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Abstract

The origin and evolution of Saturn’s rings is critical to understanding the Saturnian system as a whole. Here, we discuss the physical and chemical composition of the rings, as a foundation for evolutionary models described in subsequent chapters. We review the physical characteristics of the main rings, and summarize current constraints on their chemical composition. Radial trends are observed in temperature and to a limited extent in particle size distribution, with the C ring exhibiting higher temperatures and a larger population of small particles. The C ring also shows evidence for the greatest abundance of silicate material, perhaps indicative of formation from a rocky body. The C ring and Cassini Division have lower optical depths than the A and B rings, which contributes to the higher abundance of the exogenous neutral absorber in these regions. Overall, the main ring composition is strongly dominated by water ice, with minor silicate, UV absorber, and neutral absorber components. Sampling of the innermost D ring during Cassini’s Grand Finale provides a new set of in situ constraints on the ring composition, and we explore ongoing work to understand the linkages between the main rings and the D ring. The D ring material is organic- and silicate-rich and water-poor relative to the main rings, with a large population of small grains. This composition may be explained in part by volatile losses in the D ring, and current constraints suggest some degree of fractionation rather than sampling of the bulk D ring material.

Abstract

The beading of auroral arcs often takes place at substorm onset. It is known that auroral beads propagate more often eastward than westward at several km/s, which is difficult to explain by existing models. We investigate this issue observationally and theoretically. First, based on previous research and additional statistical analysis, we suggest that (a) auroral beads often propagate eastward in the presence of westward background convection, and (b) background ionospheric convection may be better represented by large-scale convection for westward propagation, and by meso-scale convection for eastward propagation. Then we model auroral beads as vortices of ionospheric flow, and consider the longitudinal propagation of their meridional displacement based on the conservation of vorticity. Here it is crucial that the background zonal flow has vorticity (i.e., flow shear) changing with latitude. It is found that the wave propagates either parallel or anti-parallel to the background flow depending on whether the background vorticity increases or decreases in latitude, and if its latitudinal scale is significantly smaller than the longitudinal wavelength, the phase velocity exceeds the background flow speed. The result suggests that the latitudinal structure of the background flow is crucial for the bead propagation. More specifically, the aforementioned feature (a) implies that the zonal flow associated with eastward propagation is confined in latitude, which may correspond to the preonset approach of mesoscale flows. In contrast, the large-scale ionospheric flow suggested for westward propagation as described in (b) may correspond to the global convection of the conventional growth phase.

Abstract

The connection between the magnetosphere and ionosphere is particularly dynamic during substorms. Mesoscale features in the magnetotail are consistent with substorm activity, including magnetic reconnection in the tail, flow channels, and particle injections. Observations of substorm related phenomena can be made using energetic neutral atom (ENA) imagers, in situ satellite measurements, and ground based magnetic field perturbation measurements. Analysis of the 10 October 2014 isolated substorm event is presented. Comparison of the spatial and temporal dynamics of the features seen in equatorial maps generated from ENA data are made with inner magnetosphere in situ measurements and ionospheric features with network analysis of the SuperMAG data. An MHD simulation of the event using OpenGGCM is also compared with the data.

Abstract

The atmospheric phase, which is the sum of vertical stratification and turbulent atmospheric phase, is a major challenge currently faced by small baseline subset interferometric synthetic aperture radar (SBAS-InSAR) measurements. Many previous studies have demonstrated that the former can be separated from the interferogram by establishing a functional model between it and the topography. Due to the high variability of the turbulent atmospheric phase (TAP) in the space and time domains, however, the TAP is difficult to model and remove. Recently, many stochastic models have been developed to reduce the influence of the TAP in SBAS-InSAR. To avoid the rank deficient in stochastic model method, we present a correction method using network-based variance estimation, interferogram stacking and ordinary kriging interpolation (NIO). There are three main steps in the proposed algorithm to ensure the accuracy of the correction result: (1) adaptively identify and select sufficient good-quality interferograms that contain less turbulent atmospheric noise to participate in deformation calculation; (2) further select the short temporal baseline interferogram and mask the corresponding deformation location to avoid the effect of deformation; and 3) take advantage of ordinary kriging interpolation to reduce the effects of TAP from the selected good-quality interferograms. The performance of the proposed method has been validated with a set of simulations and real Sentinel-1A SAR data in Southern California, USA.

