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Most experimental investigations on planetary-scale waves in the mesosphere and lower thermosphere (MLT) region are based on single-station or -satellite spectral analysis methods, which suffer from intrinsic spectral aliasing/ambiguity. To overcome the aliasing, the author has developed and utilized dual- and multi-station spectral methods in a series of recent works. These methods were implemented on meteor radar observations and surface magnetometer observations. In the implements, a variety of waves were discovered or investigated in terms of seasonal variations and responses to sudden stratospheric warming events, such as lunar and solar tides (migrating and non-migrating), Rossby wave normal modes, ultra-fast Kelvin waves, and secondary waves of wave–wave nonlinear interactions between the previous waves. The current paper illustrates these methods using synthetic data, comparatively reviews the methods and results in plain language, and proposes a new representation, termed the adjusted Feynman diagram, to summarize the nonlinear interactions and explain their implications.

China is a key region for understanding fire activity and the drivers of its variability under strict fire suppression policies. Here, we present a detailed fire occurrence dataset for China, the Wildfire Atlas of China (WFAC; 2005–2018), based on continuous monitoring from multiple satellites and calibrated against field observations. We find that wildfires across China mostly occur in the winter season from January to April and those fire occurrences generally show a decreasing trend after reaching a peak in 2007. Most wildfires (84%) occur in subtropical China, with two distinct clusters in its southwestern and southeastern parts. In southeastern China, wildfires are mainly promoted by low precipitation and high diurnal temperature ranges, the combination of which dries out plant tissue and fuel. In southwestern China, wildfires are mainly promoted by warm conditions that enhance evaporation from litter and dormant plant tissues. We further find a fire occurrence dipole between southwestern and southeastern China that is modulated by the El Niño-Southern Oscillation (ENSO). Fire activity in China and its associations with climate are not well quantified at a local scale. Here, the authors present a detailed fire occurrence dataset for China and find a dipole fire pattern between southwestern and southeastern China that is modulated by the El Niño-Southern Oscillation (ENSO).

Second harmonic generation is the lowest-order wave-wave nonlinear interaction occurring in, e.g., optical, radio, and magnetohydrodynamic systems. As a prototype behavior of waves, second harmonic generation is used broadly, e.g., for doubling Laser frequency. Second harmonic generation of Rossby waves has long been believed to be a mechanism of high-frequency Rossby wave generation via cascade from low-frequency waves. Here, we report the observation of a Rossby wave second harmonic generation event in the atmosphere. We diagnose signatures of two transient waves at periods of 16 and 8 days in the terrestrial middle atmosphere, using meteor-radar wind observations over the European and Asian sectors during winter 2018–2019. Their temporal evolution, frequency and wavenumber relations, and phase couplings revealed by bicoherence and biphase analyses demonstrate that the 16-day signature is an atmospheric manifestation of a Rossby wave normal mode, and its second harmonic generation gives rise to the 8-day signature. Our finding confirms the theoretically-anticipated Rossby wave nonlinearity. Rossby waves occur in rotating fluids. Here, the authors show observation of a Rossby wave second harmonic generation event in the middle atmosphere and confirm theoretically anticipated Rossby wave nonlinearity.

Understanding the interdecadal periodicity of large-scale seismic activities is critical for improving long-term earthquake forecasting, yet it remains constrained by the limited duration of instrumental records. Here we apply high-resolution spectral analysis to a millennia-long catalog of historical earthquakes in China, extending to 1831 BC. The most pronounced seismic activities occurred during the 1620–1630 s AD, a period that coincided with a major regime shift that preceded the collapse of the Chinese Ming Dynasty. High-resolution spectral analysis was performed on the historical dataset to identify robust ~ 10- and ~ 50-year periodicities in seismic frequency. The ~ 10-year periodicity exhibits a significant lagged correlation with solar (sunspot) activity, while the ~ 50-year periodicity aligns with multidecadal variability in solar irradiance, sea level fluctuations, and tropical sea surface temperatures (SSTs). We propose that enhanced solar irradiance modulates the mean state and variability of tropical Pacific climate modes, particularly the El Niño–Southern Oscillation (ENSO), which is associated with changes in inter-basin sea-level gradients and crustal stress regimes. These findings reveal a potential coupling between external solar forcing and internal climate variability in shaping seismic cyclicity at interdecadal scales, offering a novel framework for assessing long-term earthquake risks.

