High frequency drift waves with wavelengths below the ion gyroradius in equatorial spread F

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 ≳ Ω i , where Ω i 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 kr e ∼ 1, where k is the wavenumber perpendicular to the magnetic field and r e is the electron gyroradius. For these instabilities, the growth rate is γ ≳ (m e /m i ) ¼ Ω i and results in growth times less than a second. For typical equatorial Spread F ionospheric parameters, where (ν ii /Ω i ) (kr i )² ≳ 1 (ν ii is the ion‐ion collision frequency, r I is the ion gyroradius), the lower‐hybrid‐drift instability is dominant.