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Gamma‐ray glows are observational evidence of relativistic electron acceleration due to the electric field in thunderclouds. However, it is yet to be understood whether such relativistic electrons contribute to the initiation of lightning discharges. To tackle this question, we started the citizen science "Thundercloud Project," where we map radiation measurements of glows from winter thunderclouds along Japan's sea coast area. We developed and deployed 58 compact gamma‐ray monitors at the end of 2021. On 30 December 2021, five monitors simultaneously detected a glow with its radiation distribution horizontally extending for 2 km. The glow terminated coinciding with a lightning flash at 04:08:34 JST, which was recorded by the two radio‐band lightning mapping systems, FALMA and DALMA. The initial discharges during the preliminary breakdown started above the glow, that is, in vicinity of the electron acceleration site. This result provides one example of possible connections between electron acceleration and lightning initiation. Plain Language Summary: Thunderstorms are natural particle accelerators. The strong electric field inside thunderclouds accelerates relativistic electrons, which emit gamma rays via interaction with the atmosphere. High‐energy photons generated in this process have been observed as radiation enhancements called gamma‐ray glows. Winter thunderclouds along the sea of Japan are an ideal target for monitoring glows because their altitudes are usually sufficiently low for the generated gamma‐ray photons to reach the ground. We started a new citizen science "Thundercloud Project" in this area, where we distributed radiation detectors to citizen supporters to observe glows and to reveal their relationship with the aerological condition and lightning discharges. On 30 December 2021, five of those sensors detected a glow from a single thundercloud. Two of them recorded a sudden termination of the glow coinciding with a lightning flash, which was monitored by our two radio mapping systems of FALMA and DALMA. The initial discharges of the flash started at a location about 1.6 km above the glow region with an unusually fast downward progression. This paper is the first report of our citizen science project. We discuss the possibility that accelerated electrons contribute to the initiation of lightning discharges. Key Points: We started the citizen science "Thundercloud Project," a multi‐point observation campaign of gamma‐ray glows from thunderstormsOn 30 December 2021, five radiation monitors detected a 2‐km‐long size gamma‐ray glow, which suddenly terminated with a lightning flashTwo radio mapping systems of lightning identified the initiation of the discharges, which started at a location above the glow region [ABSTRACT FROM AUTHOR]

Condensation and sublimation of ices at the surface of the planet is a key part of both the Martian H2O and CO2 cycles, either from a seasonal or diurnal aspect. While most of the ice is located within the polar caps, surface frost is known to be formed during nighttime down to equatorial latitudes. Here, we use data from the Emirates Mars Infrared Spectrometer onboard the Emirates Mars Mission to monitor the diurnal and seasonal evolution of the ices at the surface of Mars over almost one Martian year. The unique local time coverage provided by the instrument allows us to observe the apparition of equatorial CO2 frost in the second half of the Martian night around the equinoxes, to its sublimation at sunrise. Plain Language Summary: The H2O and CO2 ices that form at the surface on Mars play an important role in the exchange between the atmosphere and the surface of the planet. While most of the ice is located within the two polar caps that grew and shrink seasonally, ice is also known to condensate as surface frost during the night and sublimate during the day. This nighttime surface frost deposition can be observed even at equatorial latitudes. In this paper we use data from the Emirates Mars Infrared Spectrometer onboard the Emirates Mars Mission to detect the H2O and CO2 ices at the surface of the planet at all local times over almost one Martian Year, which allows us to monitor both the seasonal and diurnal evolution of the distribution of ices at the surface of Mars. We observe that nighttime CO2 frost forms at equatorial latitudes in the second half of the night to disappear at sunrise around the Martian equinoxes. Key Points: We monitor the seasonal growth and retreat of both polar caps over MY 36We monitor the presence of CO2 ice on the surface of Mars over MY 36, through the season and through all times of dayCO2 ice appears at the surface at equatorial latitudes during the second half of the night around the equinoxes [ABSTRACT FROM AUTHOR]

Cirrus clouds provide a significant radiative forcing on the Earth's climate system. This paper looks at the connection in space and time between monthly mean lightning activity observed from the Lightning Imaging Sensor on board the International Space Station, and the global monthly mean cirrus cloud cover obtained from the MERRA‐2 reanalysis product. The correlation coefficient between the global monthly mean cloud optical thickness of the cirrus clouds (clouds at altitudes above the 400 hPa pressure levels) with the monthly mean lightning flash counts is 0.84, implying that monthly mean lightning can explain 70% of monthly variability of the global high cloud optical thickness. In addition, lightning amount explains nearly 60% of the monthly mean global area coverage of cirrus clouds. Given these statistically significant connections between lightning and cirrus clouds, we propose using global lightning data as an additional tool for monitoring monthly variability of cirrus clouds. Plain Language Summary: Cirrus clouds are one of the most important components maintaining the Earth's radiation budget. They reflect shortwave radiation from the Sun while absorbing the longwave radiation from the Earth. The net cloud radiative forcing for cirrus clouds results a warming of the climate. More/less cirrus clouds result in more/less warming of the planet. The moisture for the formation of cirrus clouds in the upper atmosphere is transported there in large part via deep convective storms, many associated with lightning activity and hence defined as thunderstorms. An increasing in cirrus clouds in a warmer atmosphere will amplify the initial warming. We explored in this study the relationship between global cirrus cloud coverage and global lightning activity. The results of the research show that 60%–70% of the monthly variability in global cirrus clouds can be explained by mean global lightning activity. Key Points: Cirrus clouds are one of the essential components in the atmosphere, with many important feedbacks on the radiation balance of EarthMonthly mean lightning can explain 70% of the monthly variability of the global high cloud optical thicknessLightning explains nearly 60% of the monthly mean global area coverage of cirrus clouds [ABSTRACT FROM AUTHOR]

The 2022 Tongan volcanic eruption released significant energy into the atmosphere. Tropospheric satellite images show that the eruption generated pressure waves that traveled globally. The Global Observation of the Limb and Disk (GOLD) mission observed significant wave‐like thermospheric temperature perturbations (>100 K) from 12 to 16 UT. These temperature perturbations' spatial curvatures and arrival times are initially similar to the tropospheric wave‐fronts but differ significantly with eastward propagation. The perturbations had a phase speed of ∼300–400 m/s and wavelengths greater than 2,400 km. Near‐concurrent Ionospheric Connection Explorer neutral wind measurements suggest that the eruption's effects reversed the direction of the prevailing thermospheric zonal winds around the perturbed regions. The eruption's global and whole atmospheric effects provide a unique opportunity to study how different atmospheric layers exchange energy and momentum during explosive events. GOLD's synoptic observations are uniquely positioned to study these effects in the middle thermosphere. Plain Language Summary: The powerful Hunga Tonga‐Hunga Ha'apai volcanic eruption on 15 January 2022, at 4:15 UT was visible from space. Satellite images in the troposphere (near 10 km altitude) show concentric pressure waves emanating from the eruption center that travel globally. Wave signatures were seen across multiple atmospheric layers. NASA's Global‐scale Observations of Limb and Disk mission observed its signatures near 150 km altitude as neutral temperature perturbations. Explosive events that affect multiple atmospheric layers allow us to observe how the different atmospheric layers exchange energy and momentum in response to an impulsive source. In this paper, we present an analysis of the eruption induced temperature perturbation in the middle thermosphere. Key Points: Global‐scale Observations of Limb and Disk mission observed global‐scale thermospheric neutral temperature perturbations (>100 K) induced by the Tongan volcanic eruptionThe phase speeds of the perturbations are around 300–400 m/s with a large wavelength (>2,400 km)The zonal winds near the peak temperature perturbation reverse from eastward to westward [ABSTRACT FROM AUTHOR]

The thermospheric neutral density response to the 7–9 September 2017 storms is investigated based on the Swarm satellite observations and the thermosphere‐ionosphere‐electrodynamic general circulation model (TIEGCM) simulation. The Swarm data depicted a prominent interhemispheric asymmetry (IHA) in the afternoon sector during the second storm, a feature that was yet explained. Driven by realistic high‐latitude electric potential and electron precipitation patterns, the TIEGCM is able to reproduce the observed storm‐time neutral density response. The TIEGCM simulation reveals that the differences in the traveling atmospheric disturbances (TADs) is largely responsible for the observed IHA in the neutral mass density response at low and middle latitudes, whereas the difference in mean molecular mass between the two hemispheres may contribute to the IHA in neutral density at higher latitudes. The IHAs in TADs and mean molecular mass are attributed to the IHA in Joule heating dissipation on the night and dawn sides. Plain Language Summary: During geomagnetic storms, thermospheric neutral density can increase drastically due to intense heat inputs. Enhanced neutral density increases the air drag on satellites which could lower their orbits and lead to faster atmosphere reentering of satellites. In addition, enhanced neutral mass density may cause larger uncertainties in orbit prediction which increases the risk of collisions with other space objects. Therefore, it is essential to improve the understanding of storm‐time neutral density variations to improve satellite operations. Recent satellite observations have revealed that thermospheric neutral mass density response can be interhemispherically asymmetric even under equinox conditions when the background density in the two hemispheres is comparable. This paper presents a case study of such an event based on data analysis and numerical simulations to investigate the physical mechanisms behind this intriguing phenomenon. The event took place on 7–9 September 2017 and the most prominent interhemispheric asymmetry of neutral density occurred in the afternoon sector. Our study found that the interhemispheric asymmetry during this event results from the differences in the generation, propagation and interaction of globally propagating atmospheric waves, namely traveling atmospheric disturbances, as well as the difference in mean molecular mass between the northern and southern hemispheres. Key Points: Thermosphere‐ionosphere‐electrodynamic general circulation model captures the observed interhemispheric asymmetry (IHA) in the neutral density response to the 7–9 September 2017 geomagnetic stormsThe IHA in the neutral density response results from the IHA in traveling atmospheric disturbances and mean molecular massThe observed IHA in the neutral density response is not caused by direct Joule heating dissipated at the same local time [ABSTRACT FROM AUTHOR]

The time delay from an interplanetary driver arriving at the magnetopause to the response in the ionosphere has never been quantified separately for different types of storm drivers. This study investigates the delay for storms driven by high‐speed streams and associated stream interaction regions (HSS/SIR), or by interplanetary coronal mass ejection sheaths and magnetic clouds (MC). The total field‐aligned current (FAC) and SME index lag the Newell coupling function (NCF) by 40 ± 10 min during storms driven by HSS/SIR and sheaths, and by 60 ± 10 min for MCs. The correlation coefficient between FAC and NCF reaches maximum value as NCF is averaged over the preceding 80 min for sheath, 90 min for HSS/SIR, and 140 min for MC storms. Plain Language Summary: The Sun causes perturbations in the solar wind, which may drive geomagnetic storms associated with strong field‐aligned currents to the ionosphere. The solar wind drivers studied in this paper are high‐speed stream/stream interaction regions (HSS/SIR), and sheath and magnetic cloud (MC) interplanetary coronal mass ejections. The exact time from the arrival of the solar wind interplanetary driver at the magnetopause to the response in the ionospheric and field‐aligned currents (FAC) have not been quantified for different types of solar wind drivers. We study this time delay during geomagnetic storms and find that it is typically 40 min for HSS/SIR‐ and sheath‐driven storms, and 60 min for MC‐driven storms. Additionally, the total FAC best correlate with the solar wind averaged over the preceding 80 min for sheath, 90 min for HSS/SIR, and 140 min for MC‐driven storms. These results may help improve the accuracy of forecasting solar wind disturbances on the high‐latitude ionosphere. Key Points: Correlation between Newell coupling function (NCF) and total field‐aligned current (FAC) is studied for different storm driversBest correlation for sheath, high‐speed stream, and magnetic cloud storms is found by integrating NCF over 80, 90, and 140 min, respectivelySheath‐driven storms are associated with the highest values of total FAC and NCF [ABSTRACT FROM AUTHOR]

