Evaluating day-to-day bottomside ionospheric variability using bistatic HF observations

The purpose of this investigation is to quantify day-to-day variability of the bottomside ionosphere using bistatic HF observations. The International Reference Ionosphere (IRI) is one of the most dominant ionospheric models that produces a reasonable climatology, however, it does not reproduce day-to-day variability which can be very significant. To understand day-to-day variations we use nearly continuously collected bistatic HF data of Coastal Ocean Dynamics Applications Radars (CODARs). The frequency modulated continuous wave (FMCW) waveforms of these radars can be used to extract group delay measurements using standard signal processing, and the virtual height of an ionospheric layer can also be estimated. The cadence of observations is approximately 1 minute for this investigation. Here, we show results from three CODAR transmitters located along the east coast of the US during the Fall of 2020 at a transmission frequency of 4.537 MHz. The receiver was located near Clemson University. As a baseline, we present results of day-to-day variability by ray tracing from the CODAR transmitters to the receiver using the IRI as the background model. For the ray tracing, we use the PHaRLAP or the Provision of High-Frequency Raytracing Laboratory for Propagation ray tracing software package. From this synthetic data, we generate the sample average and standard deviation, as an indication of the variance. We use the CODAR observations along similar links to also quantify the sample mean and variance for this month of observations. We discuss the differences that are observed between the model results versus the observations.