Solar flare and magnetotail observations show simultaneous acceleration of ions and electrons into power-law energy distributions extending to high energy. This suggests a common reconnection acceleration process but the underlying physics is not well understood.During magnetic reconnection, energetic particles undergo a universal Fermi acceleration process involving the curvature drift of particles. However, the efficiency of this mechanism is limited by the trapping of energetic particles within flux ropes. Using 3D fully kinetic simulations, we demonstrate that the flux-rope kink instability leads to field-line chaos in weak-guide-field regimes where the Fermi mechanism is most efficient, thus allowing particles to transport out of flux ropes and undergo further acceleration. As a consequence, both ions and electrons form clear power-laws which contain a significant fraction of the released energy. The low-energy bounds, which control the nonthermal energy contents, are determined by the injection physics, while the high-energy cutoffs are limited only by the system size. These results have strong relevance to observations of nonthermal particle acceleration in both the solar corona and magnetotail.

This research aims to explore the relationship between the temperatures of solar corona and planetary magnetosheaths. Based on the second law of thermodynamics, the maximum temperatures of the planetary magnetosheaths cannot be higher than that of the solar corona. A theoretical investigation has been made on the expansion of solar corona, the propagation of solar wind and the compressions of planetary magnetosheaths by the bow shocks. The method used is general and fit for the dynamics of multiple components, thermal anisotropy, and non-Maxwellian plasmas in a steady state, and approximate formulas have been obtained. It is found that, for the steady situations, the temperatures of all the planetary magnetosheaths at the subsolar points in the solar system have comparable values, which are also close to the maximum temperature of the solar corona. Secondly, a systematic statistical survey on the average temperatures of the planetary magnetosheaths have been carried out, which show that, the average plasma temperatures of the magnetosheaths of Earth and Saturn are 183eV (2.12MK) and 172eV (2.00MK), respectively. The statistical results are consistant with the theoretical results. These results are very practical for the estimations of the thermal properties of the planetary magnetospheres.

Tropical cyclones (TCs) and their economic cost risk under climate change are significant concerns globally. Previous studies on TC damage functions and risk assessment are mostly performed based on modeling TC-level damage and thus obtaining the annual average loss for a country or region. The scalability of these damage functions at finer scales has been less systematically explored. In this study, we examine how the model structure, estimated parameters, and model performance of TC damage functions vary with spatial scale. The comparisons are illustrated by fitting two types of damage functions based on reported damage data at the county, province, and TC scales. We find that the newly proposed precipitation-calibrated sigmoidal damage function significantly outperforms the wind-calibrated sigmoidal damage function at three scales of county, province and TC event. Another type of power-law damage function that integrates hazard, exposure, and vulnerability complements the typical sigmoidal damage function because it yields a better fit when estimating direct economic loss above the province scale. Our work provides an empirical assessment of the role of spatial scale and damage function in TC economic impact evaluation and demonstrates the importance of spatially scale-specific policy-making in TC risk management and climate adaptation strategies.

Headland sediment transport is dynamic and complex, but understanding the transport mechanisms is necessary for effective long-term management of downdrift beach compartments. In this study, we have develop a coastal process model using TUFLOWFV, that is used to calibrate an approximation tool for headland bypassing at the study site. The approximation tool is shown to reproduce sediment transport rates at the headland apexes accurately and efficiently. We have explored the headland sediment transport mechanism, the influence of wave height and direction, and the sensitivity in regional climate conditions. Headland sediment transport is shown to occur as ‘trickle’ bypassing under modal wave conditions or ‘sand slug’ migration under storm wave conditions that travel in either a headland-attached and a cross-embayment pathway. Bypassing during storm wave conditions produces 50% to 60% of total bypassing volume, despite only accounting for 6% of the recorded days. The results indicate that headland transport is sensitive to changes in wave direction and wave height, with the existing mean wave direction balancing sediment transport on the east and north faces of the headland. Seasonality is the most significant climatic control on headland transport, while ENSO phase is only significant for the headland apexes that are exposed to south-east wave conditions. The potential for anticlockwise rotation of the wave climate in future is explored, with greater erosion of the northern beaches of the headland likely due to a reduced supply of sediment around the eastern point of the headland and greater erosive wave power on the north side.

Serpentinization of ultramafic rocks is fundamental to modern plate tectonics and for volatile (re-)cycling into the mantle and magmatic arcs. Serpentinites are also highly reactive with CO2 such that they are prime targets for carbon sequestration. Serpentinization and carbonation of ultramafic rocks results in changes in their physical properties such that they should be detectable using geophysical surveys; this could provide constraint on the reactivity of rocks without extensive sample characterization. We constrain the physio-chemical relationships in altered ophiolitic ultramafic rocks using petrographic observations, major-element chemistry, quantitative X-ray diffraction, and physical properties on a suite of >400 samples from the Canadian Cordillera. Serpentinization results in a systematic decrease in density that reflects the increase in serpentine abundance and carbonation results in an increase in density, mostly reflecting the formation of magnesite; based on these data we present two formulations for determining extent of serpentinization: one based on major-element chemistry and the other on density. Magnetic susceptibility is variable during serpentinization; most harzburgitic samples show a 100-fold increase in magnetic susceptibility, whereas most dunitic samples and a minor proportion of harzburgitic samples show very little change in magnetic susceptibility. We use quantitative mineralogy and physical properties of the samples to constrain a model for using density and magnetic susceptibility to approximate the mineralogy of ultramafics rock. Although further work is required to understand the role of remanence in applying these models to geophysical data, this presents an advancement and opportunity to prospect for the most reactive ultramafic rocks for carbon sequestration.