Distributed Acoustic Sensing (DAS) has become a popular method of observing seismic wavefields: backscattered pulses of light reveal strains or strain rates at any location along a fiber‐optic cable. In contrast, a few newer systems transmit light through a cable and collect integrated phase delays over the entire cable, such as the Microwave Frequency Fiber Interferometer (MFFI). These integrated systems can be deployed over significantly longer distances, may be used in conjunction with live telecommunications, and can be significantly cheaper. However, they provide only a single time series representing strain over the entire length of the fiber. This work discusses theoretically how a distributed and integrated system can be quantitatively compared, and we note that the sensitivity depends strongly on points of curvature. Importantly, this work presents the first results of a quantitative, head‐to‐head comparison of a DAS and the integrated MFFI system using pre‐existing telecommunications fibers in Athens, Greece. Plain Language Summary: New technologies are being developed to measure earthquakes using fiber‐optic telecommunications cables. The most popular new method in recent years is "Distributed Acoustic Sensing," (DAS) in which pulses of light are repeatedly sent down a fiber and one measures the signals that reflect back. Shaking from an earthquake will stretch the fiber and the signature of reflected pulses will change. A new method (Microwave Frequency Fiber Interferometer [MFFI] in the paper) sends light from one end to the other and measures differences in optical phase. The new method has many advantages: it is cheaper, can be used on longer cables, and can be used at the same time as active telecommunications. As a disadvantage is lacks the high spatial resolution that DAS offers. This paper discusses the differences between the two methods and shows how to compare them, and then shows the first real‐data, head‐to‐head comparison from an earthquake observed in Athens, Greece. Key Points: We develop and test a theory for the quantitative comparison of distributed and integrated fiber‐optic strain sensingThe sensitivity of integrated measurements depends primarily on local fiber curvature and fiber heterogeneityA data‐based comparison corroborates the theory, thereby demonstrating the geophysical value of our integrated sensor system [ABSTRACT FROM AUTHOR]

The lightning channel morphology is closely related to developmental processes, which depends on the charge structure. There is currently a lack of understanding of how lightning channel morphology reveals developmental characteristics and charge structure. Based on the Lightning mapping array radiation source localization data of six lightning flashes, we studied the lightning channel morphology characterization method and obtained the spatial distribution of the fractal dimension (FD). We found that the fractal distribution had a consistent spatial and temporal correspondence with the channel development characteristics. Compared to regions with unidirectional extended channels, regions with more channel branches or direction change of channel development have a larger FD and a higher power density, where horizontal channel development is accelerated. These correspondence features reflect the inhomogeneous presence of pocket charge, which provides a new method to use lightning morphology information to reveal the characteristics of charge level distribution within thunderstorm clouds. Plain Language Summary: The charge distribution within the horizontal charge region may have a more significant effect on the extension of the lightning channel. At present, it is still difficult to directly observe the charge structure of thunderstorm clouds, especially the horizontal charge structure. Also, the morphological study of the channel is not perfect due to the limitations of observation and data acquisition. Therefore, there is currently a lack of understanding of how lightning channel morphology reveals developmental characteristics and charge structure. In the paper, the relationship between the lightning channel morphology, developmental characteristics, and charge structure was investigated for the first time based on Lightning mapping array localization data, and the spatial distribution of the lightning fractal dimension (FD) was obtained. It is found that, compared to regions with unidirectional extended channels, regions with more channel branches or direction change of channel development have a larger FD and a higher power density, where horizontal channel development is accelerated. These correspondence features reflect the inhomogeneous presence of pocket charge, which provides a new method to use lightning morphology information to reveal the characteristics of charge level distribution within thunderstorm clouds. Key Points: The spatial distribution of fractal dimension (FD) is obtained, and uesd to describe lightning channel morphologyThe channel morphology characterized by the FD is closely related to channel development velocity and radiation power densityThe channel morphology can be used to reflect the inhomogeneous state of charge level distribution [ABSTRACT FROM AUTHOR]

This paper combines the power of deep‐learning with the generalizability of physics‐based features, to present an advanced method for seismic discrimination between earthquakes and explosions. The proposed method contains two branches: a deep learning branch operating directly on seismic waveforms or spectrograms, and a second branch operating on physics‐based parametric features. These features are high‐frequency P/S amplitude ratios and the difference between local magnitude (ML) and coda duration magnitude (MC). The combination achieves better generalization performance when applied to new regions than models that are developed solely with deep learning. We also examined which parts of the waveform data dominate deep learning decisions (i.e., via Grad‐CAM). Such visualization provides a window into the black‐box nature of the machine‐learning models and offers new insight into how the deep learning derived models use data to make decisions. Plain Language Summary: This paper presents a new method to distinguish earthquakes from explosions using seismic data. The method combines features implicitly defined by a deep learning algorithm with features explicitly defined from physical models of seismic sources and elastic wave propagation. The combination of these two types of features makes our method perform better on new data sets. By visualizing the performance of our combined model, we gain insight into what the deep learning model relies on to make its decisions. Key Points: Discrimination of earthquakes and explosions can be enhanced by combining physics‐based features with those derived from machine learningVisualizing which parts of the input the deep learning model relies on can provide more insight into the processes underlying decisionsThe deep learning model focuses on different frequency bands for P and S waves to make the decision [ABSTRACT FROM AUTHOR]

Microinjection phenomena, characterized by dispersive oscillations of electron fluxes at the Pc5 period and bi‐directional pitch angle anisotropy, are frequently observed by Magnetospheric Multiscale in the dusk to midnight plasma sheet. In this paper, two such events are analyzed and the features of toroidal mode drift resonance measured meanwhile are shown in detail. The prominent observation is that the fluctuations of the electron flux and the electric field have either −90° or +90° phase difference at the resonant energy, and the phase difference rises as the energy increases. We extend the present theory for drift resonance of toroidal mode wave with only the equatorial moving electrons in a dipole field to include bouncing electrons. The predicted phase differences based on the new theory are consistent well with the observations in the microinjection events. It is thus suggested that drift resonance may act as the forming mechanism for the observed microinjections. Plain Language Summary: In the dusk to midnight plasma sheet, the fluxes of counter‐streaming energetic electrons were often observed to fluctuate at the period about 2 ∼ 6 min, with the higher energy fluxes coming to peaks earlier than those at lower energies. These phenomena were considered to be caused by repetitive injections of electrons with smaller time scale than the substorm‐associated injections, so were named "microinjections." In this paper, we analyze two microinjection events and find that they are likely to result from the drift resonance between local electrons and Ultralow‐frequency compressional toroidal waves. To verify this hypothesis, we extend the present theory for toroidal mode drift resonance from only considering electrons with 90° pitch angles to including all bouncing electrons. We find that the predicted phase relationships of electron fluxes and the wave electric field correspond well with observations. The phase difference between the electron fluxes and the electric field equals to −90° or +90° at the resonant energy and increases with energy. Therefore, we propose that the compressional toroidal mode drift resonance may work as the generation mechanism of microinjections in the two events. Key Points: Evidences of drift resonance between compressional Ultralow‐frequency waves and electrons are observed in microinjection eventsCompressional toroidal mode drift resonance theory is extended to off‐equator regionDrift resonance can be a possible mechanism for the generation of microinjection events [ABSTRACT FROM AUTHOR]

No observations show evidence of ionization in long air gap discharge during dark periods. However, for the first time, weak channel illuminations are observed during the so‐called dark periods in this paper. The length of these faint bright channels is almost constant during each dark period and is nearly equal to that of the remaining low‐density channels created by the previous discharge. The phenomenon, that is, the dark period is not really dark, is more likely to occur at the time when there is a greater change rate of the applied voltage. These findings are based on the high‐speed direct imaging and Schlieren photography together, in which the former is used to visualize the bright channels and the latter to detect the low‐density area. Plain Language Summary: Although a large amount of related experimental and theoretical researches on long positive sparks have been carried out, there are still some phenomena to be explained, such as the abrupt elongation of leader channel, and the intermittent propagation of upward positive leader. Together with the above phenomena, dark periods are often accompanied before the channel re‐illumination, during which no observations show evidence of ionization as no current and light are measured. However, for the first time, this paper shows the direct experimental evidence that channels in dark periods are not always dark in long positive sparks, but have weak illumination with a length equal to the low‐density channel left by previous discharges. It is thought that the capacitive current due to the varying applied voltage may make the left low‐density channels to emit weak light. New findings can inspire researchers to pay attention to the capacitive current in the dark period instead of directly ignoring them as before. This may help to explain the above‐mentioned mysterious phenomena to a certain extent. Key Points: More direct experimental evidence to show that discharge channels during dark periods are not always dark in long positive sparksThe length of faint bright channels in each dark period is almost constant and nearly the same as that of the remaining low‐density channelsCapacitive current due to the varying applied voltage may cause the channel illumination in dark periods [ABSTRACT FROM AUTHOR]

The shoreline development index—The ratio of a lake's shore length to the circumference of a circle with the lake's area—Is a core metric of lake morphometry used in Earth and planetary sciences. In this paper, we demonstrate that the shoreline development index is scale‐dependent and cannot be used to compare lakes with different areas. We show that large lakes will have higher shoreline development index measurements than smaller lakes of the same characteristic shape, even when mapped at the same scale. Specifically, the shoreline development index increases by about 14% for each doubling of lake area. These results call into question previously reported patterns of lake shape. We provide several suggestions to improve the application of this index, including a bias‐corrected formulation for comparing lakes with different surface areas. Plain Language Summary: Lakes vary in shape from nearly perfect circles to the almost comically convoluted. These shapes reflect their geologic (or anthropogenic) origins, and influence within‐lake ecological and chemical processes. As a consequence, the shapes of lakes are often compared, both among lakes on Earth and between Earth's lakes and those on other planetary bodies, to provide context when measuring and interpreting other characteristics. In this paper, we show that a widely‐used metric of lake shape—The shoreline development index—Is biased and produces false patterns when comparing the shape of lakes with different areas, a common analysis and primary purpose of the metric. When applying the shoreline development index, we suggest: (a) Reporting the scale at which lakes are mapped; (b) when possible, only comparing lakes mapped at the same scale; (c) explicitly considering how bias may impact interpretation of patterns of lake shape; and (d) reporting a bias‐corrected or alternative metric. Key Points: The shoreline development index is scale dependent and cannot be used to compare the shape of lakes with different surface areasPatterns of lake shape reported in global hydrographic studies are artifacts of scale dependenceBias‐corrections are possible, but introduce additional uncertainties [ABSTRACT FROM AUTHOR]

