A sudden stratospheric warming (SSW) event in the Antarctic region occurred in September 2019. During the SSW event, the quasi 6–day wave (Q6DW) was enhanced in the mesosphere, and strong 6–day oscillations are observed in ionospheric parameters, such as the equatorial electrojet (EEJ) and electron density. The 6–day variation in the EEJ has a westward–moving structure with the zonal wavenumber 1, indicating the influence of the Q6DW. In this study, we investigate the excitation mechanism of the 6–day variations in the EEJ and electron density using numerical simulations. The main results are as follows. The 6–day variations in the ionosphere are not generated by the Q6DW, but generated by the waves with periods from 10 to 14 hours. The amplitude of the 10–14 hour waves is modulated with a period of 6 days, due to the nonlinear interaction between the Q6DW and migrating semidiurnal tide. This leads to the 6–day variations in the EEJ and electron density through the E–region dynamo process. At a fixed local time, the secondary waves generated by the Q6DW-tidal interaction produce westward-moving ionospheric 6-day variations with zonal wavenumber 1, which cannot be distinguished from the ionospheric variations by the Q6DW itself. The interference of secondary waves leads to a longitudinal asymmetry in the magnitude of the ionospheric 6–day oscillation.

An exceptionally strong stationary planetary wave with Zonal Wavenumber 1 led to a sudden stratospheric warming (SSW) in the Southern Hemisphere in September 2019. Ionospheric data from ESA’s Swarm satellite constellation mission reveal prominent 6-day variations in the dayside low-latitude region at this time, which can be attributed to forcing from the middle atmosphere by the Rossby normal mode “quasi-6-day wave” (Q6DW). Geopotential height measurements by the Microwave Limb Sounder aboard NASA’s Aura satellite show a burst of Q6DW activity in the mesosphere and lower thermosphere during the SSW, which is one of the strongest in the record. The Q6DW is apparently generated in the polar stratosphere at 30-40 km, where the atmosphere is unstable due to strong vertical wind shear connected with planetary-wave breaking. These results suggest that an Antarctic SSW can lead to ionospheric variability through wave forcing from the middle atmosphere.