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Abstract

Positive leaders branch less frequently than negative counterpart, and the physical processes and properties of positive leader branching remain a mystery. We investigated 10 m laboratory discharges under four positive voltages using a high-speed video camera. Positive leaders differ from negative leaders by either directly splitting or connecting with floating bidirectional leaders to form branching, and the number of leader branches shows a positive correlation with the applied voltage, that is, the branched channels increased from 1 to 4 when the voltage increased by a factor of 1.5. Grounding points are positioned beneath the electrode and are more concentrated with lower voltage. During the stable progression of the leader, there is a slight increase in its development speed as the applied voltage rises. When the voltage is increased by 70%, the average breakdown time decreases by 40%. These characteristics provide insights into the branching mechanism of positive leaders.

Abstract

We report results from a global simulation of the September 2017 geomagnetic storm. The global model comprises the ionospheric code SAMI3 and the atmosphere/thermosphere code WACCM-X. We show that a train of large-scale EPBs form in the Pacific sector during the storm recovery phase on 8 September 2017. The EPBs are associated with storm-induced modification of the zonal and meridional winds. These changes lead to an eastward electric field which in turn causes an upward E × B drift in the post-midnight sector. A large decrease in the Pedersen conductance caused by meridional equatorward winds leads to an increase in the growth rate of the generalized Rayleigh-Taylor instability that causes EPBs to develop. Interestingly, several EPBs reach altitudes above 3,000 km.

Abstract

Damage zones are common around faults, but their effects on earthquake mechanics are still incompletely understood. Here, we investigate how damage affects rupture patterns, source time functions (STF) and ground motions in 2D fully-dynamic cycle models. We find that back-propagating rupture fronts emerge in large faults and can be triggered by residual stresses left by previous ruptures or by damage-induced pulse-to-crack transitions. Damage-induced back-propagating fronts are modulated by slip rate oscillations, amplify high-frequency radiation, and sharpen the multiple peaks in STF even in the absence of frictional heterogeneity or fault segmentation. Near-field ground motion is predominantly controlled by stress heterogeneity left by prior seismicity, and further amplified within the damage zone by trapped waves and outside it by secondary rupture fronts. This study refines our knowledge on damage zone effects on earthquake rupture and identifies their potentially observable signatures in the near and far field.

Abstract

A devastating earthquake with moment magnitude 7.5 occurred in the Noto Peninsula in central Japan on 1 January 2024. We estimate the rupture evolution of this earthquake from teleseismic P-wave data using the potency-density tensor inversion method, which provides information on the spatiotemporal slip distribution including fault orientations. The results show a long and quiet initial rupture phase that overlaps with regions of preceding earthquake swarms and associated aseismic deformation. The following three major rupture episodes evolve on segmented, differently oriented faults bounded by the initial rupture region. The irregular initial rupture process followed by the multi-scale rupture growth is considered to be controlled by the preceding seismic and aseismic processes and the geometric complexity of the fault system. Such a discrete rupture scenario, including the triggering of an isolated fault rupture, adds critical inputs on the assessment of strong ground motion and associated damages for future earthquakes.

Abstract

Using a two-way coupled magnetohydrodynamics with embedded kinetic physics model, we perform a substorm event simulation to study electron velocity distribution functions (VDFs) evolution associated with Bursty Bulk Flows (BBFs). The substorm was observed by Magnetospheric Multiscale satellite on 16 May 2017. The simulated BBF macroscopic characteristics and electron VDFs agree well with observations. The VDFs from the BBF tail to its dipolarization front (DF) during its earthward propagation are revealed and they show clear energization and heating. The electron pitch angle distributions (PADs) at the DF are also tracked, which show interesting energy dependent features. Lower energy electrons develop a “two-hump” PAD while the higher energy ones show persist “pancake” distribution. Our study reveals for the first time the evolution of electron VDFs as a BBF moves earthward using a two-way coupled global and kinetic model, and provides valuable contextual understanding for the interpretation of satellite observations.

