Mid-latitude traveling ionospheric disturbances under geomagnetically quiet conditions from Kharkiv and Millstone Hill incoherent scatter data

We report the characteristics of traveling ionospheric disturbances (TIDs) with periods of 10 120 min that are observed in the mid-latitude ionosphere during low level of geomagnetic activity (Ap 10, daily maximum Kp 3). The data of Kharkiv and Millstone Hill incoherent scatter (IS) radars were used to estimate such TID parameters as characterized by their predominant periods, relative amplitudes of variations in electron density, vertical and horizontal phase velocities and wavelengths. We focused on two different aspects: seasonal variations in TIDs during low solar activity and TID characteristics during sudden stratospheric warming (SSW) events. For the first study, we used Kharkiv IS data. We analyzed 59 TID events that occurred in 2006 2021 during low solar activity (F10.7 85) and close to solstices and equinoxes. In 83% of cases (49 events), we detected a substantive downward phase progression. This indicates that the detected TIDs were likely the manifestations of acoustic-gravity waves (AGWs) that propagate from lower altitudes. Both large-scale and medium-scale TIDs were revealed. The number of detected large-scale TIDs (22 events) was not much lower than the number of observed medium-scale TIDs (27 events). Our results disagree with well-established conclusions that large scale TIDs are usually observed during magnetically disturbed conditions. Our analysis demonstrates that the largest number of TIDs and the maximum values of their relative amplitudes were observed near the winter solstice. Some TID patterns like fishbone structures were detected. They can be an evidence for AGW dissipation and generation of next-order gravity waves. For the second study, we used simultaneous observations by Kharkiv and Millstone Hill IS radars to observe TID behavior at midlatitudes over Eastern Europe and Northeastern America during 2020 SSW event Our results reveal a significant day-to-day variability in TID intensity which can be caused by the evolution of polar vortex and stratospheric gravity waves.