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Science, Volume 384, Issue 6700, June 2024.

Science, Volume 384, Issue 6700, June 2024.

Science, Volume 384, Issue 6700, June 2024.

Science, Volume 384, Issue 6700, June 2024.

Science, Volume 384, Issue 6700, June 2024.

Abstract

Relativistic electron fluxes in the outer radiation belt exhibit significant variability during geomagnetic storms and substorms. This study investigates rapid relativistic electron enhancements (REE) in the outer radiation belt throughout the entire Van Allen Probes (RBSP) era from October 2012 to October 2019. Utilizing RBSP measurements, we identify 182 rapid REE events characterized by a factor of greater than two increase in relativistic electron fluxes within a half RBSP orbit (approximately 4.5 hr) at L = 4.5–5.5. Approximately 76% of rapid REE events occur during geomagnetic storms. Rapid REEs during storms are concentrated within the 12-hr period preceding and the 24-hr period following the end of the storm's main phase. Intense REE are more likely found in storm's main phase compared to moderate REE. Sub-relativistic and relativistic electron injections are commonly observed during rapid REE. Substorm activities (AL/AE, MPB index) and convection (AU index) are more intense before and during REE, in contrast to the intervals following REEs. The intensity of rapid REE correlates with the strength of substorms and convection. This comprehensive survey suggests that rapid REEs in the outer radiation belt are likely associated with, but not strictly tied to, geomagnetic storms. Enhanced convection and substorm appear to create favorable conditions for rapid REE. These substorms and enhanced convection are likely linked to favorable solar wind conditions for REEs, as documented in previous studies.

Abstract

The generation of medium-scale traveling ionospheric disturbances (MSTIDs) in the mid-latitude F region ionosphere, particularly in the presence of sporadic E (Es) layers or geomagnetically conjugate features, has been the focus of extensive investigation using both observational and numerical modeling approaches. Recent observations have revealed the occurrence of nighttime MSTIDs over the continental US during storm conditions even without invoking the Es instability. While this phenomenon is considered to be electrified and likely associated with the Perkins instability, the influences of storm-enhanced density (SED), electric fields, and winds on the excitation of nighttime MSTIDs remain a complicated issue and require further quantitative analysis. In this study, we develop a two-dimensional numerical model of the nighttime ionospheric electrodynamics at midlatitudes using the ionospheric ion continuity equation and the electric field Poisson equation to investigate the characteristics of MSTIDs in the SED base region during storm conditions. We demonstrate that the magnetic inclination effect can explain the lower latitude preference of the MSTIDs during magnetic storms, while the development of MSTIDs is primarily influenced by intense storm electric fields under the background ionospheric condition of large density gradients associated with SED. However, the impact of neutral winds on the MSTIDs growth varies, depending on their specific direction determined by the strongly dynamic spatiotemporal variation of the thermosphere and ionosphere during storms. Therefore, the MSTIDs stormtime scenario results from a combination of multiple important factors.

Abstract

The ionospheres of Jupiter's icy moons have been observed by in situ plasma measurements and radio science. However, their spatial structures have not yet been fully characterized. To address this, we developed a new ray tracing method for modeling the radio occultation of the ionospheres using Jovian auroral radio sources. Applying our method to Jovian auroral radio observations with the Galileo spacecraft, we derived the electron density of the ionosphere of Ganymede and Callisto. For Ganymede's ionosphere, we found that the maximum electron density on the surface was 76.5–288.5 cm−3 in the open magnetic field line regions and 5.0–20.5 cm−3 in the closed magnetic field line region during the Galileo Ganymede 01 flyby. The difference in the electron density distribution was correlated with the accessibility of Jovian magnetospheric plasma to the atmosphere and surface of the moons. These results indicated that electron-impact ionization of the Ganymede exosphere and sputtering of the surface water ice were effective for the producing Ganymede's ionosphere. For Callisto's ionosphere, we found that the densities were approximately 350 and 12.5 cm−3 on the night side hemisphere during Callisto 09 and 30 flybys, respectively. These results combined with previous observations indicated that atmospheric production through sublimation controlled the ionospheric density of Callisto. This method is also applicable to upcoming Jovian radio observation data from the Jupiter Icy Moon Explorer, JUICE.