Abstract

Whakaari/White Island has been the most active New Zealand volcano in the 21st century, producing small phreatic and phreatomagmatic eruptions, which are hard to predict. The most recent eruption occurred in 2019, tragically claiming the lives of 22 individuals and causing numerous injuries. We employed shear-wave splitting analyses to investigate variations in anisotropy between 2018 and 2020, during quiescence, unrest, and the eruption. We examined spatial and temporal variations in 3,499 shear-wave splitting and 2,656 V p /V s ratio measurements. Comparing shear-wave splitting parameters from similar earthquake paths across different times indicates that the observed temporal changes are unlikely to result from variations in earthquake paths through media with spatial variability. Instead, these changes may stem from variations in anisotropy over time, likely caused by changes in crack alignment due to stress or varying fluid content.

Abstract

Mixed-phase stratiform clouds contain numerous liquid, mixed-phase, and ice clusters, quantifying the cluster scales is potentially helpful to improve the parameterizations of microphysics and radiation models. However, the scales of hydrometeor clusters at different levels of stratiform clouds are not well understood. In this study, using airborne measurements and a large eddy simulation, we show that turbulence plays an important role in controlling the clusters with length of a few hundred meters, while the scales of larger clusters have stronger vertical variations from cloud base to top. The liquid clusters are the largest near the cloud top, while the lengths of ice clusters decrease from cloud base to top. The lengths of mixed-phase clusters depend on the glaciation process, a faster glaciation results in smaller mixed-phase clusters. The results improve our understanding on how the liquid and ice are mixed at different levels in stratiform clouds.

Abstract

The impact of an episodic river flood is intimately linked to its duration. Yet it is still unclear how often should a river be observed to accurately determine the occurrence and duration of extreme events. Here we assess flow statistics along with peak flow event detection and duration as a function of the discharge sampling period for large tributaries of the Mississippi basin using hourly gages over 2010–2022. Median event durations above high quantiles spatially vary from around 2 days upstream to 30 days downstream. Discharge mean, standard deviation, and quantiles can all be estimated within 2.5% error for sampling periods up to 8 days. A minimum temporal sampling 4× (2×) finer than peak flow event median duration is required to detect 95 ± 3% (85 ± 5%) of events and to estimate their duration within 90 ± 5% (75 ± 10%) median accuracy. Our findings have direct implications for future satellite missions concerned with capturing flood events.

Abstract

Seismic anisotropy is a powerful tool to map deformation processes in the deep Earth. Below 660 km, however, observations are scarce and conflicting. In addition, the underlying crystal scale mechanisms, leading to microstructures and crystal orientations, remain poorly constrained. Here, we use multigrain X-ray diffraction in the laser-heated diamond anvil cell to investigate the orientations of hundreds of grains in pyrolite, a model composition of the Earth's mantle, at in situ pressure and temperature. Bridgmanite in pyrolite exhibits three regimes of microstructures, due to transformation and deformation at low and high pressure. These microstructures result in predictions of 1.5%–2% shear wave splitting between 660 and 2,000 km with reversals in fast S-wave polarization direction at about 1,300 km depth. Anisotropy can develop in pyrolite at lower mantle conditions, but pressure has a significant impact on the plastic behavior of bridgmanite, and hence seismic observations, which may explain conflicting anisotropy observations.

Abstract

The processing of hundreds of Synthetic Aperture Radar (SAR) images acquired by two satellite systems: Sentinel-1 and COSMO-SkyMed reveals a decade of ground deformation for a ∼0.5 km diameter area around the summit crater of the only active carbonatitic volcano on Earth: Ol Doinyo Lengai in Tanzania. Further decomposing ascending and descending orbits when the appropriate SAR data sets overlap allow us to interpret the imaged deformation as ground subsidence with a significant rate of ∼3.6 cm/yr for the pixels located just north of the summit crater. Using geodetic modeling and inverting the highest spatial resolution COSMO-SkyMed data set, we show that the mechanism explaining this subsidence is most likely a deflating very shallow (≤1 km depth below the summit crater at the 95% confidence level) magma reservoir, consistent with geochemical-petrological and seismo-acoustic studies.