Gongga Shan (GGS) is the highest mountain on the eastern margin of the Tibetan Plateau. However, the mechanism for the uplift of Gongga Shan is still unclear due to a lack of detailed geophysical studies. Inversion of an array of magnetotelluric data at 120 sites produced a 3D resistivity model that revealed that the GGS massif is characterized by a high resistivity upper crust underlain by a westward dipping resistor at middle crustal depths that is interpreted as the underthrust Yangtze Craton (YC). A thin conductive layer is sandwiched between these two zones of high resistivity. This resistivity model is inconsistent with previously published geodynamic models. Based on the new magnetotelluric results, we propose that the uplift of Gongga Shan occurs primarily by underthrusting of the YC. Additional uplift may be due to transpression on a restraining bend of the oblique‐slip Xianshuihe faults. Plain Language Summary: Continent‐continent collisions are an important tectonic process and have controlled the formation of the modern continents. The India‐Asia collision is the best modern example and has produced both a high elevation plateau and the world's highest mountain belts. A range of tectonic processes occurs during these collisions as the crust deforms including extrusion and perhaps crustal flow. Within these collision zones, there are locations of especially rapid uplift that have not been explained with existing geodynamic models. This paper investigates this process through a study of Gongga Shan—a mountain on the eastern edge of the Tibetan Plateau, where uplift continues at a rate of 2–3 mm per year and has formed peaks greater than 7,500 m in elevation. 3D inversion of an array of magnetotelluric data has produced a well‐constrained crustal resistivity model for the GGS area. It reveals that the GGS crust is characterized by four resistivity layers including a westward dipping resistor at depth of 15–30 km overlain by a thin conductive layer. This distinctive structure implies that the basement of the Yangtze Craton is underthrust beneath the Songpan‐Ganzi Terrane, thereby resulting in uplift of the eastern margin of the Tibetan Plateau. Key Points: The middle crust beneath Gongga Shan is characterized by a westward dipping resistor at depth of 15–30 kmGeodynamic models proposed for Gongga Shan are not consistent with the resistivity structureUnderthrusting of the Yangtze Craton beneath the Songpan‐Ganzi Terrane appears to contribute to uplift [ABSTRACT FROM AUTHOR]

Oxygen chemistry plays a central role in the production of plasma in the ionosphere. While molecular oxygen (O2) is the third most abundant constituent in the thermosphere, molecular and atomic oxygen densities are difficult to quantify accurately. Consequently, modeled densities have significant uncertainties. New stellar occultation measurements from the Global‐scale Observations of Limb and Disk (GOLD) mission provide data that can quantify, with high precision, the O2 density and its daily and local time variations. These observations are compared to the Mass Spectrometer Incoherent Scatter radar family of empirical models and the physics based Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension. While GOLD observations show a strong diurnal structure with local time, the models display a semidiurnal structure in O2. The local time structure of O2 is critical for accurately modeling plasma densities in the ionosphere. Plain Language Summary: This paper compares new observations from the Global‐scale Observations of Limb and Disk mission of molecular oxygen (O2) in the lower thermosphere (130–200 km in altitude) to widely used models in the aeronomy community. The main finding is that the local time structure of the observed O2 has a minimum at 6 hr local time and a peak near 18 hr local time while the models all show a structure that has peaks at both 6 and 18 hr local time. This significant difference not only influences the total neutral thermospheric density results from the models, but may ultimately impact the calculated ionospheric plasma density and its temporal variability. Understanding ionospheric plasma densities is vital for the proper modeling of the propagation of communications and navigational signals. Key Points: Global‐scale Observation of Limb and Disk molecular oxygen (O2) density indicates a diurnal local time structure, while empirical and numerical models have a semidiurnal structure Seasonal variability in O2 density is important for understanding the local time structure Observed vertical O2 density structure has discrepancies with models, with larger differences at 170 km than 140 km [ABSTRACT FROM AUTHOR]

Recent observations from LOFAR indicate that multiple, spatially distributed corona bursts can occur in lightning processes with a timescale of 10 microseconds. The close proximity of the corona bursts in space and time poses a great observation challenge for short‐baseline (typically ≤100 m) radio interferometers. This paper reports simulations to show the interferometry results that would be obtained with such an interferometer. In particular, spatially‐separated corona bursts at fixed locations may be seen as a fast (>107 m/s) propagating source with large power variation if the resolution of the instrument is greater than the spatial separation of the bursts. The implications and suggestions for lightning interferometry studies are discussed in the paper. Plain Language Summary: Lightning evolution contains brief processes that are critical to its channel formation, but their physical mechanisms are poorly understood at present. Those processes emit very high frequency radio emissions, and radio sensor arrays have been used to detect those emissions to investigate their source processes. The latest observations from the large radio telescope LOFAR show that multiple, intense bursts of radio emission at discrete locations occur in those processes. Here we report simulations to show that such bursts are seen as a fast propagating wave for a short‐baseline lightning radio sensor array. Our study suggests that caution is required when interpreting the observations made with such a radio sensor array. Key Points: Complications of lightning radio interferometer data interpretation due to random nature of the emission sources are investigatedCorona bursts observed by LOFAR may be seen as a fast (>107 m/s) moving source with power variation by short‐baseline interferometersRequirements for radio interferometers to resolve corona bursts occurring in close spatial and temporal proximity are examined and discussed [ABSTRACT FROM AUTHOR]

The present paper reports of a field experiment over a shallow, roughness‐varying barrier reef at Maupiti island, French Polynesia. The depth‐averaged momentum balance is used to estimate the bottom drag coefficient Cd, which varies from 0.01 to 0.3, with dependence on both depth and reef structure. The depth effect on Cd is well predicted by a log dependency, as used in previous laboratory and fields observations. The present results extend the approach to a system with more wave exposure and higher roughness‐to‐depth ratio. Additionally, the statistical relationship between high‐resolution reef topography and hydrodynamical parameters is discussed. Plain Language Summary: Coral reefs are essential for human societies and ecosystems in many tropical coastlines. Having reliable hydrodynamic models of these systems is important for proper management and engineering applications. Reefs play an important role in sheltering shorelines by attenuating wave energy and slowing currents. Thus, they must be accounted for in the models. However, due to the complexity of coral geometry, there is still no reliable manner to represent or translate the physical observation of corals' geometrical structure to a friction term in the governing equations of the models. Additional representative challenges exist due to the varying importance of friction, depending on the water depth. This paper demonstrates good alignment between a metric of the physical geometry of coral to a frictional parameter used in the governing equation, the momentum balance. Additionally, the depth dependence of our results is described well by a classical equation meant to describe turbulence near a wall, log‐layer theory. In total, the results yields good agreement between terms in the momentum balance and aid in the fundamental connection of the physically‐observed and the numerically‐representation reefs. Key Points: Field estimates of bottom drag coefficient over a shallow rough coral barrier reef using a bulk momentum balance approachDrag coefficients are dependent on the local depth and colony structure, and well predicted by a log‐dependencyThe recovered data allows for a discussion of the connection between hydrodynamic roughness scale and high resolution seabed topography [ABSTRACT FROM AUTHOR]

Carbon‐14 in tree rings have suggested there had been multiple extreme solar proton events (SPEs) in the past. While the largest events such as in 774–775 CE can be significantly detected by the typical precision of accelerator mass spectrometry, smaller but possibly more frequent events have been difficult to be detected. Thus, the frequency or any characteristics of such relatively smaller events are still largely unknown. In this paper, we report that large SPEs had occurred in 1261–1262, 1268–1269, and 1279–1280 CE before the onset of the Wolf minimum based on high‐precision carbon‐14 analyses. It is suggested that they had occurred at the maximum and the declining phase of solar cycles, and that they had occurred during the transition time of solar activity into a deep minimum. We propose that this episode may provide a unique opportunity to elucidate a potential interaction between the solar dynamo and extreme solar flares. Plain Language Summary: The Sun is a magnetically active star and occasionally cause intense bursts that sometimes accompany the ejection of energetic protons, described as the solar proton events. In this paper, we report that there were three intense solar proton events in the thirteenth century, just before the onset of the Wolf grand minimum. We propose that these events may be related to the weakening of solar activity during that time. Key Points: Multiple abrupt increases in carbon‐14 content were found during the transition time of solar activity into the grand minimum stateThey occurred at solar activity maximum or at the declining phase of solar cycles, suggesting that they originate from solar proton eventsThe Wolf minimum may provide a unique opportunity to potentially deepen the understanding of the solar dynamo [ABSTRACT FROM AUTHOR]

We demonstrate a novel method for observing Large Scale Traveling Ionospheric Disturbances (LSTIDs) using high frequency (HF) amateur radio reporting networks, including the Reverse Beacon Network (RBN), Weak Signal Propagation Reporter Network (WSPRNet), and PSKReporter. LSTIDs are quasi‐periodic variations in ionospheric densities with horizontal wavelengths >1,000 km and periods between 30 and 180 min. On Nov 3, 2017, LSTID signatures were observed simultaneously over the continental United States in amateur radio, SuperDARN HF radar, and GNSS Total Electron Content with a period of ∼2.5 hr, propagation azimuth of ∼163°, horizontal wavelength of ∼1680 km, and phase speed of ∼1,200 km hr−1. SuperMAG SME index enhancements and Poker Flat Incoherent Scatter Radar measurements suggest the LSTIDs were driven by auroral electrojet intensifications and Joule heating. This novel measurement technique has applications in future scientific studies and for assessing the impact of LSTIDs on HF communications. Plain Language Summary: Large Scale Traveling Ionospheric Disturbances (LSTIDs) are variations in the ionosphere with wavelengths greater than 1,000 km, periodicities between 30 min and 3 hr, and speeds greater than about 1,400 km per hour. Auroral zone disturbances are generally cited as the energy source for LSTIDs. In this paper, we show for the first time that LSTIDs can cause variations in the distances amateur (ham) radio operators can communicate using data from the Reverse Beacon Network (RBN), Weak Signal Propagation Reporter Network (WSPRNet), and PSKReporter amateur radio networks. The LSTID signatures in the amateur radio data are in excellent agreement with LSTID observations from two well‐established instruments: the Blackstone, Virginia SuperDARN radar and a large scale network of GNSS based ionospheric Total Electron Content receivers. The observed LSTIDs appear 2–3 hr after auroral zone disturbances are detected by ground magnetometers in the SuperMAG network and the Poker Flat Incoherent Scatter Radar (PFISR) in Alaska. Results suggest that auroral zone disturbances were the ultimate cause of the observed LSTIDs. This paper provides a foundation for using large‐scale, crowd‐sourced amateur radio observations of LSTIDs as a new method for the study of LSTIDs. Key Points: Amateur radio data provides a new method for studying Large Scale Traveling Ionospheric Disturbances and HF communications impactsLarge Scale Traveling Ionospheric Disturbances are seen for the first time simultaneously in amateur radio, SuperDARN, and GNSS TEC dataObserved midlatitude Large Scale Traveling Ionospheric Disturbances are likely driven by auroral zone electrojet surges and Joule heating [ABSTRACT FROM AUTHOR]