This study combines 8 years of middle atmospheric wind data observed at 52°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. Key Points Wind spectrum reveals 6 tidal harmonics significantly higher than background noise with −3 frequency power law Coherence between two longitudinal sectors reveals that the harmonics are synchronized with the Sun Winter tidal enhancements seem to be influenced by seasonal factors rather than SSW, presenting a contrast to existing literature

Nine‐years of mesospheric wind measurements, from two meteor radars at 52°N latitude, were analyzed to study planetary waves (PWs) and tides through estimating their zonal wavenumbers. The analysis reveals that multi‐day oscillations are predominantly driven by PW normal modes (NMs), which exhibit distinct seasonal variations and statistical association with Sudden Stratospheric Warming events. Specifically, a prominent 6‐day NM emerges in April, followed by dominant 4‐ and 2‐day NMs persisting until June, with subsequent peaks of 2‐, 4‐, and 6‐day NMs extending from July to October. Furthermore, this study presents the first observational verification of the frequencies and zonal wavenumbers of over 10 secondary waves, arising from nonlinear interactions among planetary‐scale waves. A notable finding is the prevalence of non‐migrating components in the winter 24‐hr and summer 8‐hr tides, phenomena attributed to the nonlinear interactions. Our findings highlight the complexity of atmospheric nonlinear dynamics in generating diverse planetary‐scale periodic oscillations. Plain Language Summary By analyzing 9 years of wind data from two distinct longitudes, we investigated the origins of planetary‐scale atmospheric waves. Our research revealed that waves shaped by the atmosphere's physical properties drive most multi‐day wind fluctuations. These waves exhibit variability: some occur regularly between April and October, while others are associated with a meteorological event known as the sudden stratospheric warming. Interactions between these waves, tides, and other planetary‐scale phenomena generate secondary waves that are difficult to detect from a single station or single spacecraft analysis. By utilizing data from two longitudes, we identified more than 10 of these waves for the first time. These waves may explain the prevalence of non‐sun‐synchronous components in the 24‐hr and 8‐hr tides during certain seasons, which is unusual given the typical dominance of sun‐synchronous tides. Our findings underscore the extensive nonlinear behaviors of planetary‐scale waves, leading to a complex array of oscillations. Key Points Planetary wave normal modes drive multi‐day oscillations, showing April‐October seasonality and statistical SSW associations First evidence of frequency and zonal wavenumber matching for over 10 secondary waves of nonlinear interactions among planetary‐scale waves Non‐migrating components dominate the winter 24‐hr tide and summer 8‐hr tide, attributed to the nonlinear interactions

Day‐to‐day variability is a persistent feature of the ionosphere, but distinguishing contributions from the atmosphere below or geospace above remains unclear. Here, simultaneous observations of thermospheric meridional winds and ionospheric electron density from the Sanya Incoherent Scatter Radar (SYISR) reveal that the terdiurnal tide (TDT), a crucial agent in atmosphere‐ionosphere coupling, is strongly correlated with ionospheric F‐layer oscillations on a day‐to‐day basis. During a major sudden stratospheric warming in 2024, day‐to‐day variability of both the TDT amplitudes and F‐layer oscillations exhibited pronounced modulation by enhanced planetary waves (PWs). Bispectral analysis revealed nonlinear interactions between PWs, the TDT, and secondary waves at lower and upper sidebands. The ionospheric bispectrum showed peaks shifted toward shorter periods than the PWs themselves. Numerical simulations indicate that variations in magnetospheric forcing modify the ionospheric response to PW‐modulated tides. Our results clarify how the ionosphere responds to coupled lower atmospheric and geospace disturbances on daily timescales.