Abstract

To ensure the robustness of both civilian and military infrastructure, it is important to protect electric power grids, smart grids, and other electrotechnologies from known and possibly as-of-yet unknown space weather hazards. Space weather can generate intense geoelectric fields at the surface of the Earth, as well as large voltage gradients across long distances of the Earth. These voltage gradients can lead to geomagnetically induced currents (GICs), which are known to produce hazards to electric power grids. The finite-difference time-domain (FDTD) method is a powerful and versatile method that has already been applied to the study of geoelectric fields. The advantages of FDTD over other methods are that it can account for more geometrical complexities and realistic time waveforms and that it directly solves for geoelectric fields. Snell's Law predicts that any electromagnetic waves incident on the ground should essentially propagate straight downwards into the low resistivity ground. For this reason, vertical FDTD grid resolutions of 1/3 of a skin depth were usually chosen, while the horizontal grid resolution was relaxed. We find, however, that there is another important consideration for choosing an FDTD grid resolution applied to real-world scenarios: localized field variations due to currents generated by ground features. It turns out the grid resolution requirements are much stricter when taking this physics into account.

Addressing class imbalance in soil movement predictions
Praveen Kumar, Priyanka Priyanka, Kala Venkata Uday, and Varun Dutt
Nat. Hazards Earth Syst. Sci., 24, 1913–1928, https://doi.org/10.5194/nhess-24-1913-2024, 2024
Our study focuses on predicting soil movement to mitigate landslide risks. We develop machine learning models with oversampling techniques to address the class imbalance in monitoring data. The dynamic ensemble model with K-means SMOTE (synthetic minority oversampling technique) achieves high precision, high recall, and a high F1 score. Our findings highlight the potential of these models with oversampling techniques to improve soil movement predictions in landslide-prone areas.

Regional modelling of extreme sea levels induced by hurricanes
Alisée A. Chaigneau, Melisa Menéndez, Marta Ramírez-Pérez, and Alexandra Toimil
Nat. Hazards Earth Syst. Sci. Discuss., https//doi.org/10.5194/nhess-2024-100,2024
Preprint under review for NHESS (discussion: open, 0 comments)
Tropical cyclones drive extreme sea levels, causing large storm surges due to low atmospheric pressure and strong winds. This study explores factors affecting the numerical modelling of storm surges induced by hurricanes in the tropical Atlantic. Two ocean models are compared and used for sensitivity experiments. ERA5 atmospheric reanalysis forcing generally improves storm surge estimates compared to parametric wind models. Including ocean circulations reduces errors in storm surge estimates.

Science, Volume 384, Issue 6700, June 2024.

Abstract

Stratified turbulence (ST) has been proposed as a model for the dynamics of the mesosphere-lower thermosphere (MLT) region. This theory postulates that for horizontal mesoscales (∼1–400 km), the kinetic energy of horizontal winds dissipates from large to small scales with an approximately mean constant rate. In this investigation, dissipation rates are quantified using meteor-radar observations conducted in Northern Norway. The observed seasonal variability of dissipation rates exhibits maxima during the summer and winter, and minima near the equinoxes, between 80 and 95 km altitude. The results are compared with model predictions and earlier medium frequency radar, rocket, lidar, and satellite observations of MLT turbulence. The findings suggest that multi-static meteor radar measurements of ST can provide a novel way to continuously monitor turbulent dissipation rates in the MLT region.

Abstract

Accurately measuring emission factors (E s ) of biogenic volatile organic compounds (BVOCs) with consideration of intraspecific variability is vital but often overlooked. This study presents in-situ measurements of BVOC emissions from 114 Eucalyptus urophylla individuals using the LI-6800 portable photosynthesis system. We observed intraspecific variability exceeding an order of magnitude in BVOC E s . Despite this variability, our approach yielded statistically representative E s for E. urophylla, yet challenging the feasibility of extensive field measurements. By quickly screening net photosynthesis rate (P n) across a broad set of individuals and selecting those within a specific P n range, such as mean ± 0.1 × SD (standard deviation) of P n for all screened individuals, for detailed BVOC emission measurements, we achieved comparable mean E s with approximately 10% of the original sampling effort. This offers a practical solution for efficient and accurate field measurement of representative BVOC E s , significantly reducing required sample size while effectively addressing intraspecific variability.