Abstract

This paper presents the first ever observations on aspect-sensitive characteristics of 205 MHz stratosphere–troposphere (ST) radar located at a tropical station Cochin (10.04°N, 76.3°E) using volume scanning. The most significant and new observation is that the signal-to-noise ratio in zenith and off-zenith beams are nearly equal in some height region, indicating the presence of isotropic turbulence. Signal strength decreases by 0.75 dB per degree from 0 to 10 degree off-zenith, 0.9 dB per degree from 10 to 20 degree off-zenith and 0.3 dB per degree beyond 20 degree off-zenith. Different causative mechanisms are discussed on the basis of various estimated parameters associated with aspect sensitivity. Maximum aspect sensitivity is observed between 12 and 17 km, indicating the presence of dynamic instability arising due to strong wind shear and atmospheric stability. When both the square of wind shear and stability parameters are above 0.25 × 10−3 s−2, the scatterers become mostly isotropic. The study also shows a power difference in the symmetric beams as well as azimuth angle dependency. Analysis suggests that this asymmetry is due to the tilting of layers by the action of atmospheric gravity waves generated through Kelvin-Helmholtz instability. The present configuration of radar can provide a better understanding of three-dimensional structures of turbulence and instabilities.

Abstract

Recent observations beneath Kanto, Japan have shown that seismic activity and seismic attenuation within the overlying continental plate change with time due to drainage caused by slow-slip events (SSEs) along the upper boundary of the Philippine Sea plate. However, associated changes in rock properties have not been investigated. In this study, we estimate frequency-dependent shear-wave anisotropy to provide a detailed insight into the structural change associated with drainage. We perform shear-wave splitting analysis in frequency ranges of 1–4, 2–6, and 4–8 Hz for 306 earthquakes that occur during September 2009–August 2021 and recorded at the Metropolitan Seismic Observation network. Obtained time differences between fast and slow S waves (delay time) range from almost zero to 0.16–0.18 s, exhibiting spatio-temporal variation and frequency dependence. The fast S-wave polarization directions are generally consistent with the direction of the maximum horizontal compressional axis in the study region, which suggests that the observed anisotropy is probably caused by the NE–SW-oriented fractures developed under the regional stress field. The temporal variation in delay times is correlated with SSEs activity with a lag time of 0.0–0.1 year. Furthermore, comparisons between observed frequency-dependent delay times and numerical calculation of fracture-induced anisotropy suggest that the average fracture radius is almost constant (0.30–0.35 m) over time but fracture density temporally varies from 0.025 to 0.035. We infer that the fracture density is probably enhanced by opening of the NE–SW-oriented fractures during the upward migration of fluids that are expelled from the plate interface.

Abstract

To gain a deeper understanding of the extensive and varied lithospheric deformations beneath northern Oman, we examine seismic anisotropy in this region using splitting analysis of teleseismic shear wave data. Our study utilizes data from a dense network consisting of 13 permanent and 45 temporary seismic stations, which were operational for approximately 2.5 years starting from October 2013. By examining the azimuthal distribution of shear wave splitting (SWS) parameters, we were able to divide the study area into three sub-regions. The stations located to the west of the Hawasina window exhibit relatively azimuthally invariant SWS parameters suggesting a single anisotropic layer. On the other hand, most of the stations located in the central and eastern regions display variations versus back-azimuth, indicating the potential presence of depth-dependent anisotropy. The General NW-SE trend of the Fast Polarization Directions (FPDs) of the one-layer anisotropy in the west and FPDs of the upper layers in the east is concordant with the strike of the structures resulting from the collision between the continental and oceanic plates. A clear contrast in SWS parameters is observed in the Semail Gap Fault Zone (SGFZ), suggesting that the SGFZ can be a lithospheric-scale structure that hampers the intrusion of mafic magma from the southeast. Furthermore, the FPDs of the lower layer in the east exhibit an NE-SW trend, which may be indicative of the large-scale mantle flow resulting from the present-day plate motion.