A document by Eleonora Dallan. Click on the document to view its contents.

Buoyant trench sediment generates an isostatic restoring force, which opposes the slab pull of subducting plates, but the influence of this force on subduction dynamics is poorly understood. Here, we performed 3D free subduction simulations adopting a variety of sediment distributions along a trench to investigate the correlation between trench kinematics and heterogeneous buoyant force. Two endmembers, sediment-rich and sediment-starved centers, induced convex and concave trenches, respectively. The trench curvatures obtained from natural subduction zones are well constrained by our models over wide magnitudes (1-9 km) and wavelengths (100-400 km) of deficient and excess sediment. Conversely, uniform sediment distribution leads to an extremely narrow range of trench curvature. These results imply that trench sediment contributes to the diversity of trench shapes. Finally, we observed stress localization in slab hinges beneath abundant sedimentation, thereby clarifying the relationship between sediment thickness and subduction earthquakes.

We investigate the causes of large $dB/dt$ events observed by SuperMAG, by comparing with the time-series of different types of geomagnetic activity, or “convection state”, for the duration of 2010. Spikes are found to occur predominantly in the pre-midnight and dawn sectors. We find that pre-midnight spikes are associated with substorm onsets. Dawn sector spikes are not directly associated with substorms, but with auroral activity occurring within the westward electrojet region. Azimuthally-spaced auroral features drift sunwards, producing Ps6 (10-20 min period) magnetic perturbations on the ground. The magnitude of $dB/dt$ is determined by the flow speed in the convection return flow region, which in turn is related to the strength of solar wind-magnetospheric coupling. Pre-midnight and dawn sector spikes can occur at the same time, as strong coupling favours both substorms and westward electrojet activity; however, the mechanisms that create them seem somewhat independent. The dawn auroral features share some characteristics with omega bands, but can also appear as north-south aligned auroral streamers. We suggest that these two phenomena share a single underlying cause.

A document by Tom R Cowton. Click on the document to view its contents.

A document by Siddharth Chatterjee. Click on the document to view its contents.

This study examines the impact risk consequences due to asteroid disruption by a deflection mission. We use an Apophis-like scenario with a Nuclear Explosive Device (NED) deflection mission in our case studies. A Monte Carlo framework samples asteroid physical properties from probabilistic distributions based on the current knowledge of Apophis, and samples orbital states from an archival orbit solution reflecting Apophis’ 2.7% peak impact probability. Asteroid disruption is modelled at deflection time and the fragments are propagated forward to calculate if and where they impact the Earth. NASA’s Probabilistic Asteroid Impact Risk (PAIR) model estimates the impact damage in terms of affected population, and the overall scenario impact risk is calculated. Multiple case studies are explored to generate comparative data for scenarios where the asteroid is not altered, is always disrupted, or is conditionally disrupted with deflection impulse. The analysis shows that disruption increases impact risk for this Apophis scenario significantly. Even though deflection missions may cause disruption, a sufficiently strong deflection mission can be effective as risk decreases from its post-disruption peak with increasing deflection strength. Results also point to the dependence of risk changes on physical properties. Objects with a fraction of Apophisâ\euro™ mass will result in less risk when disrupted. We recommend that disruption analysis should be a critical factor in future asteroid mitigation considerations and suggest future research avenues of interest to mission design as well as planetary sciences.

Ice shelves are assumed to flow steadily from their grounding lines to the ice front. We report the detection of ice-propagating extensional Lamb (plate) waves accompanied by pulses of permanent ice shelf displacement observed by co-located GNSS receivers and seismographs on the Ross Ice Shelf. The extensional waves and associated ice shelf displacement are produced by tidally triggered basal slip events of the Whillans Ice Stream, which flows into the ice shelf. The propagation velocity of 2800 m/s is intermediate between shear and compressional ice velocities, with velocity and particle motions consistent with predictions for extensional Lamb waves. During the passage of the Lamb waves the entire ice shelf is displaced about 60 mm with a velocity more than an order of magnitude above its long-term flow rate. Observed displacements indicate a peak dynamic strain of 10-7, comparable to that of earthquake surface waves that trigger ice quakes.