Observations of a regular pulse burst (RPB) at the end of a K‐event are analyzed utilizing a simple geometric model and particle swarm optimization (PSO) to estimate the currents and propagation speeds of successive pulses of the RPB. The results show that the current of successive pulses is strongly overlapped and, for typical speeds of continuously propagating K‐events, are unphysically large (88 kA), exceeding the currents of most strokes to ground. By default, the unphysical nature of the result, coupled with very high frequency interferometer observations of an RPB in Florida, shows that the propagation speed of the pulses is significantly faster than expected, namely ∼0.6–1.8 × 108 m/s. This reduces the inferred current from 88 kA down to 6–18 kA, typical of intracloud events. The fast propagation speed of the stepping is explained by successive pulses retracing much of the path of the preceding pulses due to the successive pulses being strongly overlapped. Plain Language Summary: Regular pulse bursts (RBP) are a spectacular phenomenon in lightning studies, in which rapidly propagating, conductive channels called K‐events transition from a smoothly and continually lengthening channel to an intermittent but regularly‐spaced stepping process, with each step producing a highly‐impulsive current and electric field change, or "sferic." Here we introduce a new approach for analyzing sferic observations that models the K‐event as a series of successive segments, with the parameters of the current and extent of each pulse being determined by searching the multi‐dimensional parameter space that best fits the corresponding pulse waveforms. This was done using a process called PSO, and indicated that the RBP pulses were caused by a succession of overlapping fast breakdown events, but having unphysically strong currents. For the distant measurements of the study, the sferic amplitudes can determine only the product of current and propagation speed of the pulses, causing their current and speed values to be ambiguous. However, for the currents to have reasonable values, we are able to conclude that the speeds needed to be faster than assumed maximum value of 1.5 × 107 m/s value in the analysis. This contention is supported by observations of similar RPBs being studied by the authors. Key Points: A new, innovative method is developed for estimating the currents and extents of fast electric field change pulsesThe method is applied to the study of a regular pulse burst, showing that it consisted of a succession of fast overlapping eventsThe speed of the pulses is inferred to be significantly higher than that of continuously‐developing K‐leader [ABSTRACT FROM AUTHOR]

Arctic Ocean freshwater storage increased since the mid‐1990s, but the cause was unknown. Jahn and Laiho (2020, https://doi.org/10.1029/2020GL088854) use ensemble runs of a coupled climate model to suggest that the observed increase is anthropogenic. The paper quantifies when the anthropogenic signals should emerge from the noise of natural variability. This result contextualizes research on the Arctic Ocean freshwater system and sketches an unprecedented opportunity. Future work should elucidate mechanisms, seek to observe the anthropogenic freshwater changes, and investigate the impacts on biogeochemistry and the North Atlantic Ocean circulation. Plain Language Summary: The Arctic is a region of clear man‐made climate change. Changes in the Arctic Ocean salinity and currents have been seen, but the cause was unknown. A new paper shows that the changes are probably due to man‐made climate change. The reason is they only occur in a climate model with man‐made climate forcing. This is an important result because it helps scientists focus their research into how the changes work. It also points to a valuable opportunity to watch the Arctic Ocean respond to man‐made climate change. There might be important future impacts on North Atlantic oceanography and North Atlantic climate that scientists can now look for. Key Points: A recent paper shows that observed increases in Arctic Ocean freshwater storage are anthropogenicThe paper anticipates future anthropogenic changes, which are testable predictions for observersThere are widespread impacts of anthropogenic freshwater change, both within the Arctic and the North Atlantic Oceans [ABSTRACT FROM AUTHOR]

Satellite observations show that there is a wintertime hotspot of gravity wave activity, located mainly over the ocean, around 60°S in the stratosphere. However, the sources of the gravity waves making up this hotspot are varied and complex and remain unclear. Here we use radiosonde observations from 11 Antarctic stations and selected small islands close to 60°S to examine the horizontal directional pseudo‐momentum flux and energy density distributions of upward propagating gravity waves in the lower stratosphere. This paper shows, for the first time, that short vertical wavelength gravity waves in the lower stratosphere clearly propagate meridionally toward 60°S during the winter months. This result supports previous studies that show that this belt of gravity wave activity over the ocean is contributed to by wave sources outside 60°S. Plain Language Summary: Atmospheric gravity waves are buoyancy waves, where the restoring force is gravity, which can have horizontal wavelengths from a few tens to hundreds of kilometers. They can be generated by a variety of sources including wind flow over mountains and storms. Over the Drake Passage and out to the east around 60°S, a large hotspot of gravity waves has been observed in the wintertime stratosphere using satellite observations. The sources for this gravity wave hotspot are unclear as it mainly located over the open ocean. In this paper radiosonde observations are used to examine, for the first time, the directional variation in short vertical wavelength gravity waves close to 60°S in the lower stratosphere. It is found that during the winter months there is a meridional convergence of gravity waves toward 60°S, implying that the gravity wave sources of the hotspot are in fact be outside this region. Key Points: This paper provides evidence of a winter time convergence of gravity waves and their momentum flux toward 60°SThis is the first time this has been observed for short vertical wavelength waves in the lower stratosphere [ABSTRACT FROM AUTHOR]

Recent observations have reported that magnetosonic waves can exhibit rising‐tone structures in the frequency‐time spectrogram. However, the generation mechanism has not been identified yet. In this paper, we investigate the generation of rising‐tone magnetosonic waves in the terrestrial magnetosphere using 1‐D particle‐in‐cell (PIC) simulations, in which the plasma consists of three components: cool electrons, cool protons and ring distribution protons. We find that the magnetosonic waves excited by the ring distribution protons can form a rising‐tone structure with frequency of the structure ranging from about 0.5Ωlh to nearly Ωlh, where Ωlh is the lower hybrid frequency. It is further demonstrated that the rising frequency of magnetosonic waves can be accounted for by the scattering of ring distribution protons. Moreover, the rising‐tone timescale obtained by PIC simulation is compared with the satellite observation. Our findings provide some new insights to understand the nonlinear evolution of plasma waves in the Earth's magnetosphere. Plain Language Summary: Magnetosonic waves in the Earth's inner magnetosphere often exhibit a series of harmonics of the local proton gyrofrequency. Recent observations found that they also have some other features, such as rising‐tone spectra and quasiperiodicity in time. Since these phenomena are similar to the rising‐tone chorus and electromagnetic ion cyclotron waves, many researchers speculated the generation of rising‐tone magnetosonic waves may be also related to the nonlinear wave‐particle interaction. In spite of the speculation, the specific generation mechanism of rising‐tone magnetosonic waves has not been identified yet. In this paper, we perform 1‐D particle‐in‐cell (PIC) simulations to investigate the generation of rising‐tone magnetosonic waves. It is found that the magnetosonic waves excited by the ring distribution protons can form a rising‐tone structure. In addition, we also demonstrate that the rising‐tone magnetosonic waves can be accounted for by the scattering of ring distribution protons. Key Points: Using 1‐D PIC simulations, we have investigated the generation of rising‐tone magnetosonic waves excited by ring distribution protonsThe rising‐tone structure of magnetosonic waves can be accounted for by the scattering of ring distribution protonsPIC simulations show that the rising‐tone timescale depends on the initial ring proton density and thermal speed [ABSTRACT FROM AUTHOR]

This paper reports on the standing whistler waves upstream of Mercury's quasi‐perpendicular bow shock. Using MESSENGER's magnetometer data, 36 wave events were identified during interplanetary coronal mass ejections (ICMEs). These elliptic or circular polarized waves were characterized by: (a) a constant phase with respect to the shock, (b) propagation along the normal direction to the shock surface, and (c) rapid damping over a few wave periods. We inferred the speed of Mercury's bow shock as ∼26 km/s and a shock width of 1.87 ion inertial length. These events were observed in 20% of the MESSENGER orbits during ICMEs. We conclude that standing whistler wave generations at Mercury are generic to ICME impacts and the low Alfvén Mach number (MA) collisionless shock, and are not affected by the absolute dimensions of the bow shock. Our results further support the theory that these waves are generated by the current in the shock. Plain Language Summary: The strength of planetary bow shocks varies with the planet's heliocentric distance from the Sun. Studying the bow shocks of other planets is important for extending our understanding of collisionless‐shock physics. In the solar system, the bow shocks of Mercury are unique as they are produced by low Mach numbers and low plasma beta solar wind blowing over a small magnetized body that is 1–2 orders smaller than Earth. The standing whistler waves upstream of the bow shock of Mercury were determined through statistical analyses. Similar to the observations at Earth, these waves were rapidly damping with a proportion of the wave periods; however, the damping distance at the spacecraft frame was considerably shorter at only a few kilometers upstream in the small‐scale bow shock of Mercury. The high occurrence rate of standing whistler waves suggests that Mercury's bow shock is a natural plasma laboratory, which can be used to further investigate low MA planetary shocks during the upcoming BepiColombo mission. Key Points: First survey of standing whistler waves upstream of Mercury's bow shockStanding whistler waves are common at Mercury during interplanetary coronal mass ejectionsOur results support the theory that current in shock generates standing whistler waves [ABSTRACT FROM AUTHOR]

Both volcano‐tectonic (VTs) and deep long‐period earthquakes (DLPs) have been documented at Akutan Volcano, Alaska and may reflect different active processes helpful for eruption forecasting. In this study, we perform high‐resolution earthquake detection, classification, and relocation using seismic data from 2005 to 2017 to investigate their relationship with underlying magmatic processes. We find that the 2,787 VTs and 787 DLPs are concentrated above and below the inferred magma reservoir respectively. They both are clustered as swarms and occur preferentially during inflation episodes with no spatial migrations. However, moment release rates of DLP swarms show a stronger correlation with inflation and their low‐frequency content is likely a source instead of a path effect. Therefore, we infer that DLPs are directly related to unsteady magma movement through a complex pathway. In comparison, repeating events are observed in VTs. Thus, we conclude that they represent fault rupture triggered by magma/fluid movement or larger earthquakes. Plain Language Summary: Volcano eruption forecasting is a challenging task that often requires the deciphering of processes underlying observed signs of volcanic unrest. As seismometers become common monitoring sensors on volcanoes, the recorded ground motion is valuable for scientists to study eruption precursors. Earthquakes are commonly observed and generally inferred to be associated with stress perturbations in the shallow crust. However, earthquakes with predominantly lower‐frequency energy are sometimes observed at depth and their origin is enigmatic. In this paper, we use the existing catalog of earthquakes at Akutan Volcano in Alaska between 2005 and 2017 as templates to successfully detect more earthquakes before locating them with higher precision. We find that earthquakes at Akutan Volcano tend to occur in swarms during times when the ground inflates due to magma accumulation beneath the volcano. Some earthquakes have predominantly low‐frequency energy which suggests a different source mechanism compared to regular earthquakes. Furthermore, the largest events are more strongly correlated with surface inflation. Therefore, we conclude that these lower‐frequency earthquakes are more directly related to unsteady magma movement through a complex pathway compared to regular earthquakes which represent fault rupture triggered by magma/fluid movement or larger earthquakes. Key Points: Moment release rates of deep long‐period events correlate more strongly with inflation episodes compared to volcano‐tectonic eventsAkutan deep long‐period earthquakes are likely due to non‐stationary source effects like unsteady magma transport through complex pathwaysAkutan volcano‐tectonic earthquakes represent fault ruptures triggered by magma/fluid movements or larger earthquakes [ABSTRACT FROM AUTHOR]