Author(s): V. Horný, P. G. Bleotu, D. Ursescu, V. Malka, and P. Tomassini

With the usage of the postcompression technique, few-cycle joule-class laser pulses are nowadays available extending the state of the art of 100 TW-class laser working at 10 Hz repetition. In this Letter, we explore the potential of wakefield acceleration when driven with such pulses. The numerical …

[Phys. Rev. E 110, 035202] Published Fri Sep 06, 2024

Author(s): Jiangxu Huang, Lei Wang, Zhenhua Chai, and Baochang Shi

In this paper we first propose a phase-field model for the containerless freezing problems, in which the volume expansion or shrinkage of the liquid caused by the density change during the phase change process is considered by adding a mass source term to the continuum equation. Then a phase-field-b…

[Phys. Rev. E 110, 035301] Published Fri Sep 06, 2024

Abstract

Local empirical models of the F2 layer peak electron density (NmF2) are developed for 43 low- middle latitude ionosonde stations using auto-scaled data from Lowell GIRO data center and manually scaled data from World Data Center for Ionosphere and Space Weather. Data coverage at these stations ranges from a few years to up to 6 decades. Flare Irradiance Spectral Model index version 2 (FISM2) and ap3 index are used to parametrize the solar extreme ultraviolet (EUV) flux and geomagnetic activity dependence of NmF2. Learning curves suggest that approximately 8 years of data coverage is required to constrain the solar activity dependence of NmF2. Output of local models altogether captures well known anomalies of the F2 ionospheric layer. Performance metrics demonstrate that the model parametrized using FISM2 has better accuracy than a similarly parametrized model with F10.7, as well as than the IRI-2020 model. Skill score metrics indicate that the FISM2 based model outperforms F10.7 model at all solar activity levels. The improved accuracy of model with FISM2 over F10.7 is due to better representation of solar rotation by FISM2, and due to its performance at solar extremum. Application of singular spectrum analysis to model output reveals that solar rotation contributes to about 2%–3% of the variance in NmF2 data and FISM2 based model, while F10.7 based models overestimate the strength of solar rotation to be at 4%–7%. At solar extremum, both F10.7-based model and IRI-2020 tend to overestimate the NmF2 while FISM2 provides the most accurate prediction out of three.

Abstract

This work presents an algorithm for automatic detection of anomalous electron heating (AEH) events in the auroral E-region ionosphere using data from the Poker Flat Incoherent Scatter Radar (PFISR). The algorithm considers both E-region electron temperature and magnetically conjugate electric field measurements. Application of this algorithm to 14 years of PFISR data spanning 2010 through 2023 detected 505 AEH events. Measured electron temperatures increase linearly with plasma drift speeds. Statistical trends of AEH occurrence as a function of space weather indices (AE and F10.7) demonstrate correlations with the solar cycle and geomagnetic activity levels. The magnetic local time occurrence rates show preferences for dusk and dawn with most events in the dusk sector. Observed AEH events tend to appear in regions of relatively low electron density and do not appear inside intense auroral arcs with high electron density. Furthermore, AEH detection requires a higher electric field than predicted by the threshold for a positive growth rate of the Farley-Buneman instability (FBI), according to linear fluid theory. The implications of these findings for kinetic theories of FBI and AEH are discussed.

