Weibel Turbulence in Laboratory Experiments and GRB/SN Shocks     

Mikhail V. Medvedev 《Astrophysics and Space Science》2007,307(1-3):245-250

It has recently been realized that the Weibel instability plays a major role in the formation and dynamics of astrophysical shocks of gamma-ray bursts and supernovae. Thanks to technological advances in the recent years, experimental studies of the Weibel instability are now possible in laser-plasma interaction devices. We, thus, have a unique opportunity to model and study astrophysical conditions in laboratory experiments – a key goal of the Laboratory Astrophysics program. Here we briefly review the theory of strong non-magnetized collisionless GRB and SN shocks, emphasizing the crucial role of the Weibel instability and discuss the properties of radiation emitted by (isotropic) electrons moving through the Weibel-generated magnetic fields, which is referred to as the jitter radiation. We demonstrate that the jitter radiation field is anisotropic with respect to the direction of the Weibel current filaments and that its spectral and polarization characteristics are determined by microphysical plasma parameters. We stress that the spectral analysis can be used for accurate diagnostics of the plasma conditions in laboratory experiments and in astrophysical GRB and SN shocks.  相似文献   

Phenomenological Theory of the Photoevaporation Front Instability     

D. D. Ryutov  J. O. Kane  A. Mizuta  M. W. Pound  B. A. Remington 《Astrophysics and Space Science》2007,307(1-3):173-177

The dynamics of photoevaporated molecular clouds is determined by the ablative pressure acting on the ionization front. An important step in the understanding of the ensuing motion is to develop the linear stability theory for an initially flat front. Despite the simplifications introduced by linearization, the problem remains quite complex and still draws a lot of attention. The complexity is related to the large number of effects that have to be included in the analysis: acceleration of the front, possible temporal variation of the intensity of the ionizing radiation, the tilt of the radiation flux with respect to the normal to the surface, and partial absorption of the incident radiation in the ablated material. In this paper, we describe a model where all these effects can be taken into account simultaneously, and a relatively simple and universal dispersion relation can be obtained. The proposed phenomenological model may prove to be a helpful tool in assessing the feasibility of the laboratory experiments directed towards scaled modeling of astrophysical phenomena. PACS Numbers: 98.38.Dq, 98.38.Hv, 52.38.Mf, 5257.FG, 52.72.+v  相似文献   

EFFECT OF PERPENDICULAR A.C. ELECTRIC FIELD ON THE OBLIQUE WHISTLER MODE INSTABILITY IN THE EARTH’S MAGNETOSPHERE     

A. K. Tripathi  R. P. Singhal 《Earth, Moon, and Planets》2005,97(1-2):91-106

The elements of dielectric tensor and dispersion relation for obliquely propagating whistler waves with finite in an infinite magnetoplasma are obtained for a kappa distribution in the presence of perpendicular a.c. electric field. Integrals and modified plasma dispersion functions are reduced in power series form. Numerical calculations have been performed to obtain temporal growth rate and real frequencies of the plasma waves for magnetospheric plasma, using linear theory of dispersion relation. The effect and modification introduced by the perpendicular a.c. electric field on the temporal growth rates, real frequencies and resonance condition are discussed for kappa and Maxwellian distributions. Our results and their interpretation are compared with known whistler observations obtained by ground-based techniques and satellite observations.  相似文献   

Instabilities of the tidally induced bottom boundary layer in the rotating frame and their mixing effect     

