Global simulations of multi-frequency HF signal absorption for direct observation of middle atmosphere temperature and composition

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This paper presents the first numerical study on a new concept for the direct measurement of D-region absorption in the HF band. Numerical simulations based on the Appleton–Hartree and Garrett equations of refractive index are presented. Electron temperature as a result of HF radio pumping of the ionosphere is included in the calculations using proper numerical formulation. Both O- and X-mode radio wave polarizations are taken into consideration. A global map of HF absorption in the northern hemisphere is calculated. Detailed calculations of HF radio wave absorption as it propagates through the lower atmosphere are presented. The effect of several parameters on the amount of absorption is calculated. The best frequencies to be used for the purpose of this study are discussed. A machine learning model is developed and the capability of the model in estimation of D and E-region constituents includes $N_2$, $O$, $O_2$, as well as $T$ and $N_e$ is examined. Such a technique can also lead to global mapping of HF absorption and improve OTHR (over-the-horizon-radar) performance.