Seeding of equatorial plasma bubbles with electric fields from an Es-layer instability

Evidence is presented that supports the suggestion by Tsunoda (2006) that a polarization electric field, if generated by a sporadic-E (E-s) layer instability (Cosgrove and Tsunoda, 2002), should map to the base of the F layer and seed equatorial plasma bubbles. Seeding, which leads to bubble development, seems to be a four-step process operating in the bottomside F layer: (1) amplification of seed perturbations in a region of westward-drifting plasma, which is found below the velocity-shear node; (2) upward transference of this modulation through a rotational velocity shear to eastward-drifting plasma; (3) further seed amplification, when the F layer begins to descend; and (4) bubble growth from the seed via the Rayleigh-Taylor instability. The time available for interaction between F-region plasma and elongated channels of enhanced polarization electric field (the latter formed by the mentioned instability) appears to be crucial for seed amplification. The need for a descending F layer to increase interaction time with the eastward-drifting plasma is appealing because plasma bubbles display a propensity to appear after, and not during, the postsunset rise of the F layer. Although plausible, the need for Es presence, perhaps low solar activity, and a multistep process suggest that this mechanism may be a more occasional contributor than the collisional-shear instability (Hysell and Kudeki, 2004).