Improved modeling of midlatitude D-region ionospheric absorption of high frequency radio signals during solar X-ray flares

High frequency (HF) radio communication is widely used for real-time, medium to long range communications due to its low cost of operation and maintenance. However, HF communication is strongly dependent on the state of the ionosphere, which is sensitive to solar X-ray flares. The lowest region of the ionosphere, the D-region, is the region in which the majority of the absorption of HF radio wave energy occurs. D-region HF absorption depends on the local electron density, which is enhanced during a solar X-ray flare. HF propagation data obtained during the HF Investigation of D-region Ionospheric Variation Experiment (HIDIVE) and obtained at the Canadian Space Agency NORSTAR riometer in Pinawa, Manitoba, Canada and X-ray flux data, as reported by GOES satellites, are analyzed here for the purpose of validating and improving the performance of two HF absorption models, the operational Space Weather Prediction Center (SWPC) D-region Absorption model and the physical AbbyNormal model. The SWPC D-region absorption model is an empirical model providing real-time global predictions of D-region absorption, and the physical Absorption by the D and E Region of HF Signals with Normal Incidence (AbbyNormal) model is based on simple D-region chemistry and provides near real-time predictions of midlatitude D-region HF absorption. Analysis of the HIDIVE data revealed an absorption dependence on signal frequency of f-1.24 where f is signal frequency, and a Cos 0.9(chi) dependence on solar zenith angle, chi. These relations differ from what is used in the SWPC model, and from these relations, a new empirical model, the Empirical HIDIVE Absorption (EHA) model, is developed. The EHA model can be used to improve the SWPC model performance. NO density data obtained with the Student Nitric Oxide Explorer (SNOE) and during the Halogen Occultation Experiment (HALOE) are used to improve the method by which the AbbyNormal model defines the nitric oxide (NO) profile within the atmosphere. Improved NO profiles allows for better AbbyNormal characterization of the ionosphere and HF propagation and for better prediction of solar flare-induced HF absorption. This research is sponsored by the Air Force Office of Scientific Research and the Air Force Weather Agency.