太阳大气中磁重联的MHD数值模拟

回顾了近３０年太阳大气中磁重联过程的ＭＨＤ数值模拟工作取得的进展。着重描述了在验证理论模型,解释观测现象,以及研究各种因素对重联的影响三个方面的成果,如快速磁重联,太阳耀斑机制及色球,日冕中的各种爆发现象等。指出了在数值模拟中应注意的几个问题,并对该领域今后的发展作了简要的展望。     

线性状态等式约束下两种H∞滤波的比较

近年带有线性等式约束的H∞滤波在实际应用中越来越广泛,针对此种情况,从理论上研究了线性状态等式约束下两种H∞滤波状态估计性能的比较问题。在投影系统和H∞滤波方程的基础上,考虑线性状态等式约束条件,给出了一种含线性状态等式投影系统约束的H∞滤波。通过比较其与状态投影下的H∞滤波的估计性能,指出并证明了投影系统约束的H∞滤波的估计性能优于状态投影下的H∞滤波的估计性能。最后给出一个数值模拟例子,来验证结论。     

Kilohertz quasi-periodic oscillations difference frequency exceeds inferred spin frequency in 4U 1636−53

We report the first detection of a sharp spectral feature in a narrow-line Seyfert 1 galaxy. Using XMM-Newton we have observed 1H     and find a drop in flux by a factor of more than 2 at a rest-frame energy of ∼ 7 keV without any detectable narrow Fe K α line emission. The energy of this feature suggests a connection with the neutral iron K photoelectric edge, but the lack of any obvious absorption in the spectrum at lower energies makes the interpretation challenging. We explore two alternative explanations for this unusual spectral feature: (i) partial-covering absorption by clouds of neutral material; and (ii) ionized disc reflection with lines and edges from different ionization stages of iron blurred together by relativistic effects. We note that both models require an iron overabundance to explain the depth of the feature. The X-ray light curve shows strong and rapid variability, changing by a factor of 4 during the observation. The source displays modest spectral variability which is uncorrelated with flux.     

Site effects close to structural lineaments in eastern Sicily (Italy)

This paper analyses site effects that are applied to compute the H/V spectral ratio close to structural lineaments. Aseismic creep and seismic creep have been examined. The lineaments are located in different types of volcanic soil. The aim of the work is to verify the possibility of fault-guided seismic waves within the structures, which behave as a waveguide for trapped waves.

The measurements of ambient noise were carried out along profiles crossing the structures, and the standard Nakamura technique was applied to calculate the spectral H/V ratio. It was observed that horizontal components of ground motion were found to be significantly amplified within the shear zone.     

Inexact Newton methods and the method of lines for solving Richards' equation in two space dimensions

Richards' equation (RE) is often used to model flow in unsaturated porous media. This model captures physical effects, such as sharp fronts in fluid pressures and saturations, which are present in more complex models of multiphase flow. The numerical solution of RE is difficult not only because of these physical effects but also because of the mathematical problems that arise in dealing with the nonlinearities. The method of lines has been shown to be very effective for solving RE in one space dimension. When solving RE in two space dimensions, direct methods for solving the linearized problem for the Newton step are impractical. In this work, we show how the method of lines and Newton-iterative methods, which solve linear equations with iterative methods, can be applied to RE in two space dimensions. We present theoretical results on convergence and use that theory to design an adaptive method for computation of the linear tolerance. Numerical results show the method to be effective and robust compared with an existing approach.     

天祝台阵地电场季节变化及其两种影响因素分析

地电场是联系空间Sq电流体系、地球表面电流和内部电流活动的地球物理量,它包含了空间电流系变化产生的大地电(流)场和区域环境变化等引起的自然电场。通过研究青藏高原东北缘的天祝台阵5个固定地电场台站连续10年观测数据,得出该区域地电场变化具有明显的366±(<1) d周期。且大地电流矢量方向有明显的季节变化,每半年改变一次方向,方向变化时间一般为每年4月和9～10月;大地电流矢量的强度变化也具有周期性。通过建立简单模型进行定量化分析,认为地电场季节变化主要受到Sq年变和区域气候环境的耦合作用,主要依据为地电场长趋势变化与电离层Sq电流年变化趋势吻合。基于区域性气候、冻融深度等季节性变化模型开展计算,认为气温导致了地下浅层介质电性结构、电极附近电位方向的变化,影响了地电场的长趋势变化。本文结果有助于认识超低频地电场年变周期特点及其原因,并探索其应用前景。     

Seismic response of deep Quaternary sediments in historical center of L’Aquila City (central Italy)

On 6 April 2009 a M_w=6.1 earthquake produced severe destruction and damage over the historic center of L’Aquila City (central Italy), in which the accelerometer stations AQK and AQU recorded a large amount of near-fault ground motion data. This paper analyzes the recorded ground motions and compares the observed peak accelerations and the horizontal to vertical response spectral ratios with those revealed from numerical simulations. The finite element method is considered herein to perform dynamic modeling on the soil profile underlying the seismic station AQU. The subsurface model, which is based on the reviewed surveys that were carried out in previous studies, consists of 200–400 m of Quaternary sediments overlying a Meso-Cenozoic carbonate bedrock. The Martin-Finn-Seed's pore-water pressure model is used in the simulations. The horizontal to vertical response spectral ratio that is observed during the weak seismic events shows three predominant frequencies at about 14 Hz, 3 Hz and 0.6 Hz, which may be related to the computed seismic motion amplification occurring at the shallow colluvium, at the top and base of the fluvial-lacustrine sequence, respectively. During the 2009 L’Aquila main shock the predominant frequency of 14 Hz shifts to lower values probably due to a peculiar wave-field incidence angle. The predominant frequency of 3 Hz shifts to lower values when the earthquake magnitude increases, which may be associated to the progressive softening of soil due to the excess pore-water pressure generation that reaches a maximum value of about 350 kPa in the top of fluvial-lacustrine sequence. The computed vertical peak acceleration underestimates the experimental value and the horizontal to vertical peak acceleration ratio that is observed at station AQU decreases when the earthquake magnitude increases, which reveals amplification of the vertical component of ground motion probably due to near-source effects.     

中国地区地磁异常静日（AQD）的初步分析

本文将G.M.Brown等人对离S_q电流体系焦点较远处台站的异常静日（AQD）的H分量分析,发展为对包括S_q电流体系焦点附近台站和Z、D分量在内的AQD分析。主要分析了中国五个地磁台D、H、Z三要素静日最大值和最小值出现时间的分布,及其季节变化和逐年变化规律。结果表明,D、Z也有和H类似的AQD现象,其出现的年频度,也有与太阳黑子数反相变化的趋势。在有三个多太阳周资料的佘山台,太阳极小年AQD(Z_max)出现的频度,和随后的太阳极大年的黑子数R呈近似线性的关系。最后,本文对今后我国开展变化磁场的分析研究提出了建议。     

单台地磁夜均值资料研究半年周期变化的电磁响应

用地磁夜均值资料研究半年周期变化的电磁响应 (地磁内外场比值 Q1) ,如夜均值半年周期变化的源场分布符合 P01分布模式 ,则可用单台资料进行计算 ,有利于进行地区性深地磁测深的研究 .本文选择了质量较好的 5个台约 2 6年的连续资料 ,又搜集了可用的全球分布的台站资料进行分析计算 .结果表明 ,从夜均值估算的半年周期变化的 Q1更为可靠 .原因可能是源场模式不同 .一般的日均值资料不符合 P01模式 ,而夜均值资料则比较符合 P01模式 .本文对此作了源场分布的研究 .