Seeding of equatorial plasma bubbles with electric fields from an E s ‐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 E s 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).