三维试验粒子轨道法在磁层粒子全球输运中的应用

根据磁层粒子动力学理论, 通过偶极磁场模型验证利用三维试验粒子轨道方法模拟近地球区(r < 8R_e)带电粒子运动特征的可靠性. 在此基础上, 以太阳风和磁层相互作用的全球MHD模拟结果为背景, 利用三维试验粒子轨道方法, 对非磁暴期间南向行星际磁场背景下太阳风离子注入磁层的情形进行数值模拟, 并对北向行星际磁场背景下太阳风离子注入极尖区以及内磁层的几种不同情形进行了单粒子模拟. 模拟结果反映了南向和北向行星际磁场离子向磁层的几种典型输入过程, 揭示出行星际磁场南向时太阳风粒子在磁层内密度分布的晨昏不对称性以及其在磁鞘和磁层内的大致分布, 并得出统计规律. 模拟结果与理论预测和观测结论相一致, 且通过数值模拟发现, 行星际磁场北向时靠近极尖区附近形成的非典型磁镜结构对于能量粒子经由极尖区注入环电流区域过程有重要的影响和作用.      

月球尾迹的二维MHD模拟

采用理想的二维单流体MHD方程,对太阳风通过月球时所形成的尾迹结构进行数值模拟,得到了太阳风尾迹的粒子分布及磁场分布.模拟结果表明,在月球背阳面的本影区,太阳风粒子密度急剧下降,行星际磁场增强.当行星际磁场与太阳风流动方向平行时,尾迹被拖得很长,而磁场与太阳风流动方向垂直时,尾迹较短.     

1998年5月日冕三维结构的数值模拟

采用三维理想磙流体力学（MHD）模式，内边界条件把二维投影特征线边界方法推广应用到三维计算，有效地稳定了数值计算并保证稳态解的自洽性；初始猜解磁场由1935卡林顿周光球磁场观测数据得到，这样计算得到的1998年5月份期间日冕三维结构比较符合实际，计算结果表明：（1）计算得到的源表面非径向磁场量值在磁中性线附近不超过2μT，表明源表面磁场基本径向。（2）模拟得到的源表面径向磁场量值除了在磁中性线附近的区域外变化不大，这和观测一致。（3）由源表面磁场按平方反比的规律计算出1AU处磁场量值更接近观测值。（4）计算得到的日冕结构和观测定性一致，三维数值模拟结果表明，日冕的三维大尺度背景结构主要是由磁场决定的，在闭磁场处或者电流片附近，太阳风的密度高，速度低；在开场区，太阳风的密度低，速度高。     

球坐标系六片网格下三维定态行星际太阳风模拟

采用二阶MacCormack差分格式, 利用稳态的磁流体(MHD)方程组在球坐标系六片网格下模拟研究了行星际太阳风. 六片网格系统能有效避免极区奇性和网格收敛性. 迭代按径向方向推进求解, 很大程度上减少了计算量, 节约了计算时间. 内边界条件根据太阳与行星际观测确定, 比较测试了5种内边界条件, 模拟给出了1922卡林顿周的背景太阳风结构. 几种内边界条件所得模拟结果与行星际观测基本吻合. 太阳风速度采用McGregor 等的经验公式给出, 磁场由水平电流片(HCCS)模型得到, 密度和温度分别根据动量守恒和气压守恒得到, 研究表明采用这样的边界条件模拟结果最佳.      

SIP-AMR-CESE模式对CR2055背景太阳风的模拟

使用三维太阳行星际自适应网格守恒元解元太阳风模型（SIP-AMR-CESE MHD）,模拟从太阳表面到地球轨道附近的太阳风.该模型使用六片网格技术,同时利用PARAMESH软件包实现网格自适应.在该模型的基础上,通过增加广义拉格朗日乘子（GLM）磁场散度误差消去方法,完善网格加密放粗判据,微调加速加热形式等方法,使模拟结果与观测更好地符合.另外,通过控制不同时刻的计算区域,显著提高了模型的计算效率.在此基础上,给出了模型改进后模拟得到的CR2055太阳风稳态解与观测的对比分析.     