Recent climate and environment over the Tibetan Plateau (TP) have undergone significant changes, dominated by variations in the Westerlies and the Indian summer monsoon. However, long‐term shifts in atmospheric circulation during the transitional seasons are still lacking. Here we investigate the modern distribution of Tsuga pollen over the central‐western TP and confirm it as an indicator of variable atmospheric circulation in spring. By combining our Tsuga record from Taro Co with existing records in the Tsuga pollen source area, we suggest that a potential particle transport pathway from the southern slope of the Himalayas to the interior of the plateau appeared in the spring of the late Holocene. Our results show that the springtime atmospheric circulation over the southwestern TP during the early and late Holocene is closely related to the substantial remnants of ice sheets at northern high latitudes and the frequency of El Niño events, respectively. Plain Language Summary: The spring climate over the Tibetan Plateau (TP) is undergoing significant changes that yield profound impacts on environmental changes such as variations in vegetation phenology and alpine treeline. Knowledge of long‐term variations in atmospheric circulation during spring can improve the understanding of current climatic and environmental change and the projection of future variability. In this paper, we use an exotic pollen grain, which can be transported long distances in the air, as an indicator to trace the variability of spring atmospheric circulation over the TP. The results indicate that the spring meridional atmospheric circulation from the southern slope of the TP to its interior has been enhanced in the last four thousand years, which is mainly influenced by the frequency of El Niño events. Key Points: Spring atmospheric circulation change over the southwestern Tibetan Plateau (TP) during Holocene is reconstructedMeridional atmospheric circulation in spring over the southwestern TP is strengthened in the late HoloceneHigh‐(low‐) latitude forcing mainly influence the variations in spring atmospheric circulation during the early (late) Holocene [ABSTRACT FROM AUTHOR]

The quasi‐biennial oscillation (QBO) impacts the Madden‐Julian Oscillation (MJO) activity with a stronger MJO in QBO easterly (QBOE) than QBO westerly (QBOW) winters. However, this relationship is poorly represented in the current generation climate models. For the first time, this paper applies a stratospheric zonal‐mean nudging in a subseasonal prediction system to capture it. Two strong MJO cases in a QBO‐neutral winter are investigated. The QBO temperature and zonal wind anomalies are added separately as well as together to the stratosphere using nudging in MJO case hindcast. Only by nudging the QBO temperature anomalies while leaving the zonal wind free, can the prediction system capture the observed QBO‐MJO connection. The tropopause instability is found positively correlated to the MJO amplitude, but it cannot fully explain the captured connection. The free‐evolving zonal wind anomalies in the stratosphere due to the nudged QBO temperature are crucial for the captured connection. Plain Language Summary: The Madden‐Julian Oscillation (MJO) as a dominant subseasonal mode in the tropical troposphere is strongly modulated by the quasi‐biennial oscillation (QBO), which is the dominant interannual mode in the tropical stratosphere. However, such QBO‐MJO connection is missing in the long‐term simulations of all current climate models. Here, we show that by adding the QBO temperature signals into the MJO case simulation using the CESM2 subseasonal prediction system, the QBO‐MJO connection can be captured. Interestingly, the QBO temperature alone is not enough to explain the captured connection. The free‐evolving zonal wind generated by the nudged temperature is also crucial to capturing the QBO‐MJO relationship. These results provide a new perspective to understand the QBO‐MJO connection and improve its representation in climate models. Key Points: QBO‐MJO connection is captured in initialized MJO case studies by adding QBO temperature anomalies to the stratosphereThe free‐evolving zonal wind shear in the stratosphere is necessary to capture the QBO‐MJO connectionTropopause instability changes can not fully explain the captured QBO‐MJO connection [ABSTRACT FROM AUTHOR]

The diurnal cycle of precipitation and its associated land‐sea breeze circulation over coastal northern Australia are investigated using surface wind speed and enthalpy (latent and sensible heat) fluxes from the Cyclone Global Navigation Satellite System. Composite results during the austral summer of 2018–2022 show that diurnal precipitation propagates westward from the Cape York Peninsula into the Gulf of Carpentaria. The diurnal cycle of precipitation and surface wind speed are strongly coupled, with the wind speed maximum slightly leading the precipitation maximum. Additionally, analysis of the moist static energy budget shows that surface fluxes help support the westward‐propagation of diurnal precipitation. These results—based on novel satellite data and reanalysis—are consistent with previous studies that have examined the impact of the land‐sea breeze on the diurnal cycle of precipitation. The results provide a benchmark for model representation of this important atmosphere‐ocean‐land interaction. Plain Language Summary: The modulation of the diurnal cycle of precipitation over coastal northern Australia by surface winds was studied using the recently launched NASA Cyclone Global Navigation Satellite System (CYGNSS). Results show a strong connection between the daily cycles of precipitation and surface wind speed, with an offshore wind speed maximum leading the precipitation maximum by a few hours. Westward propagation of diurnal precipitation and wind speed signals occurs from the Cape York Peninsula into the Gulf of Carpentaria. A thermodynamic analysis supports the importance of the wind speed signal and associated surface fluxes for fostering offshore precipitation propagation. The novelty of this paper lies in the use of the new CYGNSS retrievals, which provide better space and time coverage of surface wind speed than previous satellite‐based measurements and are not as negatively affected by precipitation contamination. Key Points: Cyclone Global Navigation Satellite System wind speeds and latent heat fluxes are used to study the diurnal cycle over the northern Australia coastal regionEnhanced diurnal precipitation is associated with enhanced surface wind speed and convergenceDiurnal precipitation and surface wind speed signals propagate westward in tandem from the Cape York Peninsula [ABSTRACT FROM AUTHOR]

The typical subsolar stand‐off distance of Mars' bow shock is of the order of a solar wind ion convective gyroradius, making it highly non‐planar to incident ions. Using spacecraft observations and a test particle model, we illustrate the impact of the bow shock curvature on transient structures which form near the upstream edge of moving foreshocks caused by slow rotations in the interplanetary magnetic field (IMF). The structures exhibit noticeable decrease in the solar wind plasma density and the IMF strength within their core, are accompanied by a compressional shock layer, and are consistent with foreshock bubbles (FBs). Ion populations responsible for these structures include backstreaming ions that only appear within the moving foreshock and reflected ions with hybrid trajectories that straddle between the quasi‐perpendicular and quasi‐parallel bow shocks during slow IMF rotations. Both ion populations accumulate near the upstream edge of the moving foreshock which facilitates FB formation. Plain Language Summary: Planets in the solar system are continuously impacted by the solar wind, a plasma flow originating at the Sun and propagating through the interplanetary medium at high speeds. The solar wind also carries a magnetic field which at times contains twists or discontinuities. The discontinuities are associated with large scale electric currents that can have planar shapes. A planetary obstacle significantly modulate the solar wind plasma and the interaction of solar wind discontinuities with the modulated plasma upstream of the planet leads to formation of transient structures. Due to their relatively large size, these structures can significantly impact and destabilize plasma boundaries at lower altitudes closer to the surface. The results of this paper improve our understanding of solar wind interactions and formation of transient structures upstream of Mars. Key Points: Foreshock bubbles can form upstream of MarsSlow field rotations can cause foreshock bubbles while reflected ions from the quasi‐perpendicular bow shock contribute to their formationUnique ion kinetic scale processes exist around foreshock structures at Mars due to the different interaction size scale [ABSTRACT FROM AUTHOR]

This paper investigates the impacts of tropical Atlantic on El Niño‐Southern Oscillation (ENSO) predictability through an empirical dynamical model‐ Linear Inverse Model (LIM). By selectively including or excluding the coupling between tropical Atlantic and tropical Pacific in LIM, we find that the tropical Atlantic dynamics significantly improve the Eastern Pacific (EP)‐ENSO prediction and weaken the EP‐ENSO predictability barrier (PB), with the Equatorial Atlantic (EA) mode playing a more important role than the Tropical North Atlantic (TNA) mode. The tropical Atlantic impacts on Central Pacific (CP)‐ENSO predictability and PB are relatively smaller. Consistent with observations, the tropical Atlantic can weaken PB in most CMIP6 models. The evolution of the tropical Atlantic optimum initial structures confirms the important influence of the EA mode on the eastern tropical Pacific. Therefore, the tropical Atlantic dynamics, especially the EA mode, should be appropriately considered to improve the prediction of EP‐ENSO and weaken the EP‐ENSO PB. Plain Language Summary: El Niño–Southern Oscillation (ENSO) is one of the dominant climate modes in the tropical Pacific on interannual timescales, with significant global impacts. The tropical Atlantic sea surface temperature (SST) variability, especially in the tropical North Atlantic (TNA) and equatorial Atlantic (EA), is shown to have important impacts on ENSO variability. Using a linear dynamical model, we find that including the dynamics of the tropical Atlantic, especially the EA SST mode, can significantly improve the forecast skill of Eastern Pacific ENSO and weaken the EP‐ENSO spring predictability barrier (SPB), which is a sudden reduction of ENSO prediction skill in the boreal spring. However, Central Pacific (CP)‐ENSO's forecast skill is less influenced by the tropical Atlantic dynamic. Consistent with the observations, the topical Atlantic can weaken ENSO SPB in most CMIP6 climate models. Our results suggest that the tropical Atlantic dynamics should be appropriately considered to improve the forecast skill of EP‐ENSO. Key Points: A linear dynamical model is used to study the impact of the tropical Atlantic on El Niño‐Southern Oscillation (ENSO) predictabilityTropical Atlantic dynamics can improve Eastern Pacific (EP)‐ENSO predictability and weaken EP‐ENSO spring predictability barrier while the impact on Central Pacific‐ENSO is limitedThe Equatorial Atlantic mode plays an important role in the tropical Atlantic influence of EP‐ENSO prediction [ABSTRACT FROM AUTHOR]

