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.
Graphical Abstract
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).
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.
TIMED/Global Ultraviolet Imager (GUVI) limb measurements of far‐ultraviolet airglow emission have been used to investigate middle‐low latitude thermospheric composition and neutral temperature ...responses to the 20 and 21 November 2003 (day of year DOY 324 and 325) superstorm. Altitude profiles of O, N2 number densities and temperature, as well as O/N2 column density ratio (∑O/N2), on the storm days along the GUVI limb tracks are compared with those on DOY 323 (quiet time). The storm‐time composition and temperature responses were global and evolved continuously as the storm progressed. Specially, N2 and temperature increased almost globally at all altitudes during the storm and their perturbation structures were similar. The magnitudes of their enhancements both increased with altitude and latitude. The storm‐induced O perturbations decreased in the lower thermosphere but increased in the upper thermosphere. Transition heights of O perturbations from decrease to increase changed with latitude and time. During the storm main and recovery phases, the storm‐induced ∑O/N2 decreases were mostly related to the O depletion in the low‐middle thermosphere, whereas ∑O/N2 increases during the storm were primarily caused by N2 depletion. There was a remarkable hemispheric asymmetry in composition responses as they have different morphologies and lifetime, especially during the storm recovery phase.
Key Points
N2 density and neutral temperature increase almost globally at all altitudes during the storm with similar latitude structures
O density decreases in the lower thermosphere but increases in the upper thermosphere in the main and recovery phases of the storm
TIMED/GUVI limb measurements and first‐principles simulations from the Thermosphere Ionosphere Electrodynamics Global Circulation Model (TIEGCM) are used to investigate thermospheric atomic oxygen ...(O) and molecular nitrogen (N2) responses in the middle thermosphere on a constant pressure surface (∼160 km) to the November 20 and 21, 2003 superstorm. The consistency between GUVI observations and TIEGCM simulated composition changes allows us to utilize TIEGCM outputs to investigate the storm‐time behaviors of O and N2 systematically. Diagnostic analysis shows that horizontal and vertical advection are the two main processes that determine the storm‐induced perturbations in the middle thermosphere. Molecular diffusion has a relatively smaller magnitude than the two advection processes, acting to compensate for the changes caused by the transport partly. Contributions from chemistry and eddy diffusion are negligible. During the storm initial and main phases, composition variations at high latitudes are determined by both horizontal and vertical advection. At middle‐low latitudes, horizontal advection is the main driver for the composition changes where O mass mixing ratio Ψo decreases (N2 mass mixing ratio ΨN2 increases); whereas horizontal and vertical advection combined to dominate the changes in the regions where Ψo increases (ΨN2 decreases). Over the entire storm period, horizontal advection plays a significant role in transporting high‐latitude composition perturbations globally. Our results also demonstrate that storm‐time temperature changes are not the direct cause of the composition perturbations on constant pressure surfaces.
Key Points
Horizontal and vertical advection are the two main processes driving O and N2 changes in the middle thermosphere during the superstorm
Molecular diffusion is relatively weak in the middle thermosphere, acting to compensate for O and N2 changes caused by transport processes
Storm‐time temperature changes are not the direct cause of the composition perturbations on constant pressure surfaces
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
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
We compare coincident thermospheric neutral wind observations made by the Michelson Interferometer for Global High‐Resolution Thermospheric Imaging (MIGHTI) on the Ionospheric Connection Explorer ...(ICON) spacecraft, and four ground‐based specular meteor radars (SMRs). Using the green‐line MIGHTI channel, we analyze 1158 coincidences between Dec 2019 and May 2020 in the altitude range from 94 to 104 km where the observations overlap. We find that the two datasets are strongly correlated (r = 0.82) with a small mean difference (4.5 m/s). Although this agreement is good, an analysis of known error sources (e.g., shot noise, calibration errors, and analysis assumptions) can only account for about a quarter of the disagreement variance. The unexplained variance is 27.8% of the total signal variance and could be caused by unknown errors. However, based on an analysis of the spatial and temporal averaging of the two measurement modalities, we suggest that some of the disagreement is likely caused by temporal variability of the wind on scales ≲70 min. The observed magnitudes agree well during the night, but during the day, MIGHTI observes 16%–25% faster winds than the SMRs. This remains unresolved but is similar in certain ways to previous SMR‐satellite comparisons.
