Two‐dimensional turbulence in equatorial spread F

Abstract

The convective instability of the equatorial ionosphere is known to result in the upwelling of low‐density regions into the topside. We show here that such rising ‘bubbles’ generate a wake consisting of vortices in the background fluid with an initial scale size comparable to the bubble dimension Ls. Since there is a background plasma density gradient, density irregularities will also be created at this scale. The theory of turbulence in two‐dimensional fluids is then applied to predict that velocity and density structures will subsequently develop in the background fluid at both larger and smaller wavelengths. We suggest that this extension to longer wavelengths tends to fill in the region between bubbles with velocity and density structures and contributes to the great horizontal scale over which equatorial spread F is observed, while the wakes themselves account for the extended vertical scale. We also predict that a break in the velocity turbulence spectrum should occur near the stirring length Ls. Under some conditions a well‐defined ‘inertial subrange’ may exist where the velocity wave number spectrum should vary as k−3, and the density spectrum should vary as k−1 in this same range.