【作者】 何宏青；

【导师】 秦刚；

【作者基本信息】 中国科学院研究生院（空间科学与应用研究中心） ， 空间物理学， 2011， 博士

【摘要】 本文通过理论模型、数值模拟和飞船观测相结合的方法研究了太阳高能粒子(SEP)在三维行星际磁场中的传播过程,重点讨论了横向扩散机制在SEP传播过程中的作用.主要研究结果如下:1.源的特性对太阳高能粒子在三维行星际磁场中传播的影响.我们从不同的方面,即位置,经、纬跨度,空间演化函数,研究了粒子源对SEP在三维行星际磁场中传播过程的影响.我们计算了日球层不同观测点的粒子通量和各向异性.通过大量数值计算和理论分析,我们得到如下结果:(1)垂直扩散机制在SEP的传播中扮演了一个相当重要的角色.垂直扩散机制可以用于解释许多观测现象,特别是当飞船没有通过行星际磁力线与太阳上的源直接相连时.(2)SEP源的位置对于观测到的SEP通量和各向异性曲线具有最强的作用.观测者的行星际磁足点距离源区位置越远,观测到的粒子通量将越小,SEP事件到达观测者的时间越晚,SEP通量峰值出现的时间也越晚.位于观测者磁足点东边和西边的源产生的粒子在传播中的表现不一样,具有东―西非对称性,即使它们相对于观测者磁足点的经度距离以及其他的源特征完全一样.这种作用来自于Parker行星际磁场的方位角非对称性.当观测者的行星际磁足点与源区具有很远的经度距离时,例如,60?及以上时,第一批到达观测者的SEPs可以为朝向太阳运动.这是由于粒子首先沿着磁力线流到大的径向距离处,然后通过扩散作用横越磁力线到达连接观测者的磁力线上,进行投掷角散射,最后从外面回到观测者处. (3)源区的经、纬度范围对于SEP的通量和各向异性曲线具有较大的影响.来自较宽的源区的粒子具有较大的通量.当观测者的行星际磁力线足点位于SEP源区的中心时,对于具有不同经、纬度范围的源区,飞船处观测到的SEP事件到达时间和通量峰值时间几乎一样.但是,当观测者的行星际磁力线足点不位于源区时,较宽的源区范围将引起更早的SEP事件到达时间和通量峰值时间. (4)将SEP源区粒子注入的不同空间变化经过归一化处理之后,如果所有其他的源区特征一样,则在各种空间变化情形下,SEP的通量和各向异性曲线几乎一样.这意味着源区空间变化的具体形式在影响SEP通量和各向异性方面并不十分重要,除非源区空间分布相当奇异.2.确定太阳高能粒子平均自由程的解析方法. SEP的平均自由程是空间天气学中一个非常重要的物理参数,它由太阳风和SEP的物理特性决定.为了精确地得到一个太阳事件中SEP的平均自由程,必须用到一种所谓的数值模拟方法,即将SEP传播过程的数值模拟结果与飞船观测得到的通量和各向异性进行比对,进而确定SEP的平均自由程.但是,这种比对与模拟需要大量的计算资源,即使运用现代超级计算机也是非常耗时与低效的.因此,有必要找到一种更好的方法以便更快地得到SEP的平均自由程,尤其是在空间天气预报工作中.最近,Shalchi et al.为计算脉冲事件SEP的各向异性提供了一个近似的解析公式,其为一个关于SEP平均自由程的函数.基于此,我们引入了一种确定SEP平均自由程的所谓的解析方法,即通过将Shalchi et al.的各向异性解析公式与飞船观测进行比对以确定SEP的平均自由程.另外,我们比较了传统的数值模拟方法和新的解析方法得到的平均自由程,结果显示,经过修正之后的解析方法能够快速、较准确地得到脉冲事件的SEP平均自由程.3.反向流动粒子束数值模拟研究.最近,Tan et al.研究了Wind飞船在1AU处观测到的2001年9月24日SEP事件,发现在事件初始阶段于90?投掷角处具有通量“低谷”的反向流动粒子束现象.他们认为这是由于1 AU之外某处的一个反射边界作用的结果.当行星际磁场具有某种特定结构时,这种情形是可能存在的.在这个工作中,我们对于这种现象提供了另外一种可能的解释.我们通过数值求解带有垂直扩散效应的五维聚焦传播方程模拟了这个SEP事件的40 keV电子.我们发现在一个SEP事件的初始阶段,于90?投掷角处具有通量“低谷”的反向流动粒子束可以在未与太阳主要粒子源直接相连的Parker型磁力线上形成.邻近磁力线上的大量粒子首先通过平行扩散传播到大的径向距离处,与此同时,通过垂直扩散跨越磁力线传播,然后受到散射回到1 AU处,在那里与直接来自太阳的粒子结合在一起,最终形成了反向流动粒子束.更多还原