WSA太阳风经验模型及其应用

Wang-Sheely-Arge （WSA）模型是对准稳态太阳风的经验和物理相结合的描述,其利用观测的日面磁图作为输入,可以提前3到4天预测L1点处的太阳风速度和行星际磁场极性.WSA模型是在WS模型的基础上经过若干改进形成的实时预报模式,之后又借鉴Distance from the Coronal Hole Boundary （DCHB）模型的参数,进一步改进了太阳风速度关系式,形成了目前常见的形式.WSA经验模型由日冕磁场模型、太阳风速度关系式和一维运动学模型三部分组成.在实际应用过程中,基本步骤包括观测磁图预处理、日冕三维磁场反演、计算日冕磁场参数、计算太阳风的速度分布和将太阳风映射到1AU等环节.在发展过程中,WSA模型经历了一些细节上的调整变化,例如观测磁图数据的来源、日冕磁场模型的类型、经验速度关系中自由系数的取值等.许多研究对如何改善模型的预报效果进行了探索.     

1997年1月空间天气事件的三维MHD模拟

以1997年1月空间天气事件期间的观测为依据,在构造了比较接近真实的背景太阳风基础上,进一步利用三维时变的MHD模式,模拟了CME (日冕物质抛射)激发的扰动在行星际空间的传播过程,对地球空间环境的影响及行星际磁场南向分量B_z在1AU的时间经历.模拟结果与WIND卫星的观测进行了比较.结果表明,模拟与观测得到的扰动到达地球的时间、地球空间环境各量的变化及B_z的时间经历基本一致.     

地球弓激波的三维模拟

利用磁流体动力学(MHD)全球模拟结果,根据弓激波的跃变特性确定出弓激波位置,建立了一个新的综合考虑了快磁声马赫数、太阳风动压、行星际磁场强度以及磁层顶曲率半径的弓激波三维位型模型.将新模型与以往模型的模拟结果进行比较发现,新的弓激波全球模型结果可靠,解决了部分现有模型不能描述弓激波三维位型的问题.研究结果表明,在行星际磁场北向时,随着快磁声马赫数的增大,弓激波日下点距离减小,但是在行星际磁场南向时,快磁声马赫数的变化对弓激波日下点距离影响不大;弓激波位型在赤道面与子午面上存在明显的不对称性,而且随着行星际磁场的转向,这种非对称性也会发生相应改变;行星际磁场南向,B_z值较小时,子午面内弓激波位型已经不是简单的抛物线,出现了明显的类似于极尖区磁层顶的凹陷变化区.      

磁云边界层中的复合重联喷流观测分析

太阳风中的磁场重联通常与行星际日冕物质抛射有关.本文分析了1995年10月18日WIND飞船观测到的一例磁云前边界层中的复合重联喷流事件.该复合排空区由相邻两个不同方向的喷流构成,这两个喷流分别经过Walén关系的证认,符合行星际磁场重联排空区等离子体喷流的特征.结果表明,在磁云前端可能存在众多重联点,从而将磁云本体的磁场剥离,形成比单一重联喷流区更复杂的三维边界层结构.磁云边界层中可能发生多点多次重联,从而不表现出单点重联的排空区特征,这可能是行星际磁场重联排空区较少在ICME前端被观测到的原因之一.     

背景太阳风数值模拟的熵守恒格式

背景太阳风研究是根据行星际扰动传播情况预测空间天气状况的基础,磁流体（MHD）模拟是背景太阳风研究的重要手段.采用一种新的数值计算方式,利用Ideal GLM-MHD将计算过程中产生的磁场散度以c_h的速度向计算区域外传播,从而消去磁场散度;重构部分使用受约束的最小二乘法,将磁场散度作为约束条件添加到重构中,进一步对重构后的磁场梯度进行修正;通量计算采用满足热力学第二定律的熵守恒格式,该格式能够确保在计算过程中熵不增,保证数值稳定.研究结果表明,该方法应用于太阳风数值模拟的求解得到了更加稳定的结果.     