High-speed data transfer and high-performance imaging can be realized by using radio-waves in high-frequency bands, such as millimeter-waves and THz-waves, where wide frequency bands are available. However, the cell size would be smaller than a few hundred meters, due to large free space propagation loss and large atmospheric attenuation. Thus, many base stations, which are connected by networks, are required to offer nation-wide or global network services by such high-frequency radio-bands. The networks would be constructed by various transmission media including optical fibers and fixed wireless links, where many media converters are required. This paper reviews various technologies for seamless bridges between radio and optical links. For the time being, congestion of radio spectrum in THz bands is not significant. However, if we look at the history of radio-wave technologies, spectral congestion has been high even in newly developed high frequency bands. Even in active radio services in millimeter-wave or THz-wave bands, interference mitigation with passive services such as radio astronomy and Earth observation satellites is an important issue, as of now. This paper describes research trends of THz-wave technologies from the point of view of a figure of merit defined by a product of the carrier frequency and spectral efficiency, to discuss the significance of spectral efficiency enhancement in the high-frequency region. Analysis of power consumption of short distance radio systems is also shown to discuss expected performance of THz-wave links.

The subtropical northeastern Pacific (SNEP) acts as a bridge conveying information between the North and tropical Pacific. While most studies have investigated the SNEP sea surface temperature (SST) interannual variability, less attention has been paid to the pronounced decadal variability in this region. Here, by analyzing observational data and an ensemble pacemaker simulation, we investigate SNEP SST decadal variability in terms of atmospheric forcing processes. We find that both tropical Pacific-forced and internal (independent of tropical Pacific forcing) atmospheric variability play important forcing roles. In particular, tropical Pacific-excited Aleutian low variability, whose center of action shifts southeastward compared to the climatology, contributes to forcing SNEP SST decadal variability. This contribtution could offer an important source for the predictability of SNEP SST decadal variability.

Various indices have been defined to characterize the phase and amplitude of the Madden-Julian oscillation (MJO). One widely used index is the Outgoing Longwave Radiation (OLR) based MJO index (OMI), which is calculated using the spatial pattern of 30-96-day eastward-filtered OLR. The EOFs used to calculate the OMI in observations are prone to degeneracy and exhibit oscillations on the order of 10-20 days, despite initial filtering of the OLR. We propose a simple modification to the OMI that involves aligning the EOFs between neighboring days while retaining the spatial pattern described by the EOFs. This rotation method is implemented as a postprocessing step of the current OMI calculation and cleanly removes the spurious oscillations and degeneracy issues seen in the standard method. A similar rotation procedure can be implemented in calculations of other MJO indices.

Benthic δ18O stacks are the benchmarks by which paleoceanographic data are stratigraphically aligned and compared. However, a recent study found that between 1.8-1.9 million years ago (Ma) several Ceara Rise records differed substantially from the widely used LR04 global stack. Here, we use new Bayesian stacking software to construct regional stacks and demonstrate a geographical divergence in benthic δ18O features from 1.8-1.9 Ma. The pattern of isotopic stage features observed in the Ceara Rise is widespread throughout the Atlantic and differs notably from Pacific records. We propose that this regional difference in isotopic stages may be the result of relatively strong precession forcing and weaker obliquity forcing between 1.8-1.9 Ma. In accordance with the Antiphase Hypothesis, our results highlight a period of apparent sensitivity to regional precession forcing that is masked during most of the 41-kyr world due to the amplitude modulation of obliquity forcing.

To explore the influence of fluorine (F) on mantle minerals and its behaviors during subducting, we investigated the compressibility of F-bearing superhydrous phase B (Shy-B) using synchrotron-based single-crystal X-ray diffraction combined with diamond anvil cells up to 27 GPa and 750 K. Our results show that the presence of F can largely enhance the incompressibility of Shy-B. Based on the obtained thermal elastic parameters, density and velocity profiles are evaluated along cold and warm slabs. Our results demonstrate that addition of F enhances the density (~1.3-1.7%) and the bulk velocity (~1.0-2.4%) of Shy-B relative to OH end-member at uppermost lower mantle conditions. The decomposition of F-bearing Shy-B would lead to an abrupt increase in density (~8.9-10.5%) and a small increase in bulk velocity (~0.7-1.8%). The combined results could provide constraints for modeling the geodynamic process related to subduction and transportation of F and OH into lower mantle.

The creation of new fractures controls fault slip, produces rock damage, and contributes to the dissipation of energy during earthquakes. Pulverized rocks around faults contain grains with a wide range of sizes, but the mechanisms that produces nanoscale grains remain elusive. Using molecular dynamics simulations, we model tensile rupture propagation in alpha-quartz under conditions of stress that occur during earthquake propagation. Our results show that for rupture speeds below 15 % of the Rayleigh wave speed, the fracture propagates straight. At higher speeds, fracture propagation undergoes path instabilities with crack oscillations and microbranching. We show that microbranching can lead to nanoscale fragments. Energy dissipation occurs by the creation of fracture surfaces and material damage; the dissipated energy increases with rupture speed. This nanoscale mechanism of irreversible deformation during earthquake propagation conditions contributes to the energy budget of earthquakes, and damage production in fault zones.