Evidence is given for intense VHF and UHF radar backscatter during equatorial Spread F resulting from irregularities of 1 meter and 36 cm, respectively. The linear theory for high frequency (ω ≳ Ωi, where Ωi is the ion gyrofrequency) drift waves, generated by the drift‐cyclotron and lower‐hybrid‐drift instabilities, is presented. This linear theory is set forth as a possible explanation for the occurrence of these irregularities below the ion gyroradius. The maximum growth for these instabilities occurs for kre ∼ 1, where k is the wavenumber perpendicular to the magnetic field and re is the electron gyroradius. For these instabilities, the growth rate is γ ≳ (me/mi)¼ Ωi and results in growth times less than a second. For typical equatorial Spread F ionospheric parameters, where (νii/Ωi) (kri)² ≳ 1 (νii is the ion‐ion collision frequency, rI is the ion gyroradius), the lower‐hybrid‐drift instability is dominant.
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Huba et al. (1978) studied this question.