Three-dimensional global simulation of multiple ICMEs’ interaction and propagation from the Sun to the heliosphere following the 25–28 October 2003 solar events

This study performs simulations of interplanetary coronal mass ejection (ICME) propagation in a realistic three-dimensional (3D) solar wind structure from the Sun to the Earth by using the newly developed hybrid code, HAFv.2+3DMHD. This model combines two simulation codes, Hakamada–Akasofu–Fry code version 2 (HAFv.2) and a fully 3D, time-dependent MHD simulation code. The solar wind structure is simulated out to 0.08 AU (18 Rs) from source surface maps using the HAFv.2 code. The outputs at 0.08 AU are then used to provide inputs for the lower boundary, at that location, of the 3D MHD code to calculate solar wind and its evolution to 1 AU and beyond. A dynamic disturbance, mimicking a particular flare’s energy output, is delivered to this non-uniform structure to model the evolution and interplanetary propagation of ICMEs (including their shocks). We then show the interaction between two ICMEs and the dynamic process during the overtaking of one shock by the other. The results show that both CMEs and heliosphere current sheet/plasma sheet were deformed by interacting with each other.     

Some characteristics of propagation of flare- or CME-associated interplanetary shock waves

Many interplanetary shock waves have a fast mode MHD wave Mach number between one and two and the ambient solar wind plasma and magnetic field are known to fluctuate. Therefore a weak, fast, MHD interplanetary shock wave propagating into a fluctuating solar wind region or into a solar wind stream will be expected to vary its strength.It is possible that an interplanetary shock wave, upon entering such a region will weaken its strength and degenerate into a fast-mode MHD wave. It is even possible that the shock may dissipate and disappear.A model for the propagation of a solar flare - or CME (Coronal Mass Ejections) - associated interplanetary shock wave is given. A physical mechanism is described to calculate the probability that a weak shock which enters a turbulent solar wind region will degenerate into a MHD wave. That is, the shock would disappear as an entropy-generate entity. This model also suggests that most interplanetary shock waves cannot propagate continuously with a smooth shock surface. It is suggested that the surface of an interplanetary shock will be highly distorted and that parts of the shock surface can degenerate into MHD waves or even disappear during its global propagation through interplanetary space. A few observations to support this model will be briefly described.Finally, this model of shock propagation also applies to corotating shocks. As corotating shocks propagate into fluctuating ambient solar wind regions, shocks may degenerate into waves or disappear.     

一个基于观测的太阳风背景模型

建立由太阳光球磁场和日冕偏振亮度等观测约束的单流体太阳风模型,包括日冕和太阳风的等离子体密度、速度和磁场,温度还有待于以后处理.这里采用高山观测台(HAO)MKⅢ的日冕偏振亮度(pB)在1.36Rs上的观测概图,根据Guhathakurta在1996年发展的日冕电子密度反演模型确定日冕的电子密度分布.同时采用Wilcox太阳观测台(WSO)的光球磁场视向分量的观测概图作为底部边界,根据Zhao等在1994年发展的水平电流-电流片(HCCS)模型得到全球磁场.Phillips在1995年及McComas在2003年分别用Ulysses第一次和第二次跨极飞行的观测发现,归一化到1 AU的太阳风动量流密度除了在10°～30°的纬度范围内略低以外几乎不变.根据这一结论,结合已经得到的密度数据,就可以得到日冕和太阳风的速度.将上面的模型应用于1918卡林顿自转周稳态太阳风的研究,结果与太阳活动极小期的观测基本相符,但是与观测相比较低速高密度区偏大,因此密度模型还有待改进.     