Vertical motions across the ocean are central to processes, like CO2 fixation, heat removal or pollutant transport, which are essential to the Earth's climate. This work explores 3D conveyor routes associated with the Atlantic Meridional Overturning Circulation (AMOC). Our findings show the geometry of mixing structures in the upper and deep ocean layers by means of Lagrangian Coherent Structures. This tool identifies among others, zones linked to vertical transport and characterizes vertical transport time scales. We focus the study in two regions. The first one is the Flemish Cap region, a zone of interaction between the major AMOC components, where our analysis identifies a domain of deep waters that ascend very rapidly to the ocean surface. The second one is the Irminger Sea, where our analysis confirms the existence of a downwelling zone, and reveals a previously unreported upwelling connection between very deep waters and the ocean surface. Plain Language Summary: The Atlantic Meridional Overturning Circulation (AMOC) is a complex oceanic convective system involved in the tridimensional distribution of heat, carbon or nutrients. The horizontal transport across the AMOC is well studied, however, there are not many studies exploring mixing and transport processes across the vertical water column, that is, from the surface to the deep ocean and viceversa. To fully understand this 3D system, this paper follows a methodology that links apparently unrelated elements such as mixing and geometry. This approach is very promising to better characterize transport across the vertical column of major ocean currents and determining their impact on the global climate system. Key Points: Lagrangian Coherent Structures reveal the complex mixing geometry of 3D circulating watersSupported by this tool we are able to highlight upwelling and downwelling routes across the Atlantic Meridional Overturning CirculationThis approach seems promising to determine the role of vertical mixing on major ocean currents in the Earth's climate system [ABSTRACT FROM AUTHOR]

Heatwaves lead to catastrophic consequences on public health and the economy. Accurate and timely predictions of regional heatwaves can improve climate preparedness and foster decision‐making to alleviate the burdens due to climate change. In this paper, we propose a heatwave prediction algorithm based on a novel deep learning model, that is, Graph Neural Network (GNN). This new GNN framework can provide real time warnings of the sudden occurrence of regional heatwaves with high accuracy at lower costs of computation and data collection. In addition, its interpretable structure unravels the spatiotemporal patterns of regional heatwaves and helps to enrich our understanding of the general climate dynamics and the causal influences between locations. The proposed GNN framework can be applied for the detection and prediction of other extreme or compound climate events, which calls for future studies. Plain Language Summary: Understanding the occurrence and propagation of regional heatwaves is of vital importance to mitigate the consequence of heat extremes. A low‐cost, accurate, and timely prediction algorithm for regional heatwaves is desirable. In this study, we use the measurement data set collected from the ground weather stations, together with an advanced deep learning algorithm, to predict the occurrence of regional heatwaves in the lower 48 states of the U.S. The prediction model is trained by the daily weather observations from 91 weather stations and achieves over 90% accuracy in validation. In addition, we summarize the spatiotemporal patterns of the climate dynamics learned by the deep learning model. Our results align with the previous findings while providing a much finer resolution. The proposed modeling framework can be applied to predict other types of extreme events, such as extreme precipitation, drought, and compounded events. The analysis of the model structure will also enhance our understanding of the causal inference between climate regions in the U.S. from a brand‐new deep learning perspective. Key Points: Trained by the daily weather data from 91 ground stations over the CONUS, the Graph Neural Network (GNN) model can predict regional heatwaves with 90% accuracyWith the interpretable structure of GNN, we analyze the interplays among meteorological features and the connections between climate zonesFrom a deep‐learning perspective, the spatiotemporal patterns of climate dynamics are reproduced with a finer granularity [ABSTRACT FROM AUTHOR]

Predicting the occurrence of internal solitary‐like waves (ISWs) is important for parameterizing turbulent dissipation. ISWs in the northern South China Sea (SCS) are well understood to mainly originate from the Luzon Strait (LS). They are generally predictable due to their phase‐locked relation with tidal forcing in the LS. However, irregular occurrence of ISWs is noted from our 9‐day moored measurements on the SCS shelf. More energetic ISWs unexpectedly appeared during the neap tide than the spring tide. Their occurrence was related to subtidal shelf flows. The tidal and shelf flows had comparable magnitudes. We thus hypothesize that when the total flow meets critical conditions ISWs are generated locally over irregular seafloor. This hypothesis is supported by numerical experiments and validated by observations. Consequently, local generation of ISWs enhances both the intensity and complexity of internal wave field on the shelf, and thus the unpredictability of ISW occurrence. Plain Language Summary: Internal solitary‐like waves (ISWs) are ubiquitous in the ocean. They can propagate several hundreds of kilometers, and induce strong turbulence and mixing. Revealing generation mechanisms and propagation of ISWs is key to further investigating their influence on the distributions of energy dissipation, heat, nutrients, sediment, and pollutants, as well as the safety of submarine voyages and offshore drilling. In this paper, ISWs in the northern South China Sea (SCS), where the most energetic ISWs in the world's oceans are generated, are studied. It is widely accepted that these ISWs generally originate from the Luzon Strait (LS). They usually have regular occurrence, and are phase‐locked to tidal forcing in the LS. However, we present field measurements showing irregular occurrence of ISWs on the northern shelf of the SCS. This irregular occurrence is in striking contrast to the prominent predictability of ISWs originated from the LS. We reveal that the intermittent nature of the occurrence is due to the local generation of ISWs on the shelf; the coupling of different flow components seems to play a significant role in ISW generation on the shelf. The results reported here are expected to be applicable to other shelf regions of the world's oceans. Key Points: Field measurements documented large numbers of internal solitary‐like waves (ISWs) with irregular occurrence on the northeastern shelf of the South China SeaMore energetic and larger numbers of ISWs are observed during the neap tide than during the spring tide as modulated by shelf flowsISWs are locally generated due to the joint effects of tidal and subtidal shelf flows [ABSTRACT FROM AUTHOR]

To assess the impacts of the tropospheric ozone modulation due to El Niño on tides, we performed two simulations by using the Ground‐to‐topside Atmosphere Ionosphere model for Aeronomy model: zonally symmetric ozone density and longitudinal ozone variation of an El Niño pattern. Our results show that the ozone modulation strengthens (attenuates) the first symmetric mode of the DW2 (D0 and DW1) tide by ∼+5% (∼−1% and ∼−4%) from ∼20 to ∼100 km altitudes on average. These responses are probably attributed to the tropospheric ozone modulation. These responses are reasonable because both the ozone modulation and these first modes have a peak at the equator. Also, the vertical wavelengths of the first modes are about twice as long as the vertical thickness of the ozone modulation. Our results suggest that the ozone modulation due to El Niño influences the diurnal tides by a few percent because of the ozone modulation spatial structure. Plain Language Summary: Atmospheric absorption of solar radiation generates thermal tidal waves in the Earth's atmosphere, which propagate vertically into ∼100 km and influences the ionosphere and the thermosphere, that is, the edge of the atmosphere. Tides are strongly modulated by the climate and meteorological conditions. Previous studies show that equatorial tides are influenced by El Niño–Southern Oscillation (ENSO) because ENSO controls longitudinal variations of water vapor and rainfall. ENSO also modulates a longitudinal variation of tropospheric ozone density. Although tropospheric ozone is one of the well‐known greenhouse gases, its impact on tides over the equatorial region has not been highlighted. To assess this impact, we perform two simulations: zonally symmetric ozone density and ozone variation of El Niño pattern. This paper found that the ozone modulation of El Niño strengthens a diurnal tide with westward zonal wavenumber 2 (DW2) and attenuates diurnal tides with zonal wavenumber 0 (D0) and westward zonal wavenumber 1 (DW1). The spatial structure of the ozone modulation seems important for those diurnal tidal responses. Key Points: We investigated the impact of the ozone modulation due to El Niño on diurnal tides by using the Ground‐to‐topside Atmosphere Ionosphere model for Aeronomy modelThe tropospheric ozone modulation can influence the diurnal tides (D0, DW1, and DW2) averaged in one El Niño episode by 1%–5%The spatial structure of the ozone modulation could be important for the impacts on the diurnal tides [ABSTRACT FROM AUTHOR]

We examine coupling of fluctuations in the solar wind with electromagnetic ion cyclotron (EMIC) waves in the magnetosphere using an advanced full‐wave simulation code, Petra‐M. Dipole tilt dramatically affects the coupling process. While very little wave power can reach the inner magnetosphere without tilt effects, a tilted dipole field dramatically increases the efficiency of the coupling process. Solar wind fluctuations incident at high magnetic latitude effectively reaches the ground along the field line and mode‐convert to linearly polarized field‐aligned propagating waves at the Alfvén and IIH resonances. Therefore, solar wind compressions efficiently drive linearly polarized EMIC waves when the dipole angle is tilted toward or away from the Sun‐Earth direction. Plain Language Summary: The solar wind is emitted near the ecliptic planes, and solar wind pressure is one of the critical sources of wave activities in the Earth's magnetosphere. Since Earth's magnetic field is tilted to the ecliptic plane, compressed solar wind fluctuations can be incident over a wide range of magnetic latitudes depending on seasonal and diurnal variations. When the solar wind fluctuations reach Earth's magnetosphere, incoming wave energy can be linearly transferred to another wave mode, called a mode coupling. This paper examines the role of Earth's magnetic tilt on this mode coupling between solar wind fluctuation and electromagnetic ion cyclotron (EMIC) waves using an advanced full‐wave simulation code, Petra‐M. Without magnetic tilt, solar wind fluctuations incident at a low latitude are almost totally reflected. In contrast, solar wind fluctuations incident at high latitude can propagate efficiently into the inner magnetosphere and reach the ground. Compressional fluctuations can also be converted to linearly polarized, field‐aligned propagating waves. These results suggest that solar wind compressions can drive the linearly polarized EMIC waves and that the wave occurrence can have seasonal and diurnal dependence, which is expected to maximize at the maximum tilt of Earth's dipole into the Sun‐Earth direction. Key Points: Wave mode coupling of solar wind fluctuation to electromagnetic ion cyclotron (EMIC) waves is examined using an advanced 2D‐full wave simulation code, Petra‐MWhen the dipole magnetic field is tilted, compressional waves can reach the ground near the cusp region via mode conversionThe solar wind can drive linearly polarized EMIC waves via mode conversion more efficiently when the dipole field is tilted [ABSTRACT FROM AUTHOR]

There has been a long debate on the stable phase of iron under the Earth's inner core conditions. Because of the solid‐liquid coexistence at the inner core boundary, the thermodynamic stability of solid phases directly relates to their melting temperatures, which remains considerable uncertainty. In the present study, we utilized a semi‐empirical potential fitted to high‐temperature ab initio data to perform a thermodynamic integration from classical systems described by this potential to ab initio systems. This method provides a smooth path for thermodynamic integration and significantly reduces the uncertainty caused by the finite‐size effect. Our results suggest the hcp phase is the stable phase of pure iron under the inner core conditions, while the free energy difference between the hcp and bcc phases is tiny, on the order of 10 s meV/atom near the melting temperature. Plain Language Summary: The structure of Earth's solid inner core is a fundamental question in understanding the Earth's interior. Fe is the major element in the Earth's solid inner core, while its stable phase under inner core conditions is still under debate. The inaccuracy of present ab initio free energy calculations was too large to estimate the small free energy difference between different Fe phases, making this debate unsolved. In this paper, we developed a method to determine Fe's melting temperatures from ab initio calculations. This was achieved by utilizing a potential fitted to high‐temperature ab initio data and performing a thermodynamic integration from classical systems described by this potential to ab initio systems. This method significantly reduces the uncertainty caused by the finite size effect in the ab initio calculations. Using this method, we calculated the free energy difference and melting temperatures of hcp and bcc Fe under inner‐core boundary and center conditions. We show that the hcp phase is the stable phase of pure Fe throughout the inner core condition. However, the bcc and hcp phases show a very small free energy difference that may be altered by other elements in the inner core. Key Points: Thermodynamic integration with a potential fitted to high‐temperature ab initio data allows precise melting temperature determinationsHcp and bcc Fe's melting temperatures are computed at the same ab initio accuracy at 323 and 360 GPaBcc Fe is metastable while its free energy is only ∼10 meV/atom higher than the stable hcp phase under inner core conditions [ABSTRACT FROM AUTHOR]