K. Sakamoto  K. Akitomo 《Dynamics of Atmospheres and Oceans》2006,41(3-4):191-211

To investigate the stability of the bottom boundary layer induced by tidal flow (oscillating flow) in a rotating frame, numerical experiments have been carried out with a two-dimensional non-hydrostatic model. Under homogeneous conditions three types of instability are found depending on the temporal Rossby number Ro_t, the ratio of the inertial and tidal periods. When Ro_t < 0.9 (subinertial range), the Ekman type I instability occurs because the effect of rotation is dominant though the flow becomes more stable than the steady Ekman flow with increasing Ro_t. When Ro_t > 1.1 (superinertial range), the Stokes layer instability is excited as in the absence of rotation. When 0.9 < Ro_t < 1.1 (near-inertial range), the Ekman type I or type II instability appears as in the steady Ekman layer. Being much thickened (100 m), the boundary layer becomes unstable even if tidal flow is weak (5 cm/s). The large vertical scale enhances the contribution of the Coriolis effect to destabilization, so that the type II instability tends to appear when Ro_t > 1.0. However, when Ro_t < 1.0, the type I instability rather than the type II instability appears because the downward phase change of tidal flow acts to suppress the latter. To evaluate the mixing effect of these instabilities, some experiments have been executed under a weak stratification peculiar to polar oceans (the buoyancy frequency N²  10⁻⁶ s⁻²). Strong mixing occurs in the subinertial and near-inertial ranges such that tracer is well mixed in the boundary layer and an apparent diffusivity there is evaluated at 150–300 cm²/s. This suggests that effective mixing due to these instabilities may play an important role in determining the properties of dense shelf water in the polar regions.  相似文献   

黄土高原一次β中尺度大暴雨特征及成因分析     

井宇  陈闯  赵红兰  井喜 《陕西气象》2012,(2):1-6

利用NCEP／NCAR逐日6h分析资料和常规观测资料,分析了2011年2月25日一28日山西连续降雪天气过程（以下简称2．25降雪）。①2．25降雪过程经历了三个阶段：2月25日为回流降雪阶段,26日一27日为倒槽冷锋与回流降雪共同影响降雪阶段,28日为低空切变线影响降雪阶段。②2．25降雪过程涵盖了华北地区大到暴雪的三个类型：回流类降雪、倒槽冷锋类降雪、低空切变线类降雪。③通过温湿场分析得出,对于低空切变线类降雪,对流层中层的湿核对降雪的开始有一定的指示意义。降雪未开始之前对流层中层有湿核,随着时间的推移,湿核向低层扩展,整个对流层中低层变为高湿区,降雪开始。当对流层中层变为干区,并向低层扩展,降雪过程结束。对于回流类降雪,低层回流对回流降雪起到冷垫的作用。④通过涡度场分析得出,对流层低层的负绝对涡度中心对其东侧的降水有指示意义,如果其东侧对流层低层配合有正涡度核,降雪强度较大,维持时间较长。⑤地面层出现的负绝对涡度中心说明近地面层有小高压系统的存在,这是因为低层回流冷垫作用形成的孤立小高压体。⑥28日降雪维持机制是条件性对称不稳定。  相似文献   

一次缓慢东移的黄河气旋暴雨的诊断分析   总被引：1，自引：0，他引：1  

范学峰  王新敏  胡燕平 《气象与环境科学》2012,35(3):10-16

利用NCEP/NCAR再分析资料和常规气象资料,对2001年6月28-29日发生在河南的一次黄河气旋暴雨过程进行了诊断分析。结果表明：黄河气旋的稳定少动是造成这次暴雨的直接原因,高层辐散与中层正涡度平流对黄河气旋形成和发展起了重要作用。这次过程的水汽主要来自副高东南侧的海上,孟加拉湾的水汽也有一定的贡献。暴雨区的中低层对称不稳定的存在,导致上升运动和水汽输送的加强,造成降水的增幅。  相似文献   

湿位涡诊断分析在河南台风远距离降水中的应用   总被引：2，自引：1，他引：1  

范学峰  席世平 《气象与环境科学》2012,35(2):25-32

应用湿位涡理论,分析了两个发生在河南的台风远距离降水个例,讨论了湿位涡与台风远距离降水形成的关系。结果表明：两个个例中,河南具有有利于强降水发生的湿位涡特征;异常暴雨的发展与湿位涡的变化有很好的对应关系：湿位涡的异常区域对应着最强的降水,对流层高层ζMPV1（湿位涡的垂直分量）正值区与低层ζMPV2负值区相互作用,即高层下滑的干冷空气与中低层由东南急流输送的高温高湿空气交汇,容易储存和释放湿对流不稳定能量,有利于强降水产生。湿位涡理论在河南台风远距离降水诊断中有很好的应用前景。  相似文献   