Abstract

Kelvin Helmholtz Instabilities (KHI) result from interactions between the shocked solar wind and the Earth's magnetosphere. These are formed due to the velocity shear between the plasma in the magnetosphere and magnetosheath. The role of KHI in bringing in the shocked solar wind into the terrestrial magnetosphere has been studied extensively using MHD, Hall-MHD, hybrid and PIC simulations. Such simulations oftentimes make simplifying assumptions of the boundary layer in the magnetopause. To experimentally study the effects of KHI on the boundary layer and its effectiveness in bringing in solar wind, we analyze 43 KHI events. All these events have quasi-constant IMF orientation during its interval, thereby mitigating the effects of variation of IMF in the ongoing transient magnetopause process. In this statistical study of KHIs, we demonstrate that there is a preexisting boundary layer before KHIs begin to develop. As KHI develops to its non-linear state, the ions in the magnetosphere, magnetopause, and magnetosheath are mixed, which is demonstrated using the alpha-to-proton density ratio. As a result of this mixing, the well-defined preexisting boundary layer is replaced by a much more uniformly mixed boundary layer.

Nature Geoscience, Published online: 06 September 2024; doi:10.1038/s41561-024-01528-9

The processes that control the deformation and eventual destruction of Earth’s oldest continental crust are unclear. Mantle flow models suggest subduction played a role in the deformation of the North China Craton.

Nature Geoscience, Published online: 06 September 2024; doi:10.1038/s41561-024-01525-y

Summer snow accumulation and its albedo effect on Arctic sea ice are controlled by the Arctic Oscillation atmospheric circulation pattern, according to a combined modelling and remote sensing analysis.

Nature Geoscience, Published online: 06 September 2024; doi:10.1038/s41561-024-01513-2

Mesozoic deformation of the North China Craton occurred via lithospheric thickening followed by thinning and extension triggered by flat-slab subduction and rollback, according to four-dimensional mantle flow models of the plate–mantle system.

Abstract

At ultra-slow spreading ridges, with full spreading rates less than ∼20 mm/yr, spreading is accommodated both by highly spatially and temporally segmented magmatism, and tectonic extension along large-scale detachment faults that exhume ultramafic material to the seafloor. In the most magma-poor regions, detachment faulting alternates in polarity over time, producing a “flip-flopping” effect of subsequent detachment dips. The resulting seafloor in these regions displays a morphology termed “smooth seafloor” comprising elongate, broad ridges with peridotite/serpentinite lithologies. We conducted tomographic travel-time inversion of a 3-D wide-angle seismic data set acquired over a region of smooth seafloor around 64°30′E along the Southwest Indian Ridge (SISMOSMOOTH; Cruise MD199), to produce a seismic velocity volume through the crustal section and into the uppermost mantle. We observe patterns of velocity anomalies that correspond with variations in the bathymetry arising from the mode of spreading and are interpreted as changes in the degree of alteration with depth resulting from spatial and temporal variations in fluid-rock interaction, controlled by faulting and tectonic damage processes. The detachment faults do not show simple planar structures at depth but instead mirror the shapes of the bathymetric ridges that they exhume. Magmatic input is overall highly limited, but there is one region on the lower part of an exhumed detachment footwall where a thickness of volcanic material is observed that suggests a component of syn-tectonic volcanism, which could contribute to detachment abandonment.

Abstract

Body waves traversing the Earth's interior from a seismic source to receivers on the surface carry rich information about its internal structures. Their travel time measurements have been widely used in seismology to constrain Earth's interior at the global scale by mapping the time anomaly along their ray paths. However, picking the travel time of global seismic waves, suitable for studying Earth's fine-scale structures, requires highly skilled personnel and is often fairly subjective. Here, we report the development of an automatic picker for PKIKP waves traveling through the inner core (IC), especially nearly along Earth's diameters, based on the latest advances in supervised deep learning. A convolutional neural network (CNN) we develop automatically determines the PKIKP onset on vertical seismograms near its theoretical prediction of cataloged earthquakes. As high-quality manual onset picks of global seismic phases are limited, we employ a scheme to generate a synthetic supervised training data set containing 300,000 waveforms. The PKIKP onsets picked by our trained CNN automatic picker exhibit a mean absolute error of ∼0.5 s compared to 1,503 manual picks, comparable to the estimated human-picking error. In an integration test, the automatic picks obtained from an extended waveform data set yield a cylindrically anisotropic IC model that agrees well with the models inferred from manual picks, which illustrates the success of this pilot model. This is a significant step closer to harvesting an unprecedented volume of travel time measurements for studying the IC or other regions of the Earth's deep interior.

