Spread-F theories—a review

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Abstract

Our understanding of equatorial spread-F (ESF) phenomena has progressed significantly in the past several years. ESF phenomena involves ionospheric irregularities spanning some 5–6 orders of magnitude in scale size. The largest scale sizes, on the order of many kilometers, are thought to be caused by a plasma fluid type Rayleigh-Taylor instability mechanism on the bottomside of the night-time equatorial-F region. Plasma density bubbles (depletions) are formed on the bottomside by these ionospheric irregularities and then they rise nonlinearly to the topside, by E × B polarization motion, causing irregularities above the F-peak. The power spectral density (PSD) of these long wavelength plasma density fluctuations is proportional to k−2, where k is perpendicular to the geomagnetic field and equal to λ. Current theories of the very small scale, ≲ 10 m, radar backscatter observed irregularities (specifically at 3 m, 1 m, 36 cm, and 11 cm), during ESF, indicate that these irregularities are due to various kinetic type plasma drift wave instabilities. These instabilities are formed by a two step process in which they are driven by the steep plasma density gradients created by the primary long wavelength Rayleigh-Taylor type plasma fluid instabilities. Recent theoretical and numerical simulation studies supporting these ideas on ESF phenomena will be presented.

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