Comparisons of Simulations of Soil Moisture Variations in the Yellow River Basin Driven by Various Atmospheric Forcing Data Sets

Based on station observations, The European Centre for Medium-Range Weather Forecasts reanalysis (ERA40), the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis and Princeton University's global meteorological forcing data set (Princeton), four atmospheric forcing fields were constructed for use in driving the Community Land Model version 3.5 (CLM3.5). Simulated soil moisture content throughout the period 1951-2000 in the Yellow River basin was vali...     

利用TRMM卫星资料对人工增雨云系模式云微观场预报能力的检验

文中利用TRMM卫星测雨雷达探测反演的云水、雨水、云冰和降冰4种云参数产品及实况降水资料,对比检验该人工增雨云系业务模式对云微观场和地面降水场的预报能力.结果表明,人工增雨云系模式系统对降水的预报能力要略优于现行业务运行的GRAPES模式;人工增雨云系模式系统能较好地预报云系系统云物理微观量的垂直结构特征,模式预报的微观场与卫星监测吻合较好;在播撒窗区的水平分布上,模式预报的各水凝物分布形势和强中心位置与卫星监测一致,其大小也接近监测值;人工增雨云系模式能较好地预报云的微观场和天气形势场,可作为云系人工增雨条件决策的重要参考依据.     

江苏夏季旱涝环流演变特征分析

利用NCEP/NCAR OLR月资料和北半球500 hPa逐月高度场距平格点资料,讨论江苏夏季旱涝天气气候的环流特征.研究表明:菲律宾以西太平洋暖池区OLR低值区值强弱直接影响江苏夏季降水,OLR值越低,该区辐合越强,则其北部副热带高压偏强,有利于副高的北抬和西伸,使副高北侧的雨带偏东偏北,江苏易偏旱;反之江苏易偏涝.500 hPa在东亚地区南北呈" 、-、 "的纬向分布,中高纬度乌拉尔山和鄂霍茨克海地区夏季500 hPa高度距平场为明显的正距平区,在该地区形成双阻或单阻有利于形势的稳定,使降水持续,江苏夏季降水偏多.     

Injection and arrest of dykes: implications for volcanic hazards

Dykes are the principal channels through which magma reaches the surface in volcanic eruptions. For this reason dykes observed in the field are commonly assumed to be feeders to lava flows. The actual proportion of dykes reaching the surface is, however, poorly known. In order to develop models for the purpose of estimating volcanic hazard, this proportion must be known. This follows because such models should not only consider the probability of dykes being injected from magma chambers during periods of unrest in the associated volcanoes, but also the probability of the injected dykes being arrested. This paper presents field data on several thousand dykes from Iceland and Tenerife (Canary Islands) indicating that many, and probably most, dykes become arrested at various crustal levels and never reach the surface to feed eruptions. Using the results of analytical and numerical models, it is shown that, for common loading conditions, the stress field in the vicinity of a magma chamber may favour the injection and propagation of dykes while the stress field at a certain distance from the chamber favours dyke arrest. This means that many dykes that are injected from the chamber propagate only for a very limited distance from the chamber to the point where they become arrested. The implication is that during periods of unrest in volcanoes, the probability of volcanic eruption is only a small fraction of the probability of dyke injection from the source magma chamber.     

New magnetic field measurements of β Cephei stars and slowly pulsating B stars

We present the results of the continuation of our magnetic survey with FORS 1 at the VLT of a sample of B‐type stars consisting of confirmed or candidate β Cephei stars and Slowly Pulsating B (hereafter SPB) stars, along with a small number of normal B‐type stars. A weak mean longitudinal magnetic field of the order of a few hundred Gauss was detected in three β Cephei stars and two stars suspected to be β Cephei stars, in five SPB stars and eight stars suspected to be SPB stars. Additionally, a longitudinal magnetic field at a level larger than 3σ has been diagnosed in two normal B‐type stars, the nitrogen‐rich early B‐type star HD 52089 and in the B5 IV star HD 153716. Roughly one third of β Cephei stars have detected magnetic fields: Out of 13 β Cephei stars studied to date with FORS 1, four stars possess weak magnetic fields, and out of the sample of six suspected β Cephei stars two show a weak magnetic field. The fraction of magnetic SPBs and candidate SPBs is found to be higher: Roughly half of the 34 SPB stars have been found to be magnetic and among the 16 candidate SPBs eight stars possess magnetic fields. In an attempt to understand why only a fraction of pulsating stars exhibit magnetic fields, we studied the position of magnetic and non‐magnetic pulsating stars in the H‐R diagram. We find that their domains in the H‐R diagram largely overlap, and no clear picture emerges as to the possible evolution of the magnetic field across the main sequence. It is possible that stronger fields tend to be found in stars with lower pulsating frequencies and smaller pulsating amplitudes. A somewhat similar trend is found if we consider a correlation between the field strength and the v sin i ‐values, i.e. stronger magnetic fields tend to be found in more slowly rotating stars (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tayler instability with Hall effect in young neutron stars