Earth Observation Satellite (EOS)‐04 launched on 14 February 2022, carries a C‐band Synthetic Aperture Radar (SAR) for Agriculture, Forestry, Hydrology, and Flood mapping applications. In this paper, we have used C‐band SAR images and near‐simultaneous observations from the global precipitation measurements (GPM) to study the signatures of multiple convective rain cells. The bright patches are found on C‐band SAR imagery, which depicts the information of hydrometeors such as graupels or hails in the melting layer. For the first time, unambiguously estimated the diameter of the convective core rain cells from the C‐band SAR backscattered signal and compared with near‐simultaneous observations from GPM‐GMI and Ku‐band radar to confirm our findings. Thus, the present study demonstrates the potential of C‐band SAR for identifying the signatures of convective rain cells. Plain Language Summary: Earth Observation Satellite‐04 launched on 14 February 2022, carries a C‐band Synthetic Aperture Radar (SAR) for Agriculture, Forestry, Hydrology, and Flood mapping applications. The Convective clouds produce a significant amount of precipitation, and the signature of convective rain cells appeared as bright oceanic patches on the SAR imagery. The present study demonstrates the potential of C‐band SAR for the signatures of convective rain cells and near‐simultaneous observations from global precipitation measurements to confirm our findings. Key Points: The present study brought out the signatures of convective rain cells using C‐band Synthetic Aperture Radar (SAR) onboard Earth Observation Satellite ‐04 satelliteFor the first time, unambiguously estimated the diameter of the convective core rain cells from the C‐band SAR backscattered signalWe compared near‐simultaneous observations from GPM‐GMI and Ku‐band precipitation radar to confirm the present findings [ABSTRACT FROM AUTHOR]

Whistler‐mode waves can modulate the electron distributions through wave‐particle interactions in the Earth's inner magnetosphere. In this paper, we have investigated the effects of the whistler waves on the electron distributions by a combination of electron magnetohydrodynamics and test‐particle simulations. Our simulations show a parallel whistler wave becomes gradually oblique during its propagation from the magnetic equator to higher latitudes. Such a whistler wave causes distinct flux increases of tens of keV electrons at large pitch angles to form electron butterfly distributions. This whistler wave traps numerous resonant electrons and transports them from lower latitudes to higher latitudes through nonlinear Landau trapping. Meanwhile, these trapped electrons are efficiently accelerated to higher energies contributing to the rapid formation of electron butterfly distributions at higher latitudes. Our study suggests nonlinear Landau trapping is a potential formation mechanism of the electron butterfly distributions observed in the Earth's inner magnetosphere. Plain Language Summary: Wave‐particle interactions are known to play an important role in regulating energetic electron distributions in the Earth's inner magnetosphere. In particular, whistler‐mode waves can efficiently modulate the electron distributions by cyclotron and Landau resonant interactions. In this paper, we used a 2‐D electron magnetohydrodynamics model to simulate a whistler wave in the dipole geomagnetic field. The simulation results indicate a parallel whistler wave becomes gradually oblique during its propagation from the magnetic equator to high latitudes. Furthermore, a test‐particle simulation shows that such a whistler wave can trap and accelerate electrons from the lower latitudes to higher latitudes by nonlinear Landau trapping, which leads to the rapid formation of electron butterfly distributions at tens of keV. Previous studies suggested that nonlinear cyclotron trapping can cause electron butterfly distributions at tens of keV observed in the Earth's magnetosphere. Our study provides a new potential formation mechanism of electron butterfly distributions in the Earth's magnetosphere. Key Points: Effects of whistler waves on electron distributions have been studied by a combination of electron magnetohydrodynamics and test‐particle simulationsA whistler wave causes distinct flux enhancements of 10s keV electrons at large pitch angles to form electron butterfly distributionsNonlinear Landau trapping contributes to the rapid formation of the electron butterfly distributions [ABSTRACT FROM AUTHOR]

Life on Earth appears to have evolved under the influence of supernovae activity in the solar neighborhood. Supernovae frequency regulates the flux of cosmic ray particles arriving at the top of the Earth's atmosphere, where empirical evidence supports a close connection between cosmic rays, clouds, and climate. Burial of organic matter in marine sediments follows cosmic rays variations for more than 3.5 Gyr and in detail during the last 500 Myr. The supernovae link to the burial of organic matter may be due to climate‐induced changes in the atmospheric and oceanic circulation affecting the availability of nutrients and the bioproductivity in the oceans. A higher bioproductivity then leads to a more extensive burial of organic matter. Support for this scenario comes from a proxy of nutrient concentrations in the ocean which covaries with the supernovae frequency. The results suggest a fundamental connection between supernovae rates and life on Earth. Plain Language Summary: The study proposes a surprising link between the burial of organic matter in sediments and stellar processes. The paper has two components; the first component concerns empirical evidence: A close correlation between the fraction of organic matter buried in sediments and changes in supernovae frequency. This correlation is evident during the last 3.5 billion years (see Figure 2) and in close detail over the previous 500 Myr (see Figure 4). All astrophysical data, for example, changes in star formation and the inferred changes in supernovae frequency, are based on peer‐reviewed works, as is the carbon 13 data. The second component of the paper gives a possible justification for the observed correlations. The assumption is that changes in supernovae frequency result in climate change, which changes the mixing in oceans and river runoff and ultimately influences nutrient availability—a high nutrient concentration results in a larger bioproductivity and a larger burial of organic matter in sediments. Again empirical evidence for this connection comes from concentrations of trace elements in pyrite (a proxy of ocean nutrient concentrations) which correlate with supernovae frequency changes over the previous 500 Myr (see Figure 5). Key Points: The fraction of organic carbon buried in marine sediments covaries with changes in supernova frequency over most of the Earth's historySupernovae influence Earth via a link between atmospheric ionization, clouds and climate. High cosmic ray flux leads to a cold climateClimatic induced ocean circulation regulates nutrients to the biosphere. A larger bioproductivity results in increased organic matter burial [ABSTRACT FROM AUTHOR]

Downstream of Cape Hatteras, the Gulf Stream (GS) is bounded to the north by a sharp temperature front known as the North Wall (NW). Previous studies have generally assumed that variations of the NW and GS are equivalent. Using satellite sea surface height to identify the GS and the 15 °C isotherm at 200‐m depth to represent the NW, this paper examines their similarities and differences during 1993–2016. The NW and GS are geographically close and vary similarly only to the west of 71°W. Downstream of that, they rapidly diverge—and the variances of their latitudes increase by more than a factor of 2—as the GS flows past the New England Seamounts. Evidence is presented to show that the difference in properties of the NW and the GS is related to the presence of mesoscale eddies in the region separating them. Plain‐Language Summary: To the east of Cape Hatteras where the Gulf Stream leaves the continental shelf, a sharp gradient in ocean temperature, known as the North Wall, is seen just north of the current. Fluctuations in the position of the North Wall have large impacts in the distribution of heat in the North Atlantic and on plankton populations. There is a widespread assumption in the literature that variations of the North Wall and the Gulf Stream are similar. Results presented in this paper show that variations of these two features become increasingly different as the current proceeds eastward and the divergence in their properties is due to the presence of turbulent eddies, which arise after the Gulf Stream interacts with the New England Seamounts. These results suggest that recognition of distinct behaviors of the Gulf Stream and the North Wall is needed for a better understanding of their dynamics and their impact on climate. Key Points: The Gulf Stream and its North Wall are close to each other west of 71°W but depart sharply in their properties farther downstreamDifferences in the behaviors of the Gulf Stream and the North Wall are related to mesoscale eddies between themRecognition of distinct properties of Gulf Stream and the North Wall is necessary for understanding their dynamics and impact on climate [ABSTRACT FROM AUTHOR]

Several experiments and field observations have demonstrated seismic waves‐induced permeability increases. However, a few experiments have revealed significant decreases in the permeability of fractured sandstone after shaking produced by dynamic stresses. In this paper, we analyze the tidal responses of the water level in two wells (Junan and Dingyuan) situated on and near, respectively, the Tan‐Lu fault in China. The results sampled by a single well indicate that seismic waves could both increase and decrease the permeability. In addition, our results indicate that the earthquakes that decreased the permeability in a single well were distributed in certain azimuthal zones. Given the azimuthal distribution of distant earthquakes, the observed permeability decrease could be attributed to the seismic wave‐induced clogging of fractures that compose the flow paths in the shallow crust. Our research contributes to an enhanced overall understanding of the effects of earthquakes on the permeability of the shallow crust. Plain Language Summary: Most hydrological phenomena caused by earthquakes can be explained by permeability changes. Many experiments and field observations have demonstrated coseismic increases in permeability. However, coseismic decreases in permeability have seldom been reported. In this paper, we analyze the tidal responses of the water level in two wells. The results indicate that seismic waves could both increase and decrease the permeability. In addition, we find that the earthquakes that decreased the permeability in a single well were distributed in certain azimuthal zones. Given the azimuthal distribution of distant earthquakes, the observed permeability decrease could be attributed to the seismic wave‐induced clogging of fractures that compose the flow paths in the shallow crust. Furthermore, we discuss the possible process of particle remobilization in a natural fracture network caused by seismic waves, as this process may be used to explain the azimuthal distribution of earthquakes that decreased the permeability. Our research contributes to an enhanced overall understanding of the effects of earthquakes on the crustal permeability. In the future, earthquake‐induced increases and decreases in permeability should be considered when discussing controls on permeability for exploring underground resources of groundwater, geothermal energy, oil and gas, and for safely storing nuclear and toxic waste underground. Key Points: Seismic waves could decrease besides increase the crustal permeabilityDecreases in permeability may depend on the earthquake azimuthThe decrease in the permeability could be attributed to the clogging of fractures [ABSTRACT FROM AUTHOR]

