Quantifying the day-to-day variability in the bottomside ionosphere using Bistatic HF observations

The purpose of this investigation is to quantify the day-to-day variability of the bottomside ionosphere to better understand the contribution that traveling ionospheric disturbances have on day-to-day variability. To quantify day-to-day variations we use nearly continuously collected bistatic HF data from Coastal Ocean Dynamics Applications Radars (CODARs). The frequency modulated continuous wave (FMCW) waveforms of these radars can be used to extract group delay measurements from which the virtual height of an ionospheric layer can also be estimated. The cadence of observations is approximately 2 minutes for this investigation. We show results from several CODAR transmitters located along the Eastern coast of the United States. All ranging in transmission frequencies from 4 - 13 MHz collected from September to December of 2020-2023. The receiver was located near Clemson University. Here we present results showing the dominant frequencies of the virtual height oscillations as a function of day of month found using the Lomb-Scargle Periodogram. We correlate the day-to-day observations of the dominant frequencies with standard metrics of geomagnetic indices and standard seasonal variation to determine if a correlation exists.