Collapse calculations indicate that the hot young neutron stars rotate differentially so that strong toroidal magnetic field components should exist in the outer shell where also the Hall effect appears to be important when the Hall parameter = ω_Bτ exceeds unity. The amplitudes of the induced toroidal magnetic fields are limited by the current‐induced Tayler instability. An important characteristics of the Hall effect is its distinct dependence on the sign of the magnetic field. We find for fast rotation that positive (negative) Hall parameters essentially reduce (increase) the stability domain. It is thus concluded that the toroidal field belts in young neutron stars induced by their differential rotation should have different amplitudes in both hemispheres which later are frozen in. Due to the effect of magnetic suppression of the heat conductivity also the brightness of the two hemispheres should be different. As a possible example for our scenario the isolated neutron star RBS 1223 is considered which has been found to exhibit different X‐ray brightness at both hemispheres (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Jitter radiation images, spectra and light curves from a relativistic spherical blastwave

We consider radiation emitted by the jitter mechanism in a Blandford–McKee self-similar blastwave. We assume the magnetic field configuration throughout the whole blastwave meets the condition for the emission of jitter radiation and we compute the ensuing images, light curves and spectra. The calculations are performed for both a uniform and a wind environment. We compare our jitter results to synchrotron results. We show that jitter radiation produces slightly different spectra than synchrotron, in particular between the self-absorption and the peak frequency, where the jitter spectrum is flat, while the synchrotron spectrum grows as  ν^1/3  . The spectral difference is reflected in the early decay slope of the light curves. We conclude that jitter and synchrotron afterglows can be distinguished from each other with good quality observations. However, it is unlikely that the difference can explain the peculiar behaviour of several recent observations, such as flat X-ray slopes and uncorrelated optical and X-ray behaviour.     

Probing magnetic turbulence by synchrotron polarimetry: statistics and structure of magnetic fields from Stokes correlators

We describe a novel technique for probing the statistical properties of cosmic magnetic fields based on radio polarimetry data. Second-order magnetic field statistics like the power spectrum cannot always distinguish between magnetic fields with essentially different spatial structure. Synchrotron polarimetry naturally allows certain fourth-order magnetic field statistics to be inferred from observational data, which lifts this degeneracy and can thereby help us gain a better picture of the structure of the cosmic fields and test theoretical scenarios describing magnetic turbulence. In this work we show that a fourth-order correlator of specific physical interest, the tension force spectrum, can be recovered from the polarized synchrotron emission data. We develop an estimator for this quantity based on polarized emission observations in the Faraday rotation free frequency regime. We consider two cases: a statistically isotropic field distribution, and a statistically isotropic field superimposed on a weak mean field. In both cases the tension force power spectrum is measurable; in the latter case, the magnetic power spectrum may also be obtainable. The method is exact in the idealized case of a homogeneous relativistic electron distribution that has a power-law energy spectrum with a spectral index of   p = 3  , and assumes statistical isotropy of the turbulent field. We carry out numerical tests of our method using synthetic polarized emission data generated from numerically simulated magnetic fields. We show that the method is valid, that it is not prohibitively sensitive to the value of the electron spectral index p , and that the observed tension force spectrum allows one to distinguish between e.g. a randomly tangled magnetic field (a default assumption in many studies) and a field organized in folded flux sheets or filaments.     

On the incidence of magnetic fields in slowly pulsating B, β Cephei and B-type emission-line stars

We have obtained 40 high-resolution circular spectropolarimetric measurements of 12 slowly pulsating B (SPB) stars, eight β Cephei stars and two Be stars with the Echelle Spectropolarimetric Device for the Observation of Stars at CFHT (ESPaDOnS) and Narval spectropolarimeters. The aim of these observations is to evaluate recent claims of a high incidence of magnetic field detections in stars of these types obtained using low-resolution spectropolarimetry by Hubrig et al. The precision achieved is generally comparable to or superior to that obtained by Hubrig et al., although our new observations are distinguished by their resolution of metallic and He line profiles, and their consequent sensitivity to magnetic fields of zero net longitudinal component. In the SPB stars, we confirm the detection of magnetic field in one star (16 Peg), but find no evidence of the presence of fields in the remaining 11. In the β Cep stars, we detect a field in  ξ¹  CMa, but not in any of the remaining seven stars. Finally, neither of the two B-type emission-line stars shows any evidence of magnetic field. Based on our results, we conclude that fields are not common in SPB, β Cep and B-type emission-line stars, consistent with the general rarity of fields in the broader population of main sequence B-type stars. A relatively small, systematic underestimation of the error bars associated with the UV Focal Reducer and Low Dispersion Spectrograph for the Very Large Telescope (FORS1) longitudinal field measurements of Hubrig et al. could in large part explain the discrepancy between their results and those presented here.