Study and mitigation of temperature-composition ambiguity in the inversion of Arecibo ion-line spectra measured in CLP experiments

In the inversion of backscattered incoherent scatter radar (ISR) Doppler spectra we encounter a well-known ion temperature-composition ambiguity problem, i.e., different combinations of ion temperatures and compositions can result in similar spectral shapes, which significantly degrades the inversion performance, especially during the low SNR situations. We present a study of the inversion of the ion-line spectra measured at Arecibo using the CLP (coded long pulse) mode with a focus on how this ambiguity problem can be mitigated. We conducted a ``landscaping’’ study in the multi-dimensional space of the ionospheric parameters describing the misfit between the measured and modeled spectra, with examples taken from daytime and nighttime measurements to find multiple local minima, of which only one can be identified as the physical solution using suitable prior information. A new inversion program using the Levenberg-Marquardt algorithm was developed to filter out the non-physical solutions based on our observations derived from our landscaping study. The algorithm is applied to the Sept 23-26, 2016 CLP experiment dataset on 200 - 600 km altitude range from Arecibo observatory, and the inversion results are compared with the ones from the ULP experiment during the same time period, and the Madrigal regional model. The method for removing the pedestal from CLP ion-line spectra and comparisons with results obtained with a swarm optimization approach are also presented.