小尺度地磁场勒让德多项式建模方法

根据地面实测地磁场数据,提出一种基于勒让德多项式的对小尺度地磁场的模型建立方法,结合数据预处理、最小二乘拟合方法,采用高阶的模型描述典型小尺度区域的地磁场,绘制出地磁图.通过实验验证,模型拟合实验点的数据与真实数据的磁场总强度平均误差小于30 nT,并且反映出小尺度区域中磁场的精细结构,有效地抑制了边界效应,大大提高了地磁场建模能力.另外讨论了勒让德多项式建模方法的优缺点以及使用范围.这为地磁场在空间延拓、探矿、地下结构分析提供了重要的方法和理论基础.     

2017年11月23日重庆武隆5.0级地震前地磁谐波振幅比异常研究

近年来,地磁谐波振幅比方法在国内得到较广泛的应用,并成为一种较为有效的地震预测方法。本文利用谐波振幅比方法,对2017年11月23日重庆武隆5.0级地震震中附近台站的地磁三分量数据进行计算,并分析了各台站10~60min周期谐波振幅比值异常特征。研究发现,异常台站几乎均位于震源机制解的拉张区(P波向下的区域),具有象限分布特征;异常台站NS向谐波振幅比值变化存在分层现象,短周期(浅部)的变化与长周期(深部)变化成相反相位,从电磁学机理看,浅部与深部之间存在EW向的面电流;近震中台站的异常低点时间存在由长周期(深部)向短周期(浅部)迁移的现象。     

倾子资料三维共轭梯度反演研究

在对倾子响应和共轭梯度算法深入分析的基础上,我们实现了倾子资料三维共轭梯度反演算法.基于倾子资料的三维共轭梯度反演研究,探讨了利用倾子资料进行三维反演定量解释的方法.通过对理论模型合成数据进行反演试算,验证了所实现的倾子资料三维共轭梯度反演算法的有效性和稳定性.该反演算法可用于对大地电磁测深和地磁测深(地震地磁台站进行的三分量地磁观测)所整理出的倾子资料进行三维定量反演,获得地下三维模型的电阻率结构.     

太阳风引起的地磁扰动的参数辨识

作为太阳风引起的地磁扰动的系统辨识一文的姊妹篇,本文利用参数系统辨识方法,对太阳风-磁层耦合系统做了进一步的探讨.根据地磁扰动机制的理论研究,地磁扰动被认为是太阳风能量耦合进入磁层以及由此引起的一系列磁层内能量的输运和耗散的结果.在此基础上,对太阳风-磁层系统的非线性情况进行了讨论,建立了Dst指数与太阳风参量的非线性函数关系,给出了由此得到的地磁扰动计算结果.同时还利用1967年2月3—12日和1980年12月16—23日的两次事件,对已得到的函数关系进行了验证和讨论.验证结果不仅对模型的合理性、普适性做了很好的说明,也表明这种非线性函数关系作为定量的地磁预报模型,具有一定的可信度和实用性.     

2013年岷县—漳县6.6级地震地磁垂直分量日变化低点时间空间分布特征

2013年6月16日, 中国大陆西部出现地磁低点位移。 异常出现后一个月内, 位于低点位移分界线附近发生7月22日岷县、 漳县交界6.6级地震。 本文通过采用实测低点时间减去理论低点时间的方法剔除低点时间经度效应, 并以剔除经度效应后低点时间为0 h的等值线作为低点位移分界线, 2013年岷县—漳县6.6级地震震中处于6月16日地磁低点位移分界线上。 对异常期间的中国大陆地区地磁垂直分量日变形态进行统计, 发现其存在显著的分区特征, 低点时间为正的区域内日变化形态基本呈现W型, 低点时间为负的区域内日变化形态主要呈现V字型特征。 低点位移异常日期间, 中国大陆存在显著的高梯度带, 本文采用梯度值定量描述低点时间空间变化程度。 在6月13日正常日期间, 中国大陆地区未出现梯度值大于1.5 h/(°)的区域, 在6月16日异常日期间, 出现梯度值大于1.5 h/(°)的区域, 2013年岷县—漳县6.6级地震位于高梯度区边缘。     

中国地磁台环境场的计算与分析

根据国际参考地磁场(IGRF)模型,计算出1945年5月—1990年5月中国部分地磁台的计算年均值,并与相应观测年均值比较,计算出地磁台各个地磁要素的磁异常值以及地磁台异常场的模量,本文的计算结果将为认识地磁台的地磁场环境、地磁台站资料评比提供依据。     

The effect of expansion ratio on the critical Reynolds number in single fracture flow with sudden expansion

