Observational Verification of High-Order Solar Tidal Harmonics in the Earth's Atmosphere

This study combines 8 years of middle atmospheric wind data observed at 52(degrees)N latitude from two radars in different longitudinal sectors to investigate solar tides. The power spectral density of horizontal winds exhibits a -3 power law within the frequency range 2.0 < f < 7.0 cpd (equivalent to periods 3.6 - 12.0 hr). Particularly noteworthy are the 4.8- and 4-hr tides, exhibiting signal-to-noise ratios ranging between 13 and 16 dB, surpassing the 0.01 significance level. This challenges their previous oversight in literature, possibly due to inadequacies in prevailing noise models. Cross-spectra between longitudinal sectors emphasize the dominance of sun-synchronous components in the six lowest-frequency tides. Composite spectra indicate that tidal enhancements during SSWs resemble regular seasonal variations. Intriguingly, year-to-year spectral variations suggest that these enhancements are more influenced by seasonal dynamics than by SSW, contrasting with established literature. These findings underscore the need to reevaluate tidal harmonics and consider appropriate noise models in future studies. Plain Language Summary Tides are ubiquitous in celestial systems, influencing celestial objects diversely when one orbits another. Extensive studies have explored the tidal effects on processes such as planetary habitability, climate fluctuations, meteorological patterns, geophysical activities, geological hazards, heat and mass circulation, and certain biological behaviors. However, most existing literature focuses on the lowest-frequency tidal harmonics, with limited attention given to higher-frequency ones. In the Earth's atmosphere, the exact count of solar tidal harmonics remains uncertain, and an ongoing debate persists regarding the existence of higher-frequency harmonics, arising potentially from difficulties in distinguishing them from sporadic regional buoyancy waves. Here, we provide evidence for the statistically significant existence of the first six orders of tidal harmonics, extracted from 8 years of middle atmospheric wind observations. Spectral coherence between two distinct longitudinal sectors signifies that the six harmonics primarily correspond to sun-synchronous tides synchronized with the Sun. The presence of higher-frequency tides suggests that tidal effects are characterized by greater complexity than currently understood.