Excitation Mechanism of Ionospheric 6-Day Oscillation During the 2019 September Sudden Stratospheric Warming Event

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 wave number 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 hr. The amplitude of the 10-14 hr 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 wave number 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.