Abstract

We developed an enhanced Kalman-based approach to quantify abrupt changes and significant non-linearity in vertical land motion (VLM) along the coast of Chile and the Antarctic Peninsula using a combination of multi-mission satellite altimetry (ALT), tide gauge (TG), and GPS data starting from the early 1990s. The data reveal the spatial variability of co-seismic and post-seismic subsidence at TGs along the Chilean subduction zone in response to the Mw8.8 Maule 2010, Mw8.1 Iquique 2014, and Mw8.3 Illapel 2015 earthquakes that are not retrievable from the interpolation of sparse GPS observations across space and time. In the Antarctic Peninsula, where continuous GPS data do not commence until ∼1998, the approach provides new insight into the ∼2002 change in VLM at the TGs of +5.3 ± 2.2 mm/yr (Palmer) and +3.5 ± 2.8 mm/yr (Vernadsky) due to the onset of ice-mass loss following the Larsen-B Ice Shelf breakup. We used these data to constrain viscoelastic Earth model parameters for the northern Antarctic Peninsula, obtaining a preferred lithosphere thickness of 115 km and upper mantle viscosity of 0.9 × 1018 Pa s. Our estimates of regionally-correlated ALT systematic errors are small, typically between ∼±0.5–2.5 mm/yr over single-mission time scales. These are consistent with competing orbit differences and the relative errors apparent in ALT crossovers. This study demonstrates that, with careful tuning, the ALT-TG technique can provide improved temporal and spatial sampling of VLM, yielding new constraints on geodynamic models and assisting sea-level change studies in otherwise data sparse regions and periods.

Quantum data assimilation: a new approach to solving data assimilation on quantum annealers
Shunji Kotsuki, Fumitoshi Kawasaki, and Masanao Ohashi
Nonlin. Processes Geophys., 31, 237–245, https://doi.org/10.5194/npg-31-237-2024, 2024
In Earth science, data assimilation plays an important role in integrating real-world observations with numerical simulations for improving subsequent predictions. To overcome the time-consuming computations of conventional data assimilation methods, this paper proposes using quantum annealing machines. Using the D-Wave quantum annealer, the proposed method found solutions with comparable accuracy to conventional approaches and significantly reduced computational time.

Abstract

In this paper, we study the ionization-recombination processes in the lower ionosphere during solar flares of various classes in June 2014 and September 2017. For the first time, ionization and recombination rates of the ionospheric D-region were estimated using the experimental data on variations in the amplitude and phase characteristics of very low frequencies (VLF) signals and two-channel data from the GOES satellite (0.05–0.4 nm and 0.1–0.8 nm). The empirical two-parameter Wait-Ferguson model was used to calculate temporal changes in the electron concentration Ne during solar flares of different classes. GOES satellite data and black body model were used to estimate the X-ray fluxes in the wavelength spectral range λ ≤ 0.3 nm. Joint analysis of the temporal evolution of the Ne vertical profile in the lower ionosphere and X-ray fluxes in different wavelength ranges was carried out. As a result, the values of the ionization and recombination rate coefficients were obtained, and the spectral ranges of radiation that have the greatest impact on Ne at given heights were determined. Calculated ionization and recombination rate coefficients and ranges were successfully verified using VLF data during different solar flares. The results obtained in this work can be used in future studies for a more accurate assessment of the response of lower ionosphere to solar flares of various classes.

Abstract

In this study, the Extreme Hourly Precipitation Areas (EHPAs) of three extreme levels (i.e., between the 95th and 99th percentiles, between the 99th and 99.9th percentiles, beyond the 99.9th percentile) in the Pearl River Delta over South China are identified; then the related events and associated Convective Cores (CCs) are tracked, and their macro-and-microphysical characteristics are analyzed using multi-year dual-polarization radar observations. Results show that >90% of EHPAs are smaller than 10 km2, and 65%–75% of EHPA events last only one hour. They tend to be more localized and persist longer with increasing hourly-precipitation extremity. The EHPAs overlap with the CCs during 50%–64% of the EHPAs' life span. Their occurrence frequencies are nearly quadrupled after the monsoon onset over South China Sea (SCS), with a major (secondary) peak at about 1400 LST (0600 LST) in the diurnal variations. The CCs are non-linear shaped with about 65% being meso-γ-scale and embedded within mostly meso-β or α-scale 20 dBZ regions. The CCs generally contain active warm-rain processes and about 70% possess moderate-to-intense mixed-phase microphysical processes. The ratios of ice water path to liquid water path are about 0.37, and coalescence dominates (about 68%) the liquid-phase processes. The average size of raindrop is slightly larger than the “maritime-like” regime and the average concentration is much higher than the “continental-like” regime. These CCs' characteristics roughly resemble those of the convection producing extreme instantaneous precipitation, except for a larger horizontal scale and less evident variations with the increasing hourly-precipitation extremity.

