Quantitative determination of HF radio-induced optical emission production efficiency at high latitudes

摘要

A beam-swinging experiment carried out at the High Frequency Active Auroral Research Program under relatively stable ionospheric conditions has allowed the spatial dependence of HF-induced artificial optical emissions at 630.0 nm to be determined in a quantitative manner for the first time. A forward model accounting for the beam power distribution and beam motion, effective lifetime of the emissions, and integration and sample times of the imager was used to separate the spatial dependence from the many other variables in the experiment. These results show the well-known magnetic zenith spot to account for just over half of an average maximum emission productivity of 4 R/MW and to be superimposed on a broader plateau of enhanced emission productivity. The broader distribution is centered on a point partway between the vertical and the magnetic zenith and drops off ～15° from the center of the distribution. Optical emissions from beam positions centered far from vertical are effectively suppressed and are limited to the edge of the beam overlapping the region of higher emission productivity. A simple empirical description of emission generation efficiency captures the average spatial behavior of the 630.0 nm emissions and produces realistic synthetic optical images. Ray tracing through a 4-D data-based ionosphere model showed that HF waves are typically able to reach the upper hybrid resonance over a much wider range of angles than optical emissions are observed, suggesting that the emission cutoff results from the specific excitation mechanism rather than refraction.