Abstract

On 15 January 2022, the largest eruption of the Hunga-Tonga volcano in recorded history produced a plume registered by multi-parametric instruments around the world. However, the far-field hydrogeological responses to Lamb waves from this eruption remain underexplored. We studied the responses of groundwater to the volcanic eruption in the far-field over 8,700 km, including 274 wells. Results show that the Lamb waves with a speed of 316 m/s affects the groundwater system, leading to similar fluctuations in well water level (WL) and opposite phase fluctuation in borehole strain. Different wells exhibit diverse responses in WL amplitudes, possibly for heterogeneities in local aquifer systems. Gain values of 5 wells that simultaneously measure atmospheric pressure, borehole air pressure, borehole strain and WL are consistent with results obtained through cross-power spectrum estimation. This work demonstrates a novel response in far-field groundwater systems induced by Lamb waves and expects application for aquifer parameter estimation.

Abstract

The diurnal variability of mixed layer (ML) overturning instabilities remains poorly understood due to the challenge in capturing their rapid evolutions across large spatiotemporal ranges. Using high-resolution data from 52 gliders in the South China Sea, we examine the diurnal modulations of ML overturning instabilities. The results of the 3-month field observation show that negative potential vorticity occupies ∼16% of the ML and facilitates several types of forced overturning instabilities, especially symmetric instability (SI). Surface heat fluxes are identified to primarily modulate the diurnal variability of these overturning cells, where nighttime surface cooling is found to energize SI with an ∼2-hr phase lag. As a result, over 60% of forced submesoscale overturning cells tend to restratify the ML at night. These findings quantitatively highlight the modulation of diabatic atmospheric forcing in submesoscale restratification, which should be considered in submesoscale parameterizations of ocean and climate models.

Abstract

Cloud organization impacts the radiative effects and precipitation patterns of the cloud field. Deviating from randomness, clouds exhibit either clustering or a regular grid structure, characterized by the spacing between clouds and the cloud size distribution. The two measures are coupled but do not fully define each other. Here, we present the deviation from randomness of the cloud- and void-chord length distributions as a measure for both factors. We introduce the LvL representation and an associated 2D score that allow for unambiguously quantifying departure from well-defined baseline randomness in cloud spacing and sizes. This approach demonstrates sensitivity and robustness in classifying cloud field organization types. Its delicate sensitivity unravels the temporal evolution of a single cloud field, providing novel insights into the underlying governing processes.

Abstract

Summertime Greenland blocking (GB) can drive melting of the Greenland ice sheet, which has global implications. A strongly increasing trend in GB in the early twenty-first century was observed but is missing in climate model simulations. Here, we analyze the temporal evolution of GB in nearly 500 members from the CMIP6 archive. The recent period of increased GB is not present in the members considered. The maximum 10-year trend in GB in the reanalysis, associated with the recent increase, lies almost outside the distributions of trends for any 10-year period in the climate models. GB is shown to be partly driven by the sea surface temperatures and/or sea ice concentrations, as well as by anthropogenic aerosols. Further work is required to understand why climate models cannot represent a period of increased GB, and appear to underestimate its decadal variability, and what implications this may have.

Abstract

Transient creep in olivine aggregates has been studied by stress-relaxation experiments at pressures of 1.7–3.6 GPa and at temperatures of ≤1020 K in a DIA apparatus. Time-dependent deformation of olivine at small strains (<0.07) was monitored with an ∼1 s of time resolution using a combination of a high-flux synchrotron X-ray and a cadmium telluride imaging detector. The observed deformation was found to follow the Burgers creep function with the transient relaxation time ranging from 50 (±20) to 1,880 (±750) s. We show that the Burgers creep for olivine cannot account for the low viscosities in early post-seismic deformation reported by geodetic observations (<7 × 1017 Pa·s). In contrast, the time-dependent increase in viscosity observed in late post-seismic deformation (1018−1020 Pa·s) is explained by the Burgers rheology, suggesting that the combination of the Burgers model and another model is needed for the interpretation of post-seismic deformation.

Abstract

Dense and broad-coverage ocean-bottom observation networks enable us to obtain near-fault displacement records associated with an offshore earthquake. However, simple integration of ocean-bottom strong-motion acceleration records leads to physically unrealistic displacement records. Here we propose a new method using a Kalman filter to estimate coseismic displacement waveforms using the collocated ocean-bottom seismometers and pressure gauges. First, we evaluate our method using synthetic records and then apply it to an offshore Mw 6.0 event that generated a small tsunami. In both the synthetic and real cases, our method successfully estimates reasonable displacement waveforms. Additionally, we show that the computed waveforms improve the results of the finite fault modeling process. In other words, the proposed method will be useful for estimating the details of the rupture mechanism of offshore earthquakes as a complement to onshore observations.