Unstable Dynamical Properties of Spiral Cloud Bands in Tropical Cyclones          下载免费PDF全文

HUANG Hong  ZHANG Ming 《Acta Meteorologica Sinica》2009,23(4):485-493

A nondivergent barotropic model （Model 1） and a barotropic primitive equation vortex model （Model 2） are linearized respectively in this paper. Then their perturbation wave spectrums are computed with a normal mode approach to study the instability problem on an appointed tropical cyclone （TC）-like vortex, thereby, the dynamic instability properties of spiral cloud bands of TCs are discussed. The results show that the unstable mode of both models exhibits a spiral band-like structure that propagates away from the vortex outside the radius of maximum winds. The discrete modal instability of the pure vortex Rossby wave can account for the generation of the eyewall and the inner spiral band. The unstable mode in Model 2 has three parts, i.e., eyewall, inner and outer spiral bands. This mode can be interpreted as a mixed vortex Rossby-inertia gravitational wave. The unbalanced property of the wave outside the stagnation radius of the vortex Rossby wave is one of the important reasons for the formation of the outer spiral band in TCs. Accordingly, the outer spiral band can be identified to possess properties of an inertial-gravitational wave. When the formation of unstable inner and outer spiral bands is studied, a barotropic vortex model shall be used. In this model, the most unstable perturbation bears the attributes of either the vortex Rossby wave or the inertial-gravitational wave, depending on the vortex radius. So such perturbations shall be viewed as an unbalanced and unstable mixed wave of these two kinds of waves.  相似文献   

INSTABILITY OF SYMMETRIC TYPHOON CIRCULATION AND ADAPTIVE OBSERVATION   总被引：1，自引：0，他引：1  

高守亭  周菲凡  刘黎平 《热带气象学报(英文版)》2009,15(2):162-166

This study presents a new way to identify the sensitive areas for typhoon adaptive observations, that is, sensitive areas are determined by invoking the negative anomalies of moist potential vorticity (MPV). It is found that the areas of negative MPV are the symmetric instability areas and can be taken as sensitive areas for typhoon adaptive observations. Three typhoons in 2008, Nuri, Fung-wong, and Fengshen, were simulated with the help of MM5 model. It is shown that these typhoons are well simulated in the first 12 hours. Based on these investigations, the calculations of MPV are carried out sequentially. The result shows that the negative maxima of MPV are always around the typhoon eyes for all the cases, which means that the sensitive areas are also near them all the time.  相似文献   

赤道不稳定波对海气相互作用影响的数值模拟分析   总被引：1，自引：0，他引：1  

陈鲜艳  胡博  木本昌秀  李维京 《大气科学》2009,33(1)

赤道不稳定波 (tropical instability waves) 存在于热带东太平洋赤道附近, 通常于每年的春末夏初出现, 以约0.6 m/s速度向西传播, 波周期为20～40天左右, 波长约为1000～2000 km.本文利用一个全球高分辨率海气耦合模式对赤道不稳定波在赤道附近的热量输送进行分析, 表明赤道不稳定波产生指向赤道的热通量, 从而部分抵消了热带东太平洋地区由Ekman辐散和温度平流导致的强冷却效应, 维持热带地区的热量平衡.其对赤道冷舌区的增暖作用可以消除和减弱气候模式中热带东太平洋地区的系统性冷偏差, 能使冷舌的强度和分布得到合理的改善, 对气候模式的改进和发展具有潜在贡献.赤道不稳定波还可以改变赤道海洋上空低层大气层结稳定度, 导致近地层强的风场辐合辐散, 并进一步影响大气混合层的温度、 风场等气象要素.模拟分析结果还表明, 赤道不稳定波对大气强迫产生二次响应, 改变赤道上空逆温层的垂直位移和逆温强度.研究赤道不稳定波对热带海洋气候及其海气相互作用机理的理解具有重要意义.  相似文献