Abstract

The question of stratospheric aerosol type following the Raikoke eruption is revisited. Raman lidar measurements suggest the aerosols are predominately smoke, while Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) results indicate the aerosols are predominately sulfate aerosols. The suggested mechanism of smoke particles self-lofting into the stratosphere is inconsistent with observations in 2020, when more severe Siberian fires failed to invoke a response even vaguely similar to 2019. A side-by side comparison of the Sarychev and Raikoke eruptions invalidates model calculations that suggest sulfate aerosols should be at levels too low to explain the observed aerosol loading. Structure in infrared absorption spectra provides conclusive evidence of composition, a unique fingerprint for identifying aerosol type. Such information cannot be misinterpreted so long as there is sufficient resolution and spectral coverage. ACE-FTS infrared aerosol spectra often have an order of magnitude stronger absorption than that of background sulfate aerosols. These spectra can be accurately reproduced by laboratory measured sulfate aerosol spectroscopic information, providing unambiguous identification of the aerosols as sulfate. Visual inspection of thousands of infrared aerosol spectra from the period following the Raikoke eruption indicates the aerosols in the lower stratosphere are predominately sulfate, with no indication of smoke. The lidar study's identification of the aerosols as smoke was based primarily on observed lidar ratios that were more consistent with a material that absorbed significantly at the lidar wavelengths, inconsistent with expectations for sulfate aerosols. However, this could indicate the presence of a substance dissolved in the sulfate aerosols absorbing at those wavelengths rather than smoke particles.

Abstract

Based on satellite observations in the Arctic stratosphere at latitudes from 61° to 66°N in the second half of 2019, Boone et al. (2022, https://doi.org/10.1029/2022jd036600) provide the impression that the aerosol in the upper troposphere and lower stratosphere (UTLS) over the entire Arctic consisted of sulfate aerosol originating from the Raikoke volcanic eruption in the summer of 2019. Here, we show that this was most probably not the case and the aerosol layering conditions were much more complex. By combining the stratospheric aerosol typing results of Boone et al. (2022, https://doi.org/10.1029/2022jd036600) with lidar observations at 85°–86°N of Ohneiser et al. (2021, https://doi.org/10.5194/acp-21-15783-2021) of a dominating wildfire smoke layer in the UTLS height range, we demonstrate that the Arctic UTLS aerosol most likely consisted of Siberian wildfire smoke in the lower part and sulfate aerosol in the upper part of the aerosol layer which extended from 7 to 19 km height and was well observable until May 2020. The smoke- and sulfate-related aerosol optical thickness (AOT) fractions were about 0.7–0.8 and 0.2–0.3, respectively, according to our analysis. The sulfate AOT is in good agreement with model-based predictions of the Raikoke sulfate AOT.

Abstract

The South Atlantic Ocean hosts several well-studied volcanic ridges and seamount chains, but the origin of their associated mantle plumes is debated. Reduced seismicity on the southern Mid-Atlantic Ridge (MAR) suggests anomalously ductile thermomechanical conditions at 52°S and 47.5°S. These low seismicity patches extend 120–560 km along-axis, and correspond with axial high spreading ridge morphology, geochemical anomalies, and mantle wave speed patterns likely associated with the Shona and Discovery plumes. Bathymetric data show that the northern extent of the Shona swell is associated with increased volcanism, elevated axial bathymetry, and a series of northward-propagating rifts, with the overall swell geometry suggesting a buoyancy flux of 0.4–0.5 Mg s−1. The nearby Bouvet Island may be a product of a branch of the larger Shona plume swell, which has influenced crustal accretion on the southern MAR for the past 24 million years.