Stabilisation of BGK modes by relativistic effects

Context. We examine plasma thermalisation processes in the foreshock region of astrophysical shocks within a fully kinetic and self-consistent treatment. We concentrate on proton beam driven electrostatic processes, which are thought to play a key role in the beam relaxation and the particle acceleration. Our results have implications for the effectiveness of electron surfing acceleration and the creation of the required energetic seed population for first order Fermi acceleration at the shock front. Aims. We investigate the acceleration of electrons via their interaction with electrostatic waves, driven by the relativistic Buneman instability, in a system dominated by counter-propagating proton beams. Methods. We adopt a kinetic Vlasov-Poisson description of the plasma on a fixed Eulerian grid and observe the growth and saturation of electrostatic waves for a range of proton beam velocities, from 0.15c to 0.9c. Results. We can report a reduced stability of the electrostatic wave (ESW) with increasing non-relativistic beam velocities and an improved wave stability for increasing relativistic beam velocities, both in accordance with previous findings. At the highest beam speeds, we find the system to be stable again for a period of ≈160 plasma periods. Furthermore, the high phase space resolution of the Eulerian Vlasov approach reveals processes that could not be seen previously with PIC simulations. We observe a, to our knowledge, previously unreported secondary electron acceleration mechanism at low beam speeds. We believe that it is the result of parametric couplings to produce high phase velocity ESW’s which then trap electrons, accelerating them to higher energies. This allows electrons in our simulation study to achieve the injection energy required for Fermi acceleration, for beam speeds as low as 0.15c in unmagnetised plasma

Zircon from the East Orebody of the Bayan Obo Fe–Nb–REE deposit, China, and SHRIMP ages for carbonatite-related magmatism and REE mineralization events

Extremely U-depleted

Best of both worlds: combining SHRIMP and CA-TIMS methods in refining geochronological determinations for timescale calibration

Accurate and precise calibration of the geological timescale is a fundamental aspect of geoscience. The advent of the 'chemical abrasion' technique (e.g., Mattinson, 2005) for zircon preparation prior to thermal ionisation mass spectrometry (CA-TIMS) has significantly improved concordance and coherence of single-zircon analyses across a range of rock types, enhancing the geological accuracy of the interpreted crystallisation ages. This development affords an excellent opportunity to revisit and re-evaluate the accuracy of SHRIMP 206Pb/238U dating of natural zircon, and particularly the hypothesis that carefully targeting the 'best' areas of the 'best' grains with the ion beam should yield crystallisation ages closely comparable to those determined via CA-TIMS (cf. Kryza et al., 2012). This study examines a series of 10 samples, largely assumed to be airfall ash beds, within the coal-rich Middle to Late Permian successions of the Sydney and Bowen basins of eastern Australia. Recent work focussed on understanding mass extinctions and climate change across the critical Late Permian-Early Triassic boundary in eastern Australia has been hampered by the endemic nature of the local faunal assemblages, which has precluded reliable high-resolution biostratigraphic correlations to global stratotypes in China and elsewhere

Provenance history of the Bangemall Supergroup and implications for the Mesoproterozoic paleogeography of the West Australian Craton

The 4-10 km-thick Bangernall Supergroup, comprising the Edmund and Collier Groups, was deposited between 1620 and 1070 Ma in response to intracratonic extensional reactivation of the Paleoproterozoic compressional Capricorn Orogen. The supergroup can be further divided into six depositional packages bounded by unconformities or major marine flooding surfaces. U-Pb dating of over 1200 detrital zircon grains from 19 samples representative of each of the major sandstone units within these packages has failed to identify any zircon populations attributable to syndepositional magmatism. However, this extensive dataset provides a provenance history of the Bangernall Supergroup, which is here integrated with paleocurrent data which indicates that all source areas were located within the Mesoproterozoic West Australian Craton, with the main source area for the northern Bangemall Supergroup being the Gascoyne Complex and southern Pilbara Craton. All samples have prominent age modes in the 1850-1600 Ma range, indicating significant contribution from the northern Gascoyne Complex and coeval sedimentary basins. Some samples also display prominent modes in the 2780-2450 Ma range, consistent with derivation from the Fortescue and Hamersley Groups of the southern Pilbara Craton. The Edmund Group has age-spectra in which the dominant modes become older upwards, recording unroofing of the underlying basement from the Gascoyne Complex to the Archean granites and greenstones of the Pilbara Craton. In contrast, the Collier Group records unroofing of the underlying Edmund Group, with possible additional contribution from the Pilbara Craton and Paterson Orogen, and is characterized by age-spectra in which the dominant modes become younger upwards. These data imply that the West Australian Craton remained intact throughout the Mesoproterozoic assembly of Rodinia, and was the only source of detritus for the Bangernall Supergroup. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.</p

Biblioteca Popular del Municipio : resumen del movimiento de la biblioteca durante el mes de julio de 1886

Hf model ages of zircons provide information on the timing of new crust extraction from mantle. However, many model ages are hybrid due to mixing in the sedimentary environment. Zircons with mantle δ18O values (5.0-5.6‰) [1] are more likely to preserve Hf model ages that reflect actual crust forming events.\ud \ud Hf and O isotopes have been analysed in 3.9-2.8 Ga detrital zircons from the ~2.8 Ga Mesoarchean cover succesion in the Slave craton, Canada [2]. The zircons with mantle-like δO18 form two linear arrays in zircon crystallization age versus initial εHf plots consistent with two episodes of crust formation at ~3.45 Ga and ~3.75 Ga. Slopes of the linear arrays correspond to ¹⁷⁶Lu/¹⁷⁷Hf ratios of ~0.022 suggesting that the sources of the magmas from which zircons crystallized was mafic in composition. The zircons with the lowest initial εHf through time all have elevated δ18O values, but they also scatter around a linear array with the slope corresponding to ¹⁷⁶Lu/¹⁷⁷Hf ratio of ~0.022 consistent with mafic crust deriavtion from the mantle at ~4.4-4.5 Ga. Zircons that crystallized during crust formation and crustal recycling events both show a range of initial εHf and often elevated δ18O consistent with derivation of magmas from heterogenous crust composed of both unaltered, igneous and weathered, probably sedimentary rocks. Mafic crust generated in three episodes in the Slave Province was the source of magmas throughout the Archean, and zircons from Gondwana indicate that similar unaltered mafic crust could have had even longer residence times of over 1.5 Ga [3]

Aspects of electron acoustic wave physics in laser backscatter from plasmas

Recent experimental results from the Trident laser confirm the importance of kinetic effects in determining laser reflectivities at high intensities. Examples observed include scattering from low frequency electron acoustic waves (EAWs) and the first few stages of a cascade towards turbulence through the Langmuir decay instability. Interpretive and predictive computational capability in this area is assisted by the development of Vlasov codes, which offer high velocity space resolution in high energy regions of particle phase space and do not require analytical pre-processing of the fundamental equations. A direct Vlasov solver, capable of resolving these kinetic processes, is used here to address fundamental aspects of the existence and stability of the electron acoustic wave, together with its collective scattering properties. These simulations are extended to realistic laser and plasma parameters characteristic of single hot-spot experiments. Results are in qualitative agreement with experiments displaying both stimulated Raman and stimulated electron acoustic scattering. The amplitude of simulated EAWs is greater than that observed experimentally and is accompanied by a higher phase velocity. These minor differences can be attributed to the limitations of a one-dimensional collisionless model