Influences of sudden stratospheric warmings on the ionosphere above Okinawa

Abstract

Abstract. We analyzed the ionosonde observations from Okinawa (26.7° N, 128.1° E; magnetic latitude: 17.0° 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 (S2, O1, M2, MK3) 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.

Quasi‐Diurnal Lunar Tide O1 in Ionospheric Total Electron Content at Solar Minimum

“…It seems that the relative O 1 amplitude in the TEC map is smaller than 10% at the place of Okinawa (Figure 7). In agreement with the present study, Hocke et al (2024) found that O 1 was enhanced from December to March. They showed that the maximal O 1 amplitudes occurred just after the onset of major SSWs.…”

“…The same period of the O 1 component was also found in a spectral analysis of a long-term time series of ground magnetometer data in Honolulu (Love & Rigler, 2014). In addition, the same period has been observed in ionospheric F 2 region electron density maximum NmF2 data at Okinawa (Hocke et al, 2024). The period of O 1 is different from a lunar day (24.84 hr) since the gravitational potential depends on both, the Moon and the Sun.…”

“…In so far, we are sure that the observed O 1 variation in GNSS TEC of the present study is not an aliasing effect. The O 1 variation was also observed by an ionosonde in Okinawa which does not depend on satellite orbits (Hocke et al, 2024).…”

“…Thus, we focus on the discussion of the behavior of the relative O 1 amplitude in the following. The present study was motivated by the findings of Hocke et al (2024) who found strong lunar tides O 1 and M 2 in peak electron density data NmF2 above Okinawa with relative amplitudes of about 10%. It seems that the relative O 1 amplitude in the TEC map is smaller than 10% at the place of Okinawa (Figure 7).…”

“…The lunar tidal potential of the O 1 tide is about 41.5% of those of the semidiurnal M 2 tide (Tarpley, 1971). An indication of a strong ionospheric O 1 tide was given by Hocke et al (2024) who found that the relative amplitude of O 1 in NmF2 is up to 12% for O 1 in winter above Okinawa. This was slightly higher than for the coincident observation of the semidiurnal lunar M 2 tide which is much better investigated than the O 1 tide in the ionosphere.…”