Measurements of F1- Region Ionosphere State Variables at Arecibo Through Quasi Height-Independent Exhaustive Fittings of the Incoherent Scatter Ion-Line Spectra

Abstract

We discuss an exhaustive search approach to fit the incoherent scatter spectrum (ISS) in the F₁-region for molecular ion fraction (f_m), ion temperature (T_i), and electron temperature (T_e). The commonly used “full profile” approach for F₁-region measurements parameterizes the molecular ion fraction as a function of altitude and fits all the related heights for the state variables. In our approach, we fit the ISS at each height for f_m, T_i, T_e, and ion velocity (V_i) independently. Our exhaustive search method finds all the major local minima at each altitude. Although a parameterized function is used to guide the algorithm in finding the best solution, the fitting parameters retain their local characteristics. Despite that fitting f_m, T_i, and T_e without constraints requires Doppler shift to be accurately determined and the ISS signal-to-noise ratio higher than the full-profile method, simulations show that T_i, T_e, and f_m can be recovered within a few percent accuracy with a moderate signal-to-noise ratio. We apply the exhaustive search approach to the Arecibo high-resolution incoherent scatter radar data taken on 13 September 2014. The derived ion and electron temperatures are sensitive enough to reveal thermosphere gravity waves commonly seen in the electron density previously. Our method is more robust than previous height-independent fitting methods. Comparison with another Arecibo program indicates our results are likely more accurate. Simultaneous high-resolution measurements of T_i, T_e, f_m, V_i, and electron concentration (N_e) in our approach open new opportunities for synergistic studies of the F₁-region dynamics and chemistry.