Abstract

Models of active deformation of the Earth's crust are predominantly represented with dislocations having a downdip continuation into the lower crust, where the fault slips continuously. This model predicts surface strain accumulation concentrated near the fault during the interseismic period. In an alternative model, faults do not extend beneath the elastic portion of the crust and are accompanied by a wide zone of distributed shear underneath, predicting a more constant strain rate lacking concentrations at the faults. We use high-precision GPS data collected across the northern and central Walker Lane, USA— a region of complex faulting near the western edge of the Basin and Range Province to evaluate which model is appropriate. Despite the existence of dense continuous and semi-continuous geodetic networks that have been surveyed for ∼20 years, the horizontal velocities reveal no evidence of localized strain accumulation across the fault surface expressions. Instead, deformation within the Walker Lane is uniformly linear, suggesting that the surface deformation reflects distributed shear within the ductile crust rather than focused deformation at faults. This suggests no downdip extension of the faults below the seismogenic layer. The shear zone is 172 ± 6 km wide in the northernmost Walker Lane narrowing to 116 ± 4 km in the central Walker Lane. The total velocity budget across the shear zone is 7.2 ± 0.1 mm/yr in the north, increasing to 10.1 ± 0.1 mm/yr in the central Walker Lane. We conclude that assuming the presence of lower crustal dislocations when estimating geodetic faults slip rates may be inappropriate.

Abstract

The eastward extrusion of the Tibetan Plateau materials has caused intricate tectonic deformations and frequent seismic activities in the Sanjiang lateral collision zone (SLCZ). To reveal crust structures and deformation mechanisms, we investigate high-resolution structural features of crustal depth (≤40 km). A 3-D S-wave velocity and azimuthal anisotropy model is constructed by the direct tomography method with Rayleigh phase velocity at periods of 2–40 s from multiple temporary seismic arrays and regional permanent network. In the middle-to-lower crust, an obvious low-velocity zone is confined by the large-scale fault systems of Jinhe-Qinghe fault and Chenhai fault (CHF) to the northeast and east, Lancangjiang fault (LCJF) and Red River fault (RRF) to the west, with strong N-S-oriented anisotropy, which evident differs from the ENE-WSW-oriented weak anisotropy in the high-velocity zone on the northeastern side. We consider that the weak material may be obstructed by large faults and the high-velocity zone, resulting in complex crustal deformation and tectonic boundary. The crustal low-velocity materials beneath the Tengchong volcano (TCV) are probably separated with those from the Tibetan Plateau. The low-velocity beneath the Chuxiong basin (CXB) may be combinations of partial melts and fluid derived from shear deformation and deep material upwelling. The segmented anisotropy at the NW end of the RRF suggests complex deformation by crustal flow, emphasizing the important influence of faults on anisotropic pattern. The complex anisotropy in the fault intersection of the Lijiang-Xiaojinhe fault and RRF also highlights the important role of these faults in shaping crustal deformation.