Plain Language Summary
Although Earth's atmosphere becomes less dense at high altitudes where it transitions to space, the wind speed grows faster, often exceeding 100 m/s (225 mph). One barrier to better predictions of conditions in the near‐Earth space environment is obtaining knowledge of the wind in the thermosphere, the uppermost layer of the atmosphere. Measurements of the thermospheric wind are difficult to make and historically sparse. ICON, a new NASA mission launched in October 2019, carries the MIGHTI instrument to measure the wind from 90 to 300 km altitude. In this study we compare the observations of MIGHTI to those of meteor radars, which measure the wind from the ground by analysis of radio waves reflected by meteor trails. The results indicate good agreement between the datasets when they measure the wind at the same time and place. Specifically, with 1158 coincidences over the first 6 months of the ICON mission, the correlation is 0.82 and the average difference is 4.5 m/s. This study is important because it validates the MIGHTI data, giving confidence for subsequent studies using its data. It also quantifies limits to the agreement between space‐based and ground‐based winds, which is useful information for future studies combining them.
Key Points
Coincident wind measurements by ICON‐MIGHTI and specular meteor radars are strongly correlated (r = 0.82)
The mean discrepancy between the datasets is 4.5 m/s, validating the MIGHTI v03 zero reference
The RMS discrepancy is 26 m/s, which is attributed to inherent data errors and variability on time scales ≲70 min
Upper mesospheric winds observed by the Svalbard specular meteor radar (16.01°E,78.16°N) are analyzed to study the tidal variabilities during the 2009 sudden stratospheric warming (SSW). We report a ...textbook case of nonlinear interactions between planetary waves (PWs) and the SW2 tide (SWm denotes semidiurnal westward propagating tidal mode with zonal wave number m). The Lomb‐Scargle algorithm, bispectrum, wavelet spectra, and Manley‐Rowe relations are combined to explore the frequency match, phase coherence, energy budget, and wave number relations among the interacting waves and their temporal evolution. Our results suggest that (1) 5, 10, 16 day PW normal modes interact with SW2 generating significant sidebands (S2Ss) at frequencies lower and higher than SW2, known as SW1 and SW3 enhancements, respectively; (2) SW2 is the main energy supplier for both SW1 and SW3, hence shrinks in the interactions; (3) whereas the PWs export relatively negligible energy to SW3 but accept energy from SW2 in generating SW1, therefore, the PWs is not subject to the interactions but controlled by external dynamics, which might in turn act as a key in switching on/off the SW1 and SW3 interactions independently; (4) the SW1 enhancement could be explained as a byproduct of the planetary wave amplification by stimulated tidal decay (PASTIDE); (5) PASTIDE contributes energy to the secondary PW in the late SSW stage reported in previous studies; and (6) one SW1 component associated with the 16 day PW is very close to the semidiurnal lunar mode in frequency, which might contaminate the estimation of the lunar tidal amplification in previous studies.
Plain Language Summary
Nonlinear interaction is a prototypical behavior of waves, as fundamental as Doppler shift, interference, and diffraction. Nonlinear interactions between atmospheric planetary scale waves have been reported in numerous studies. In the current work, we apply for the first time Manley‐Rowe relations to determine the energy flow in the atmospheric interactions and roles of the interacting waves. Particularly, we report and analyze a textbook case of nonlinear interactions between planetary waves (PWs) and the migrating solar semidiurnal tide. Our analysis suggests that the secondary waves, well known as zonal wave 1 and wave 3 enhancements, are fed on the energy mainly from the tide. Although the energy flow through PWs is relatively negligible, our investigations suggest that the energy budget of PWs might act as a key in turning on/off the interactions. When PWs accept energy in the interaction, the wave 1 is generated, whereas when PWs export energy, the wave 3 is generated. In the combined case, PWs could accept energy from the tide and meanwhile export energy to wave 3, generating both wave 1 and wave 3. The energy transport from the tide to PWs in generating wave 1 might supply the secondary PW in the late sudden stratospheric warming stage reported in previous studies.
Key Points
A textbook case of nonlinear interactions between planetary waves and SW2 tide results in asynchronous SW1 and SW3
Manley‐Rowe relations explain the energy flow topology and the roles of interacting waves; SW1 is accompanied by secondary PW
SW1 contaminates the estimation of M2 amplification; we suggest tentative scenarios for interpreting the SW1/SW3 asynchrony
PDF
