Connections Between Gravity Waves, the Polar Vortex and Traveling Ionospheric Disturbances

We present evidence that, during the winter season, hotspots in stratospheric and mesospheric gravity waves (GWs) seen over Europe are strongly correlated with daytime Medium Scale Traveling Ionospheric Disturbances (MSTIDs) above the US. The roles of the stratospheric and mesospheric polar vortex, the average zonal wind component and total wind vectors in the propagation of these GWs are also examined. Travelling Ionospheric disturbances (TIDs) are perturbations in the background plasma density and are ubiquitous in many types of ionospheric data during both geomagnetically disturbed and geomagnetically quiet times. They are highly variable in space and time, and are a source of uncertainty in high frequency radio systems and Global Navigation Satellite Systems (GNSS). Medium scale TIDs, or MSTIDs can significantly affect HF communication, and cause disruptions to GNSS signals that can make geolocation difficult. Therefore, understanding them and being able to predict their occurrence is important not only from a scientific but also from a technological perspective. We use data from the Atmospheric Infrared Sounder (AIRS) on board the Aqua satellite and Cloud Imaging and Particle Size (CIPS) instrument on board the AIM satellite to locate stratospheric and mesospheric GW hotspots over Europe (at an altitude of ? 35 km and 50 - 55 km respectively) during Arctic winters of 2019 - 2020 and 2020 - 2021. We show that these GW hotspots are strongly correlated with TIDs over a wide range of latitudes (25 - 60? N) over the central US. The state of the polar vortex is shown to play a major role in the propagation of GWs. A strong polar vortex dominates throughout the entire winter of 2019 - 2020, whereas a weak vortex develops during the winter of 2020 - 2021 due to a Sudden Stratospheric Warming (SSW). We show that the effect of GW hotspots over Europe on MSTIDs over the mid-latitude US is significant only when the polar vortex is strong for a period longer than that observed during years with a Sudden Stratospheric Warming (SSW). From our observations, this implies a strong vortex lasting longer than a period of 30 days. In addition, the mesospheric wind system is shown to correlate highly with the amplitude of MSTIDs over mid-latitude North America and is theorized to modulate the propagation of GWs to higher altitudes.