Flow in a single fracture (SF) is an important research subject in groundwater hydrology, hydraulic engineering, radioactive nuclear waste repository and geotechnical engineering. An abruptly changing aperture is a unique type of SF. This study discusses the relation between the values of the critical Reynolds number (Re_c) for the onset of symmetry breaking of flow and the expansion ratio (E) of SF, which is defined as the ratio between the outlet (D) and inlet (d) apertures. This study also investigates the effect of inlet aperture d on Re_c for flow in an SF with abruptly changing apertures (SF‐ACA) using the finite volume method. Earlier numerical and experimental results showed that flow is symmetric in respect to the central plane of the SF‐ACA at small Reynolds number (Re) but becomes asymmetric when Re is sufficiently large. Our simulations show that the value of Re_c decreases with the increasing E, and the relationship between the logarithm of Re_c and E can be described accurately using either a quadratic polynomial function or a logarithmic function. However, the relationship of Re_c and d for a given E value is vague, and Re_c becomes even less sensitive to d when E increases. This study also reveals that the hydraulic gradient (J) and flow velocity (v) follow a super‐linear relationship that can be fitted almost perfectly by the Forchheimer equation. The inertial component (J_i) of J increases monotonically with Re, whereas the viscous component (J_v) of J decreases monotonically with Re. The Re value corresponding to equal inertial and viscous components of J (named as the transitional point Re) decreases when E increases, and such a transitional point Re should be closely related to the critical Reynolds number Re_c, although a rigorous theoretical proof is not yet available. Copyright © 2015 John Wiley & Sons, Ltd.     

磁暴期间高纬顶部电离层离子上行特征——DMSP卫星观测

本文利用DMSP F13和F15卫星观测数据,对2001—2005年58个磁暴(-472 nT≤Min.Dst≤-71 nT)期间高纬顶部电离层离子整体上行特征进行了统计研究.观测表明,磁暴期间,顶部电离层离子上行主要发生在极尖区和夜间极光椭圆区.在北半球,磁正午前,高速的离子上行(≥500 m·s-1)多集中在65° MLat以上;午后,高速离子上行区向低纬度扩展,上行速度要略高于午前;在南半球,磁午夜前,DMSP卫星在考察区域内几乎所有的纬度上都观测到了高速上行的离子;午夜后,各纬度上观测到上行离子的速度明显降低.离子上行期间,DMSP卫星在极区顶部电离层高度上也频繁地观测到电子/离子增温,且电子增温发生的频率要远高于离子增温.O+密度变化分析显示,DMSP卫星磁暴期间观测到的上行离子更多地源于顶部电离层高度.这些结果表明电子增温在驱动暴时电离层离子整体上行过程中起着重要作用.     

The Dependence of the Geoeffectiveness of Interplanetary Flux Rope on Its Orientation,with Possible Application to Geomagnetic Storm Prediction

Interplanetary magnetic clouds (MCs) are one of the main sources of large non-recurrent geomagnetic storms. With the aid of a force-free flux rope model, the dependence of the intensity of geomagnetic activity (indicated by Dst index) on the axial orientation (denoted by θ and φ in GSE coordinates) of the magnetic cloud is analyzed theoretically. The distribution of the Dst values in the (θ, φ) plane is calculated by changing the axial orientation for various cases. It is concluded that (i) geomagnetic storms tend to occur in the region of θ<0°, especially in the region of θ≲−45°, where larger geomagnetic activity could be created; (ii) the intensity of geomagnetic activity varies more strongly with θ than with φ; (iii) when the parameters B ₀ (the magnetic field strength at the flux rope axis), R ₀ (the radius of the flux rope), or V (the bulk speed) increase, or |D| (the shortest distance between the flux rope axis and the x-axis in GSE coordinates) decreases, a flux rope not only can increase the intensity of geomagnetic activity, but also is more likely to create a storm, however the variation of n (the density) only has a little effect on the intensity; (iv) the most efficient orientation (MEO) in which a flux rope can cause the largest geomagnetic activity appears at φ∼0° or ∼ 180°, and some value of θ which depends mainly on D; (v) the minimum Dst value that could be caused by a flux rope is the most sensitive to changes in B ₀ and V of the flux rope, and for a stronger and/or faster MC, a wider range of orientations will be geoeffective. Further, through analyzing 20 MC-caused moderate to large geomagnetic storms during 1998 – 2003, a long-term prediction of MC-caused geomagnetic storms on the basis of the flux rope model is proposed and assessed. The comparison between the theoretical results and the observations shows that there is a close linear correlation between the estimated and observed minimum Dst values. This suggests that using the ideal flux rope to predict practical MC-caused geomagnetic storms is applicable. The possibility of the long-term prediction of MC-caused geomagnetic storms is discussed briefly.     

近12000年以来北京地区地球磁场变化机理探讨

距今12000年以来北京地区地球磁场长期变化由长周期(大于1000年)和短周期(约500年)两部分组成，长周期和短周期分量分别受控于漂移场和稳定场的变化.与日本地区相比，北京地区地球磁场最显著的特征是在距今(5110-4670)±110年之间曾发生短极性漂移事件，这样的短极性地磁事件可能与地球外核流体运动的异常变化有关.