| Summary: | This study examines the role of traveling planetary waves in producing day-to-day variability of the ionosphere. Traveling planetary waves in the middle atmosphere, especially those having periods around 6, 10, 16 days, are generally regarded as the manifestation of atmospheric normal modes. A sudden stratospheric warming (SSW) is sometimes accompanied by enhanced traveling planetary waves, which can be understood in terms of the dependence of normal modes on the zonal mean state of the atmosphere. The September 2019 Antarctic SSW is one example. Following the SSW, a large quasi-6-day wave (Q6DW) is observed in the mesosphere and lower thermosphere. During the same time, Swarm satellites in near-polar orbits observe westward-propagating 6-day perturbations with zonal wavenumber 1 in ionospheric parameters such as the electron density and equatorial electrojet intensity. The whole atmosphere model GAIA is able to reproduce these ionospheric variations under constant geomagnetic and solar activity conditions. Using GAIA, it is demonstrated that tides modulated by the Q6DW are responsible for the 6-day ionospheric variations during the September 2019 SSW. Planetary wave modulations of tides and their ionospheric effects are also discussed for SSWs in other seasons.
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