Author(s): S. Bolaños, M. J.-E. Manuel, M. Bailly-Grandvaux, A. S. Bogale, D. Caprioli, S. R. Klein, D. Michta, P. Tzeferacos, and F. N. Beg

We present an experimental investigation of the formation stage of a collisionless shock when the flow velocity is aligned with an ambient magnetic field utilizing laser-driven, super-Alfvénic plasma flows. As the flows interact, electromagnetic streaming instabilities develop. Proton deflectometry …

[Phys. Rev. E 110, L033201] Published Thu Sep 05, 2024

Abstract

In the present work we have modeled diffuse auroral emissions in Jupiter using the recent observations received by JUNO orbiter. Resonant wave-particle interaction by electron-cyclotron harmonic (ECH) waves has been invoked as the mechanism for production of diffuse aurora. Energetic electrons trapped on closed field lines are diffused into the loss-cone via pitch-angle diffusion. Electron precipitation fluxes have been calculated. Electrons entering into the atmosphere undergo collisions with atmospheric constituents atomic H and molecular H2 producing electromagnetic emissions. Four excitations have been considered. These excitations are: HLy-α from excitation of atomic H, HLy-α from dissociative excitation of molecular H2, Lyman and Werner bands of H2. Volume excitation rates have been calculated for these excitations. Height integrated volume excitation rates have been obtained to give auroral intensities. Numerical calculations have been performed at five L-shells; L = 10, 12, 15, 18 and 20. Maximum auroral intensities is obtained at shell L = 10. At higher shell L = 20 the intensity value reduces to a minimum. The intensities in Rayleigh (R) for HLy-α from H, HLy-α from H2, Lyman and Werner bands of H2 are calculated. Comparing these intensities with the diffuse auroral intensities observed at Saturn, it is found that the intensities at Jupiter are higher than the values predicted for Saturn. We have also calculated volume ionization rates for atomic H producing H+, dissociative ionization of H2 producing H+, and ionization of H2 producing H2 +. The continuity equation is solved to obtain the electron density Outcomes are discussed.

Abstract

The interplanetary magnetic field (IMF) north-south component, B z , plays a crucial role in the interaction between the solar wind and the Earth's magnetosphere. We analyze 98 intervals in which B z changed from >3 nT to <−3 nT in 5 min and for which these rapid southward turnings (STs) were surrounded by consistently northward or southward IMF. We separate out events in proximity of interplanetary coronal mass ejections and corotating interaction regions. We find that IMF magnitude, solar wind dynamic pressure and proton density (but also flow speed in ICME-related events) near the turnings are enhanced above their medians. We analyze the maximum responses of the SML, SMU, SYM-H, and PCN magnetospheric indices and their timescales, along with the occurrence of geomagnetic phenomena. We find that most STs were followed by either substorms (60.20%) or enhanced convection (37.76%). While SML has similar median minima (∼−460 nT) and timescales (∼56 min) for substorm and convection events, SMU has noticeable differences. STs were followed by geomagnetic storms (SYM-H ≤ −50 nT) in 46.94% of events within 12 hr, with more storms following ICME-related turnings. PCN has peaks (median 3.8 mV/m) around 30 min after the turning, and larger ones (median 4.9 mV/m) later. Stronger solar wind driving and magnetospheric responses are observed for ICME-related events. The correlation between the geomagnetic and solar wind parameters around STs reveals a more direct link between solar wind driving and geomagnetic response for STs than at other times.

Abstract

We study the geomagnetic storm of 9 October 2012, where it had been generally accepted that the resulting prominent outer radiation belt electron acceleration throughout the storm is due to whistler-mode chorus waves. This storm has been studied previously by two-dimensional Fokker–Planck numerical simulations with data-driven quasi-linear (QL) diffusion rates. However, possible nonlinear (NL) resonant interaction effects on electron flux dynamics haven't been looked at yet. This study aims to fill this gap by demonstrating that theory-informed rescaling of QL diffusion rates accounting for contributions of NL resonant interactions helps to reproduce better observed increase of electron fluxes by diffusion simulations. We use machine learning, uncertainty quantification (UQ), physics-perturbed ensemble of VERB simulations and Van Allen Probes observations to identify optimal rescaling of quasi-linear diffusion rates.