Impact of Stratospheric Gravity Waves and Polar Vortex on Traveling Ionospheric Disturbances

Traveling ionospheric disturbances (TIDs) are important aspect of space weather due to their impact on radio communications and energy transfer. They are ubiquitous in many types of ionospheric data during both geomagnetically disturbed and geomagnetically quiet times and are highly variable in space and time. With the goal of improved ionospheric predictability, understanding the sources of TIDs has been an active area of research for the last decade. This study summarizes observational evidence about connections between stratospheric and mesospheric gravity waves (GWs), middle atmospheric winds, and mid-latitude TIDs. We focus on the wintertime season in the Northern Hemisphere, when GW activity has a seasonal maximum, and we interpret observations during several recent winters in the context of Arctic polar vortex strength. We utilize observations of GWs at 35 km altitude by the Atmospheric InfraRed Sounder (AIRS) on NASA’s Aqua satellite to characterize stratospheric GW activity, observations of GWs at 50-55 km by the Cloud Imaging and Particle Size (CIPS) onboard the Aeronomy of Ice in the Mesosphere (AIM) satellite to describe mesospheric GW activity, and MERRA-2 horizontal winds to characterize the strength of the polar vortex. To characterize TID activity, we use GNSS TEC data generated by the MIT Haystack Observatory, and focus on geographic areas with best data availability (continental US and Europe). Our results indicate that the state of the polar vortex plays a major role in the propagation of GWs and TID activity. Most consistent results are seen during periods of weak polar vortex (sudden stratospheric warmings), when activity in both GWs and TIDs is reduced. During strong polar vortex conditions, hotspots in stratospheric GWs (~35 km) evolve with altitude and are imprinted on the ionosphere over a wide range of latitudes (25-60^oN) and longitudes. Highest correlations between GWs and TIDs are seen not directly above hotspots in stratospheric GWs, but several tens of degrees westward, suggesting that mesospheric and lower thermospheric winds play a major role in the upward propagation of GWs and the location of geographic region with amplified TID activity.