at some longitude, or several can appear as a wave train; the latter is referred to as large-scale wave structure (LSWS)).
The second dark band would be the result of a secondary plume that is known to develop in the western wall of the initial bottomside upwellings caused by a wind-driven gradient drift instability.
Besides a dependence on PSSR strength, other possible factors include (1) a possible D2D variability in seeding, and (2) a variability in whether an upwelling has been imprinted onto the bottomside of the equatorial F layer by the time of SS F. An EPB would likely be launched at a longitude where SS F intercepts an upwelling.
