Estimating traveling ionospheric disturbance propagation direction and speed using bistatic HF oceanographic radars

The purpose of this investigation is to quantify the day-to-day variability of the bottomside ionosphere to understand the contribution that traveling ionospheric disturbances (TIDs) have on day-to-day ionospheric variability. To quantify the 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 - 5 MHz. The receiver was located near Clemson University. We present results showing the dominant periods of the virtual height oscillations as a function of day of month found using the Lomb-Scargle periodogram. We find a consistent ~4-5 hour period of oscillation that occurs day-to-day across most seasons for the years of 2022 and 2023. We present statistical results showing the dominant direction and speed of these propagating structures, along with the spatial extent of these structures to test whether these might be large-scale TIDs. As part of this statistical investigation, we present a case study from 06 October 2020, which was used to validate our estimations of velocity and direction of propagation. For those days, there is a clear signature of a 4 hour disturbance in virtual height that is captured at multiple midpoints corresponding to several CODAR-receiever pairs. This investigation makes it possible to understand the spatial-temporal characteristics of these oscillations at HF over regional spatial scales.