Influences of sudden stratospheric warmings on the ionosphere above Okinawa

We analyzed the ionosonde observations from Okinawa (26.7 degrees N, 128.1 degrees E; magnetic latitude: 17.0 degrees N) for the years from 1972 to 2023. Okinawa is in the northern low-latitude ionosphere, where the influences of sudden stratospheric warmings (SSWs) on the ionosphere are expected to be stronger than in the mid- and high-latitude ionospheres. We divided the dataset into winters with major SSWs in the Northern Hemisphere (SSW years) and winters without major SSWs (no-SSW years). During the SSW years, the daily cycle of the F2-region electron density maximum (NmF2) is stronger than in the no-SSW years. The relative NmF2 amplitudes of solar and lunar tidal components (S 2 , O 1 , M 2 , MK 3 ) are stronger by 3 % to 8 % in the SSW years than in the no-SSW years. The semidiurnal amplitude, averaged across 29 SSW events, has a significant peak at the central date of the SSW (epoch time 0 of the composite analysis). The SSW influence is not strong: the semidiurnal amplitude is about 38.2 % in the SSW years and about 34.0 % in the no-SSW years (relative to the NmF2 of the background ionosphere). However, there is a sharp decrease in the amplitude of about 10 % after the SSW peak is reached. The amplitude of the diurnal component does not show a single peak at the central date of the SSW. We present the maximal semidiurnal amplitudes of the SSWs since 1972. The SSW of 31 December 1984 has the strongest amplitude (162 %) in the ionosphere above Okinawa (with a high geomagnetic activity, Ap, of 37 nT). The most surprising finding of the study is the strong lunar tides with relative amplitudes of about 10 % and the discovery of a terdiurnal lunar tide (5 %) in the NmF2 during the SSW years. The periods of the ionospheric lunar tides align with the periods of ocean tides and lunisolar variations in the atmosphere.