In the present communication, characteristics of mean winds and planetary waves in the mesosphere lower thermosphere (MLT) region during sudden stratospheric warming (SSW) events using observations from four meteor wind radars located at high, middle, low and equatorial latitudes are discussed. The response of the respective MLT regions to three SSW events that occurred during 2008–09, 2009–10 and 2011–12 winters are investigated. SSW signatures in the MLT zonal and meridional winds over the high latitude station Andenes ( 69 . 3 ∘ N , 16 . 0 ∘ E ) are found to have significant differences from event to event. Mean wind reversals in the high latitude MLT are found to be preceding the corresponding signatures at 60 ∘ N , 10 hPa by a few days. Zonal and meridional wind reversals extend to the MLT region over the mid latitude location Socorro ( 34 . 1 ∘ N , 106 . 9 ∘ W ). However, MLT region over the low latitude station Thumba ( 8 . 5 ∘ N , 77 ∘ E ) as well as the equatorial station Kototabang ( 0 . 2 ∘ S , 100 . 3 ∘ E ) are found to be having a minimal response as far as mean winds are concerned. Apart from mean winds, planetary wave activity in the MLT region over the observational sites are examined, which show a systematic progression of planetary waves from high to equatorial latitudes during major as well as minor SSW events. To elucidate the origin of the observed planetary waves in the MLT region, the stratospheric winds are analyzed. Results suggest that the observed planetary waves have originated in the high-mid latitude middle atmospheric region. The present study provides observational evidence for secondary planetary wave generation in the high-mid latitude middle atmosphere and their equatorial propagation in the MLT as predicted by previous numerical modelling studies. Significance of the present study lies in employing a network of meteor radar observations to investigate the SSW signatures in the MLT region over high, middle, low and equatorial latitudes, simultaneously.

Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) electron density profiles are used to investigate the nighttime midlatitude ionospheric trough (MIT). We find that at midnight the longitudinally deepest MIT occurs to the west of the geomagnetic pole in both the Northern and Southern Hemispheres during the equinox seasons and local summer. The deepest MIT could be ascribable to the enhanced depletion caused by horizontal neutral wind. In the early evening, the eastward neutral wind prevails in the midlatitude F region, which blows the plasma downward where the declination is eastward in the Northern Hemisphere but westward in the Southern Hemisphere, both lying to the west of the geomagnetic pole. The downward drift would enhance the plasma depletion for more molecular composition at lower altitude. In addition, we find for the first time that the location of nighttime MIT minimum oscillated with a periodicity of 9 days and an amplitude of about 1°–1.5° geomagnetic latitude during 2007–2008, associated with the recurrent high‐speed solar wind. Our results shed new light on the empirical description and numerical simulation of MIT. Key Points Deepest MIT occurs to the west of the geomagnetic pole in the N and S Hemispheres The deepest MIT could be ascribed to the enhanced depletion by horizontal wind MIT is found to oscillate periodically at 9 days about 2 to 3 deg from peak to peak

Ionospheric topside O+${O}^{+}$diffusive flux is derived using Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation data, to investigate its global distribution and also its role in winter nighttime enhancement (WNE) of electron density. The flux of the winter hemisphere maintains downward throughout the night. It is much larger between 30°$30{}^{\\circ}$and 50°$50{}^{\\circ}$geomagnetic latitudes and keeps increasing until 22:00–00:00 LT. It peaks at 60°$60{}^{\\circ}$ W and 60°$60{}^{\\circ}$ E–120°$120{}^{\\circ}$ E geographic longitudes during the December solstice, and at 180°$180{}^{\\circ}$ E during the June solstice. These features are similar to those of WNE in NmF2. Furthermore, the derived flux is applied as the upper boundary condition to run the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). The simulated spatial‐temporal variations of WNE are consistent with the observations. The results indicate that downward plasma diffusion from the plasmasphere is the major mechanism of WNE, and the simulation quantifies its contribution. Plain Language Summary Solar radiation ionizes the atmosphere to produce the ionosphere. However, ionospheric electron density in the midlatitude of the winter hemisphere has been observed to increase at night with the absence of photoionization, which is referred to as winter nighttime enhancement (WNE). Many studies have suggested that the plasma causing WNE comes from the overlying plasmasphere via downward diffusion, but so far the global distribution of ionospheric topside diffusive flux has not been systematically examined because it cannot be measured directly. In this study, the topside O+${O}^{+}$diffusive flux is derived based on observational data. The flux is downward at night and varies with geographical location and local time. These characteristics are similar to those of WNE. Furthermore, the theoretical model Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) is used to conduct a modeling of WNE, with the upper boundary condition modified by incorporating the derived diffusive flux. WNE is well reproduced, providing direct evidence that downward plasma diffusion is the major mechanism of WNE. This study provides new insight into the physical processes in the nighttime ionosphere, and has implication for future development and improvement of ionospheric models. Key Points Global distribution of ionospheric topside O+ diffusive flux is derived for the first time using COSMIC radio occultation data The Thermosphere Ionosphere Electrodynamics General Circulation Model simulation driven by the derived flux effectively reproduces the midlatitude electron density enhancement in winter nighttime First global‐scale evidence indicating downward plasma diffusion as the dominant mechanism for electron density enhancement is provided