On the energy distribution of suprathermal electrons produced by HF heating in the F2 region

Using the High Frequency Active Auroral Research Program (HAARP) imager, measurements of intensities at 630.0 and 557.7 nm were carried out during HF heating in the F region in February 2002 (Pedersen et al. Geophysical Research Letters 30(4) (2003) 1169). Analysis of the red and green line emission observations confirms that both emissions can be observed only when suprathermal electrons are produced during HF heating. Using an artificial airglow model based on a Maxwellian energy distribution of suprathermal electrons, it is possible to determine the minimum ratio of red to green line intensity as a function of altitude. The model also facilitates estimating the effective temperatures and fluxes of fast electrons that excite artificial airglow. However, the associated energy flux is so large that the heating of background thermal electrons becomes very significant. Using this model, it is impossible to obtain agreement between the calculated and measured intensities of the green and red lines because heated thermal electrons additionally excite the O(1) state while O(1S) excitation is negligible. If the suprathermal electron distribution is fitted by a power law instead, it is possible to decrease this disagreement. In this case, excitation of the 630.0 nm emission is provided by suprathermal electrons together with background thermal electrons heated by suprathermal electrons.