Ionospheric Signatures of Secondary Waves From Quasi‐6‐Day Wave and Tide Interactions

Abstract

The state of the ionosphere significantly changes from day to day. The ionospheric weather is controlled not only by forcing from above (e.g., solar radiation and energy deposition from the magnetosphere) but also by forcing from below, through upward-propagating waves, such as planetary waves, tides, and gravity waves (e.g., Liu, 2016). Sudden stratospheric warmings (SSWs) are extreme meteorological events that disturb the whole atmosphere (e.g., Chau et al., 2012;Pedatella et al., 2018) and thus provide opportunities to study vertical atmospheric coupling processes including their influences on the ionosphere. Most previous studies on the ionospheric response to SSWs focused on northern hemisphere events (e.g., Goncharenko et al., 2010Goncharenko et al., , 2013Oberheide et al., 2020). In the southern hemisphere, SSWs are not as frequent or as intense because of weaker wave forcing from the troposphere as the result of smaller topographical differences and land-sea contrasts. The September 2002 SSW was the only "major" warming event recorded in the southern hemisphere, according to the SSW classification developed for northern hemisphere events (Krüger et al., 2005). Identifying ionospheric effects of the September 2002 SSW was, however, difficult because of geomagnetic storms that took place around the same time (Olson et al., 2013).As recently reported by Yamazaki, Matthias, et al. (2020, hereafter Y20), there was an Antarctic SSW in September 2019 under relatively quiet geomagnetic activity conditions. Although the September 2019 SSW was a "minor" warming without the polar vortex breakdown at 10 hPa (∼32 km altitude), it involved an unprecedentedly large increase in the stratospheric polar temperature by more than 50 K per week, which is comparable with major SSWs in the northern hemisphere. Using geopotential height (GPH) data from the Aura satellite, Y20 showed that during the SSW, the quasi-6-day wave (Q6DW) was unusually strong in the mesosphere and lower thermosphere (MLT) region, with the amplitude being approximately four times as large as the seasonal climatological value. The Q6DW is a westward-propagating planetary wave with zonal wavenumber one and period around 6 days, which is occasionally observed in the middle atmosphere (e.g., Forbes & Zhang, 2017;Riggin et al., 2006). Y20 also showed ionospheric variations with a period of ∼6 days in low-latitude electric currents and plasma densities at the time of enhanced Q6DW activity during the Abstract A sudden stratospheric waring occurred in the southern hemisphere during September 2019, accompanied by an exceptionally strong quasi-6-day wave (Q6DW). We examine the ionospheric response using global total electron content (TEC) maps, with a focus on the short-period variability (5-48 h). A Fourier analysis of the TEC data reveals ionospheric variations associated with the secondary waves due to the non-linear interaction between the Q6DW and atmospheric tides. The largest signatures among them are related to the ∼29-h standing oscillation, which is att...

Understanding the Excitation of Quasi‐6‐Day Waves in Both Hemispheres During the September 2019 Antarctic SSW

“…Figure 2 and Figure S2 in Supporting Informtion indicate that the Q6DWs in the upper MLT observed in 2019 are the most prominent from 2005 to 2020 in both hemispheres. These observations are highly consistent with the results reported in previous studies (Yamazaki, 2018; Yamazaki et al., 2020; Yamazaki & Miyoshi, 2021), which indicate the Q6DWs at 0.001 hPa is the strongest in Aura/MLS observations at mid‐latitudes in both hemispheres since August 2004. Thus, the enhancement of Q6DWs in both hemispheres in the autumn of 2019 cannot be fully explained by their climatological activities.…”

“…Based on the long-term measurement of the Aura/MLS geopotential heights from 2005 to 2020, a significance test is performed on the monthly mean Q6DWs in September. As shown in Figure 2 results reported in previous studies (Yamazaki, 2018;Yamazaki et al, 2020;Yamazaki & Miyoshi, 2021), which indicate the Q6DWs at 0.001 hPa is the strongest in Aura/MLS observations at mid-latitudes in both hemispheres since August 2004. Thus, the enhancement of Q6DWs in both hemispheres in the autumn of 2019 cannot be fully explained by their climatological activities.…”

GOLD Synoptic Observations of Quasi-6-Day Wave Modulations of Post- Sunset Equatorial Ionization Anomaly During the Sudden Stratospheric Warming

“…Specifically, the PC#1 is likely linked to plasma transport by in-situ neutral winds that are modulated by upward propagating tides; those relevant tidal components interact with the Q6DW at lower altitudes. A similar coupling paradigm has been demonstrated (Gan et al, 2017(Gan et al, , 2020aForbes et al, 2018;Miyoshi and Yamazaki, 2020;Yamazaki and Miyoshi, 2021) in prior studies. A recent study by Forbes et al (2020) also suggested that the Q6DWs can be generated in the thermosphere via a two-step nonlinear wave-wave interaction.…”

Quasi‐5‐Day Oscillations During Arctic Major Sudden Stratospheric Warmings From 2005 to 2021

“…Planetary‐scale oscillation with periods of 4–7 days is known as the quasi‐5‐day oscillation (Q5DO), which plays a significant role in the disturbances of stratospheric temperatures, mesospheric winds, and ionospheric total electron contents (e.g., Gan et al., 2015; Gong et al., 2018; Huang et al., 2017; Yamazaki, 2018; Yamazaki & Miyoshi, 2021). Previous studies mainly investigated the westward propagating Q5DO at central periods of 5‐day, 6‐day, and 6.5‐day with a zonal wavenumber of 1 (W1), which were discussed as “quasi‐5‐day waves (Q5DW)”, “quasi‐6‐day waves (Q6DW)” and “6.5‐day waves (6.5DW)”, respectively (Belova et al., 2008; Day et al., 2012; Iimura et al, 2015; Lieberman et al., 2003; Merkel et al., 2003; Merzlyakov et al., 2013; Pancheva et al., 2018; Takahashi et al., 2006; Wu et al., 1994).…”