【Abstract】 In this dissertation, we have studied the propagation of solar energetic par-ticles (SEPs) in three-dimensional interplanetary magnetic fields with combina-tion of theoretical models, numerical simulations and spacecraft observations.We have mainly discussed the effects of perpendicular diffusion on propagationof SEPs. The main research results are as follows:1. Effects of sources on propagation of SEPs in three-dimensional inter-planetary magnetic fields. By investigating source locations, coverage of latitudeand longitude, and spatial variations with calculation of SEP ffux and anisotropyprofiles at different observation locations in heliosphere, we study the effects ofsources on propagation of SEPs. The following are main conclusions in thiswork: (1) Perpendicular diffusion mechanism plays a very important role in SEPpropagation. It can be used to explain many observational phenomena, partic-ularly when a spacecraft is not directly connected to the solar source by theinterplanetary magnetic field (IMF) lines. (2) The location of SEP source hasthe greatest effects on observed SEP ffux and anisotropy profiles. The fartherthe IMF footpoint of the observer is away from the source, the smaller the par-ticle ffux will be observed, and the later the onset and the peak of SEP ffux willappear. Particles from a source on east side or west side relative to the IMFfootpoint of the spacecraft will appear differently, even though the longitudinalseparation and other source characteristics are the same. This effect results fromthe azimuthal asymmetry of the Parker interplanetary magnetic field. When theIMF footpoint of the observer is very far away from the source in longitude, e.g.,by as large as 60 degrees, the first arriving SEPs could be moving towards thesun. These are the particles that first stream out along field lines to large radialdistances, diffuse across field lines onto the line that connects the observer, getscattered in pitch angle and then come back to the observer from outside. (3)The coverage of source in latitude and longitude also has large effects on the SEPffux and anisotropy profiles. Particles coming from a wider source tends to have large ffuxes. When the IMF footpoint of the observer is at the centers of theSEP sources, the onset and the peak ffuxes are almost the same for sources ofdifferent coverages in longitude and latitude. However, when the IMF footpointof the observer is not located inside the source region, a wider source coveragewill result in an earlier onset and peak ffux. (4) The ffux and anisotropy profilesin the cases with different normalized spatial variation of SEP source injectionappear almost the same if all the other conditions remain the same. It indicatethat the form of spatial variation is not very important in affecting the SEP ffuxand anisotropy, unless its distribution is extremely strange.2. An analytical method to determine SEPs’mean free path. SEPs’meanfree path, determined by physical properties of SEPs as well as those of solar wind,is a very important physical parameter in space weather. To accurately obtainthe mean free path of SEPs for a solar event, a so-called simulation method byfitting time-profiles of both ffux and anisotropy between spacecraft observationsand numerical simulations of SEPs transport process has to be used. However,such kind of fitting and simulations need a large amount of calculation resources,so they are time-consuming even with modern super-computers. It is necessaryto find a better way to get mean free path of SEPs quickly, especially in spaceweather forecast. Recently, Shalchi et al. provided an approximate analyticalformula of SEPs’anisotropy time-profile as a function of particles’mean freepath for impulsive events. In this work, we use a so-called analytical method todetermine SEPs’mean free path by fitting the anisotropy time-profiles betweenthe Shalchi et al.’s analytical formula and spacecraft observations. In addition,we compare the mean free path obtained with the traditional simulation methodwith that obtained with the new analytical method to show that the analyticalmethod, with some modifications, can give us a good approximation of SEPs’mean free path quickly for impulsive events.3. The simulation study on counter-streaming particle beam. Recently, Tanet al. studied the 2001 September 24 SEP event observed by the Wind space-craft at 1 AU and found that there is a counter-streaming particle beam witha deep depression of ffux at 90-degree pitch angle during the beginning of theevent. They suggested that it is a result of a reffecting boundary at some distance outside of 1 AU. While this scenario could be true under some specific config-uration of interplanetary magnetic field, in this work, we offer another possibleexplanation to it. We simulated the SEP event by solving the five-dimensionalfocused transport equation numerically for 40 keV electrons with perpendiculardiffusion. We find that a counter-streaming particle beam with deep depressionat 90-degree pitch angle can form on Parker magnetic field lines that do notdirectly connect to the main particle source on the sun in the beginning of anSEP event. It can happen when a significant number of observed particles comefrom adjacent field lines through parallel transport to large radial distance first,hopping across field lines through perpendicular diffusion, and then getting scat-tered back to 1 AU, where they combine the particles directly coming from thesun to form a counter-streaming particle beam.更多还原