The Cassini Orbiter made the first in situ measurements of the upper atmosphere and ionosphere of Saturn in 2017. The Ion and Neutral Mass Spectrometer (INMS) found molecular hydrogen and helium as well as minor species including water, methane, ammonia, and organics. INMS ion mode measurements of light ion species (H+, H2+, H3+, and He+) and Radio and Plasma Wave Science instrument measurements of electron densities are presented. A photochemical analysis of the INMS and Radio and Plasma Wave Science data indicates that the major ion species near the ionospheric peak must be heavy and molecular with a short chemical lifetime. A quantitative explanation of measured H+ and H3+ densities requires that they chemically react with one or more heavy neutral molecular species that have mixing ratios of about 100 ppm. Plain Language Summary: Solar ultraviolet radiation is absorbed by Saturn's upper atmosphere and produces electrically charged molecules (or ions) and electrons in a region called the ionosphere. The Ion and Neutral Mass Spectrometer (INMS) instrument onboard the Cassini Orbiter made the first direct measurements of the composition of Saturn's ionosphere when the Cassini spacecraft flew through the upper atmosphere in the equatorial region. This paper reports on INMS measurements of the abundances of ionospheric protons, charged molecular hydrogen and helium, and protonated molecular hydrogen as functions of altitude and latitude. The paper also describes a photochemical analysis of the data indicating the presence in the upper atmosphere of heavy molecular species such as water, methane, and ammonia, thought to come from the rings. The deduced mixing ratio of these species relative to hydrogen is about a part in 10,000, which is consistent with measurements made using the neutral mode of the INMS. The simple chemical analysis was also able to reproduce the densities of electrons measured by the Radio and Plasma Wave Science instrument onboard Cassini. This paper contributes to our growing appreciation of the strong effect that the rings have on Saturn's atmosphere. Key Points: A heavy molecular species in Saturn's atmosphere is needed to explain the measured ion compositionMeasured ion and electron densities in the main Saturn ionosphere can be explained by solar ionizing radiationBoth neutral and plasma measurements made at Saturn by Cassini suggest a strong ring‐atmosphere interaction [ABSTRACT FROM AUTHOR]

The European Space Agency Earth Explorer mission Aeolus with the first spaceborne Doppler Wind Lidar onboard provides global coverage of wind profiles twice per day. This paper discusses the impact of assimilating Aeolus winds on the quality of tropical analyses and forecasts using the observing system experiments of the European Centre for Medium‐Range Weather (ECMWF). Presented examples show that Aeolus wind profiles bring changes to the Kelvin wave structure in the layers with a significant vertical shear during the easterly phase of the quasi‐biennial oscillation in the period May to September 2020. Comparing Kelvin waves in analyses and forecasts with and without Aeolus winds, it is argued that improved ECMWF forecasts in the tropical tropopause layer are due to vertically propagating Kelvin waves. Plain Language Summary: The tropics are the region with the largest uncertainties in the initial states for numerical weather prediction, called analyses. Analysis uncertainties are largest in the tropical upper troposphere and the lower stratosphere (UTLS). One of the reasons is a lack of wind profiles which are more useful than temperature profiles in the tropics. This classical effect was described by Smagorinsky as "Not all data are equal in their information‐yielding capacity. Some are more equal than others." ESA's ongoing Aeolus mission provides the first global wind profile observations from space. Despite their small number and relatively large random error, Aeolus winds have a positive impact on the quality of global weather forecasts, especially in the UTLS. In this paper, we discuss the impact of the Aeolus winds in UTLS focusing on the vertically propagating Kelvin waves, which are a major contributor to tropical variability. Several case studies are presented using the ECMWF model and data assimilation with and without Aeolus winds. The studied period May to September 2020 was characterized by a weakening easterly phase of the quasi‐biennial oscillation (QBO). Results suggest that a stronger impact of Aeolus winds in May than later in summer was associated with the QBO and the background flow. Key Points: Impact of assimilating Aeolus winds in the ECMWF system from May to September 2020 is coupled to the easterly QBO phaseAeolus assimilation modifies the representation of vertically propagating Kelvin waves in the tropical UTLSForecast improvements in May 2020 could be associated with the alteration in the upward‐propagating Kelvin waves [ABSTRACT FROM AUTHOR]

This paper evaluates sea surface temperature (SST) biases of coupled models participating in Coupled Model Intercomparison Project Phase 5 (CMIP5) and CMIP6. Overall, CMIP6 models perform better than CMIP5 ones in reproducing SST climatology, with lower multi‐model ensemble mean (MME) globally averaged absolute bias (1.17 vs. 1.31 K). MME bias in global mean annual SST shifts from cooling (−0.09 ± 0.52 K) to warming (0.23 ± 0.60 K). Regionally, in CMIP6 cooling biases over the Northwest Pacific and North Atlantic are reduced by 20% and 18%, while warming biases over the Northeast Pacific, Southeast Atlantic and Southern Ocean are increased by 25%, 16% and 107% respectively. These changes are mainly attributed to the combined effects from aggravated positive (or alleviated negative) bias in clear‐sky surface downward longwave radiation, and alleviated negative bias in cloud radiative effect, partially reduced by enhanced cooling bias in clear‐sky surface downward shortwave radiation. Plain Language Summary: As the primary approach to projecting future climate change, state‐of‐the‐art climate models still suffer pronounced biases in climatological annual mean sea surface temperature (SST), such as cold biases over the Northwest Pacific and North Atlantic, and warm biases over the Northeast Pacific, Southeast Pacific, Southeast Atlantic and Southern Ocean. We have evaluated the changes in mean‐state SST biases between the Coupled Model Intercomparison Project Phase 5 (CMIP5) and CMIP6. CMIP6 models perform better in reproducing SST climatology with lower absolute bias, which is attributed to the process‐level improvement. Overall, annual global mean SST bias shifts from cold (−0.09 ± 0.52 K) to warm (0.23 ± 0.60 K), which is mainly due to the regionally alleviated cooling biases or aggravated warming biases. This warmer shift is contributed by the increased positive (or decreased negative) bias in clear‐sky surface downward longwave radiation and decreased negative bias in cloud radiative effect. Key Points: Coupled Model Intercomparison Project Phase 6 (CMIP6) models perform better than CMIP5 ones, with significantly lower global‐mean absolute bias in annual sea surface temperature (SST)Global‐mean SST bias is with a warmer shift (+0.32 K) in CMIP6, with salient regional cold biases alleviated and warm biases aggravatedReduced cold bias in cloud radiative effect and positive change in bias in clear‐sky surface downward longwave together account for the shift [ABSTRACT FROM AUTHOR]

The PM2.5 (fine particulate matter with diameter ≤2.5 μm) concentration anomalies over the North China Plain (NCP) in early and late winter sometimes show a subseasonal reversal, which brings a great challenge for precise control of air pollution, and mechanisms are not well understood. This paper reveals the key role of the Barents Sea sea‐ice in this reversal. In early winter, a negative Scandinavian‐like pattern causes an anticyclonic anomaly over Northeast Asia and thus leads to the positive PM2.5 concentration anomaly over the NCP. In addition, anomalous warm advection associated with the positive North Atlantic Oscillation‐like pattern accelerates winter sea‐ice loss in the Barents Sea, especially in late winter, which increases the surface turbulent heating flux. These heating causes a negative Polar/Eurasian‐like pattern that induces a cyclonic anomaly over Northeast Asia and eventually leads to a negative PM2.5 concentration anomaly over the NCP in late winter. Vice versa. Plain Language Summary: In this study, we find that there is a significant seesaw pattern of PM2.5 concentration anomalies over the North China Plain (NCP) in early and late winter. Further results reveal that variations in Barents Sea sea‐ice play a crucial role. More specifically, a negative (positive) Scandinavian‐like pattern in early winter tends to induce an anticyclonic (a cyclonic) anomaly over Northeast Asia. This anticyclonic (cyclonic) anomaly leads to a higher (lower) PM2.5 concentration anomaly over the NCP in early winter. In addition, a North Atlantic Oscillation‐related warm (cold) temperature advection promotes (inhibits) the sea‐ice loss in the Barents Sea, especially in late winter. The Barents Sea‐ice loss (increment) in late winter tends to heat (cool) the lower‐level atmosphere and thus triggers an anticyclonic (a cyclonic) response in the upper troposphere there. The anticyclonic (cyclonic) response in the Barents Sea region as a part of the negative (positive) Polar/Eurasian‐like pattern induces a cyclonic (an anticyclonic) anomaly over Northeast Asia, which leads to a lower (higher) PM2.5 concentration anomaly over the NCP in late winter. These findings could be of great value for the subseasonal predictions of PM2.5 concentrations over the NCP in winter. Key Points: PM2.5 concentration anomaly over the North China Plain shows an visibly subseasonal reversal in early and late winter on interannual scaleThe Scandinavia‐like pattern and Polar/Eurasian‐like pattern are responsible for the seesaw pattern of PM2.5 concentration anomaliesThe sea‐ice variations in the Barents Sea play a key role in modulating such atmospheric circulations [ABSTRACT FROM AUTHOR]

By constructing an empirical model of the spectral and latitudinal distribution of ion cyclotron waves on the basis of Cassini datasets, we investigate the resonant interactions between ion cyclotron waves and radiation belt electrons at Saturn. Calculations based on quasi‐linear bounce‐averaged diffusion coefficients show that at Saturn ion cyclotron waves can efficiently pitch angle scatter >∼1 MeV to tens of MeV electrons into the loss cone thereby inducing precipitation loss, while the mixed and momentum scattering effects are typically negligible. The resultant electron loss timescales range from a few to tens of minutes, which in fact decrease significantly with increasing L‐shell at L = 4–6. We also find that the kinetic effects introduced by pick‐up ring particles cause distinct changes in pitch angle scattering efficiency for lower energy electrons (<3 MeV at L = 5). Our results demonstrate that ion cyclotron waves play a significant role in the dynamics of Saturn's radiation belt electrons. Plain Language Summary: Ion cyclotron waves are a common electromagnetic wave mode in the planetary magnetospheres. At Saturn, ion cyclotron waves are usually observed with wave frequencies near the gyro‐frequency of water‐group ions (e.g., O+, OH+, and H2O+). They are known to be excited by a ring distribution of the pick‐up water‐group ions which are extracted from the extended neutral clouds. In this paper, we investigate the resonant interactions between ion cyclotron waves and radiation belt electrons at Saturn. By constructing an empirical model of the spectral and latitudinal distribution of ion cyclotron waves based on Cassini observations, we calculate the bounce‐averaged electron diffusion coefficients and resultant electron loss timescales. Our results suggest that Saturn's ion cyclotron waves can cause efficient precipitation loss of radiation belt electrons by scattering them into the loss cone. The corresponding loss timescales range from a few to tens of minutes, decreasing with increasing radial distance from Saturn. Our results confirm the important role of ion cyclotron waves in the dynamics of Saturnian radiation belt electrons. Key Points: The resonant interactions between ion cyclotron waves and radiation belt electrons at Saturn are investigatedIon cyclotron waves can efficiently pitch angle scatter >∼1 MeV to tens of MeV electrons into the loss cone for precipitation lossThe resultant electron loss timescales range from a few to tens of minutes, which decrease significantly with increasing L‐shell over L = 4–6 [ABSTRACT FROM AUTHOR]