Heater-induced altitude descent of the EISCAT UHF ion line enhancements: Observations and modelling

On 12 November 2001, artificial optical annuli were produced using the EISCAT high-frequency (HF) ionospheric heating facility. This unusual phenomenon was induced using O-mode transmissions at 5.423 MHz with 550 MW effective isotropic radiated power and the pump beam dipped 9° south of the zenith. The pump frequency corresponds to the fourth electron gyroharmonic frequency at 215 km altitude. The EISCAT UHF radar observed a persistent pump-induced enhancement in the ion line backscatter power near the HF reflection altitude. The optical and radar signatures of HF pumping started at ∼230 km and descended to ∼220 km within ∼60 s. This effect has been modelled using the solution to differential equations describing pump-induced electron temperature and density perturbations. The decrease in altitude of the ion line by ∼10 km and changes in electron density have been modelled. The results show that a maximum electron temperature enhancement of up to ∼5700 K can be achieved on average, which is not sufficient to explain the observed optical emissions.