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

We discuss an exhaustive search approach to fit the incoherent scatter spectrum (ISS) in the F1‐region for molecular ion fraction (fm), ion temperature (Ti), and electron temperature (Te). 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 fm, Ti, Te, and ion velocity (Vi) 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 fm, Ti, and Te 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 Ti, Te, and fm 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 Ti, Te, fm, Vi, and electron concentration (Ne) in our approach open new opportunities for synergistic studies of the F1‐region dynamics and chemistry. Plain Language Summary: Incoherent scatter spectra (ISS) can be used to derive a range of ionosphere state variables. Fitting the ISS for ion composition, and ion and electron temperatures is challenging in the ionosphere F1 region because the cost function has only minute differences for multiple solutions. A prevalent approach for F1 region spectral fitting is to assume a parameterized composition profile and fit all heights simultaneously. While such a "full profile" approach allows a general determination of the molecular ion profile, localized variations are smoothed out. In this study, we discuss an exhaustive search approach to derive the state variables that are largely height‐independent. The approach is applied to the Arecibo incoherent scatter radar data taken on 13 September 2014. The derived ion and electron temperatures are sensitive enough to reveal thermospheric gravity waves that are commonly seen in the electron density previously. We discuss the accuracy of our results in the context of previous measurements and other methods. The accuracy and sensitivity of simultaneous measurements of multiple state variables in our approach open new opportunities for synergistic studies related to the F1‐region ionosphere. Key Points: A new exhaustive search method is presented to derive molecular ion fraction and other state variables from incoherent scatter spectraThe method can measure electron and ion temperatures with a few percent accuracy for a moderate signal‐to‐noise ratioApplication to Arecibo data reveals gravity wave structures in electron and ion temperatures, ion velocity, and electron density [ABSTRACT FROM AUTHOR]