The accuracy of present models of the high altitude polar magnetosphere

The Polar satellite has explored the high-latitude, high-latitude magnetosphere out to 9 Earth radii (Re). The magnetic field data returned from this mission can be used both to provide data for new empirical models and to test existing models. Tests include comparing the observed location of the polar cusp with its position in the empirical models and comparing the strength of the magnetic field in the surrounding region. Near the cusp the magnetosphere is quite sensitive to solar wind conditions. In particular the energy density of the cusp plasma depends on the pressure of the solar wind applied to the interface of the cusp and the sheath. The applied pressure in turn depends on the shape of the magnetopause and the orientation of that interface, both controlled by the direction of the interplanetary magnetic field. Magnetohydrodynamic (MHD) models provide a coarse picture of the magnetosphere at high latitudes. While generally quite realistic, these too require testing against observations because even the MHD models must make some simplifying assumptions.     

Prompt effects of solar wind variations on the inner magnetosphere and midlatitude ionosphere

It is well known that the solar wind can significantly affect high-latitude ionospheric dynamics. However, the effects of the solar wind on the middle- and low-latitude ionosphere are much less studied. In this paper, we report observations that large perturbations in the middle- and low-latitude ionosphere are well correlated with solar wind variations. In one event, a significant (20–30%) decrease of the midlatitude ionospheric electron density over a large latitudinal range was related to a sudden drop in the solar wind pressure and a northward turning of the interplanetary magnetic field, and the density decrease became larger at lower latitudes. In another event, periodic perturbations in the dayside equatorial ionospheric E × B drift and electrojet were closely associated with variations in the interplanetary electric field. Since the solar wind is always changing with time, it can be a very important and common source of ionospheric perturbations at middle- and low-latitudes. The relationship between solar wind variations and significant ionospheric perturbations has important applications in space weather.     

Martian obstacle and bow shock: origins of boundaries anisotropy

The origin of the anisotropy in the shape of the Martian obstacle and bow shock is analyzed using Mars Global Surveyor observations. The influence of MHD or ion pick-up effects on Martian obstacle position was to be small found, however, localized Martian crustal magnetization increases the thickness of the downstream planetary magnetotail by 500–1000 km in agreement with earlier Phobos 2 observations. A new analytical model is presented for Martian obstacle shape variation for different solar wind ram pressure. Elongation of the Martian BS cross-section in the direction perpendicular to IMF was confirmed while the shift of this cross section in the +Y direction of Martian interplanetary medium reference frame was discovered. The shift of BS cross section in the direction of interplanetary electric field was not revealed thus not conforming the idea that mass-loading play some role in BS control.     

Cone model-based SEP event calculations for applications to multipoint observations

The problem of modeling solar energetic particle (SEP) events is important to both space weather research and forecasting, and yet it has seen relatively little progress. Most important SEP events are associated with coronal mass ejections (CMEs) that drive coronal and interplanetary shocks. These shocks can continuously produce accelerated particles from the ambient medium to well beyond 1 AU. This paper describes an effort to model real SEP events using a Center for Integrated Space weather Modeling (CISM) MHD solar wind simulation including a cone model of CMEs to initiate the related shocks. In addition to providing observation-inspired shock geometry and characteristics, this MHD simulation describes the time-dependent observer field line connections to the shock source. As a first approximation, we assume a shock jump-parameterized source strength and spectrum, and that scatter-free transport occurs outside of the shock source, thus emphasizing the role the shock evolution plays in determining the modeled SEP event profile. Three halo CME events on May 12, 1997, November 4, 1997 and December 13, 2006 are used to test the modeling approach. While challenges arise in the identification and characterization of the shocks in the MHD model results, this approach illustrates the importance to SEP event modeling of globally simulating the underlying heliospheric event. The results also suggest the potential utility of such a model for forcasting and for interpretation of separated multipoint measurements such as those expected from the STEREO mission.