On mechanisms for HF pump-enhanced optical emissions at 557.7 and 630.0 nm from atomic oxygen in the high-latitude F-region ionosphere

Abstract. The EISCAT Heating facility was used to transmit powerful high frequency (HF) electromagnetic waves into the F-region ionosphere to enhance optical emissions at 557.7 and 630.0 nm from atomic oxygen. The emissions were imaged by three stations of the Auroral Large Imaging System in northern Sweden and the EISCAT UHF incoherent scatter radar was used to obtain plasma parameter values. The ratio of the 557.7 to 630.0 nm column emission rates changed from I₅₅₇₇/I₆₃₀₀ ≈ 0.2 for the HF pump frequency f₀ = 6.200 MHz ≈ 4.6 f_e to I₅₅₇₇/I₆₃₀₀ ≈ 0.5 when f₀ = 5.423 MHz 4 f_e, where f_e is the ionospheric electron gyro frequency. The observations are interpreted in terms of decreased electron heating efficiency and thereby weaker enhancement at 630.0 nm for f₀ = 5.423 MHz 4 f_e. The emissions at 557.7 nm are attributed to electron acceleration by upper hybrid waves of meter-scale wavelengths that can be excited with f₀ = 5.423 MHz 4 f_e.