Monte Carlo simulation of pair creation using petawatt lasers

Irradiating high-Z targets such as gold with ultra-intense lasers creates electron-positron pairs. In particular, the positron density in the plasma created by this procedure is higher than that obtained via other laboratory-based methods, with theoretical maximum densities exceeding 10¹⁸ cm⁻³. All of the significantly contributing processes are well-known and hence we can study this phenomenon using Monte Carlo simulation. We focus on the latter part of this procedure, the passage of high-energy electrons through the target creating pairs. In particular, we discuss the usefulness of CERN’s GEANT4 Monte Carlo code in simulating this process. Once this code is successfully calibrated, we will use it to perform parameter studies, and design future targets to optimize the positron yield.     

Organic sedimentary deposits in Titan’s dry lakebeds: Probable evaporite

We report the discovery of organic sedimentary deposits at the bottom of dry lakebeds near Titan’s north pole in observations from the Cassini Visual and Infrared Mapping Spectrometer (VIMS). We show evidence that the deposits are evaporitic, making Titan just the third known planetary body with evaporitic processes after Earth and Mars, and is the first that uses a solvent other than water.     

Wide-band high linearity active dipole for low frequency radio astronomy

We have developed an active dipole that is intended for use in new generation low frequency array applications. The preamplifier of the active dipole has very high linearity (input IP2 = 70 dBm, input IP3 = 31 dBm) and low noise temperature (100?C360 K). The frequency dependence of the dipole impedance and the match between the dipole and preamplifier have been optimized to achieve Galactic noise limited operation. The ratio between the antenna temperature due to Galactic noise and the noise temperature of the preamplifier is 10 ± 1.5 dB over the whole 10 to 70 MHz range. The total cost of the active cross-dipole is 220 euro.     

Impact cratering in sandstone: The MEMIN pilot study on the effect of pore water

Abstract– Planetary surfaces are subjected to meteorite bombardment and crater formation. Rocks forming these surfaces are often porous and contain fluids. To understand the role of both parameters on impact cratering, we conducted laboratory experiments with dry and wet sandstone blocks impacted by centimeter‐sized steel spheres. We utilized a 40 m two‐stage light‐gas gun to achieve impact velocities of up to 5.4 km s^?1. Cratering efficiency, ejection velocities, and spall volume are enhanced if the pore space of the sandstone is filled with water. In addition, the crater morphologies differ substantially from wet to dry targets, i.e., craters in wet targets are larger, but shallower. We report on the effects of pore water on the excavation flow field and the degree of target damage. We suggest that vaporization of water upon pressure release significantly contributes to the impact process.     

Farside explorer: unique science from a mission to the farside of the moon

Farside Explorer is a proposed Cosmic Vision medium-size mission to the farside of the Moon consisting of two landers and an instrumented relay satellite. The farside of the Moon is a unique scientific platform in that it is shielded from terrestrial radio-frequency interference, it recorded the primary differentiation and evolution of the Moon, it can be continuously monitored from the Earth–Moon L2 Lagrange point, and there is a complete lack of reflected solar illumination from the Earth. Farside Explorer will exploit these properties and make the first radio-astronomy measurements from the most radio-quiet region of near-Earth space, determine the internal structure and thermal evolution of the Moon, from crust to core, and quantify impact hazards in near-Earth space by the measurement of flashes generated by impact events. The Farside Explorer flight system includes two identical solar-powered landers and a science/telecommunications relay satellite to be placed in a halo orbit about the Earth–Moon L2 Lagrange point. One lander would explore the largest and oldest recognized impact basin in the Solar System— the South Pole–Aitken basin—and the other would investigate the primordial highlands crust. Radio astronomy, geophysical, and geochemical instruments would be deployed on the surface, and the relay satellite would continuously monitor the surface for impact events.     

Statistical characteristics of the received signal for stochastic surface models

This paper describes the stochastic model of the scattered electromagnetic field. Unlike common-used functional-determined models the proposed is characterised by amplitude/phase fluctuation of the received signal.     

Low-cost inundation modelling at the reach scale with sparse data in the Lower Damodar River basin,India

Abstract

Data unavailability is the main reason for limited applications of hydrodynamic models for predicting inundation in the developing world. This paper aims to generate moderately high-resolution hybrid terrain data by merging height information from low-cost Indian Remote Sensing satellite (IRS) Cartosat-1 stereo satellite images, freely-available Shuttle Radar Topograph Mission (SRTM) digital elevation model (DEM) data, and limited surveyed channel cross-sections. The study reach is characterized by anabranching channels that are associated with channel bifurcation, loops and river islands. We compared the performance of a simple 1D–2D coupled LISFLOOD-FP model and a complex fully 2D finite element TELEMAC-2D model with the hybrid terrain data. The results show that TELEMAC-2D produced significantly improved simulated inundation with the hybrid terrain data, as compared to the SRTM DEM. LISFLOOD-FP was found unsuitable to work with the hybrid DEM in a complicated fluvial environment, as it failed to efficiently divert water in the branches from the main channel.

Editor D. Koutsoyiannis; Associate editor A. Viglione     

Normal modes of black hole accretion disks

This article studies the hydrodynamical problem of normal modes of small adiabatic oscillations of relativistic barotropic thin accretion disks around black holes (and compact weakly magnetic neutron stars). Employing Wentzel-Kramers-Brillouin (WKB) techniques, we obtain the eigenfrequencies and eigenfunctions of the modes for different values of the mass and angular momentum of the central black hole. We discuss the properties of the various types of modes and show for the first time that modes covering the whole extension of the disk (full-disk p-modes) can exist within the studied thin disk model. However, these modes have relatively short wavelengths.     

A generalized two-disk dynamo model

Abstract

A generalized two-disk dynamo model is considered that includes mechanical friction; this model is intended to simulate in its broad character the behavior of the geodynamo. Fixed points, limit cycles and chaotic attractors are located for different input parameters of the model. The chaotic regimes are of several kinds as are the “routes to chaos”. Several approximate models, helpful for studying the dynamo are discussed. A number of essential differences from the well-known Rikitake dynamo are demonstrated.     

Vortex evolution

Abstract

Friedmann's equation and the potential vorticity equation are generalised for turbulent motion. The generalised equations incorporate some new phenomena connected with turbulent transport of mass. It is proved that, if ?×[S×Ω+S(?·S)]≠0 where Ω is the absolute vorticity of the velocity and S is the turbulent density flux, then the Helmholtz-Kelvin theorem concerning the conservation of the velocity circulation around a closed path is violated and the potential vorticity is not a Lagrangian adiabatic invariant. The effects of this turbulent transport of mass on the creation or dissipation of vorticity discussed here is not equivalent to effects of baroclinicity or viscosity. Some possible implications of the new circulation theorem in geophysical and astrophysical fluid dynamics are discussed.