日冕物质抛射伴生现象的数值研究

【作者】 汪红娟；

【导师】 林隽；

【作者基本信息】 中国科学院研究生院（云南天文台） ， 天体物理， 2009， 博士

【摘要】 日冕物质抛射(Coronal Mass Ejections,简称CME)是太阳大气和太阳系中最剧烈、尺度最大的能量释放和活动现象。当CME发生时,在很短时间内,将携带的巨大能量和物质,大量的高能射线和高能粒子抛到行星际空间,对空间天气造成强烈扰动,给我们完全依赖的各种现代化技术,如卫星、通讯、遥感、电力等造成严重的影响。所以CME这一研究课题与人们的日常生活息息相关。CME是日冕大尺度磁场平衡遭到破坏的产物,当这种爆发产生时,往往会伴随着很多观测现象,如莫尔顿波、EIT波、暗区(dimming)以及各型射电暴等。这些伴生现象在很大程度上丰富了日冕物质抛射的研究内容,对它们的研究为我们更全面、更深入地理解日冕物质抛射提供了很好的切入点。在对CME的研究过程中,人们一直对CME的一些伴生现象抱有极大的兴趣,因为对它们的了解为我们最终完整地理解CME提供了很多线索。本文利用目前被天文领域广泛应用的ZEUS程序,从数值模拟的角度对CME的一些伴生现象,主要是针对莫尔顿波、EIT波以及暗区,做了进一步的研究与探讨。我们在两种不同密度分布的背景场中,对上述伴生现象,做了较系统和深入的数值研究。在研究莫尔顿波和EIT波的过程中,我们从另外一个角度,即速度的散度和速度的旋度对它们分别做了多方位的数值实验。我们发现在两种不同的背景场中,当磁绳以足够快的运动速度积压前方的气体和磁场并使得相关扰动的传播速度最终超过当地的磁声波速度时,都会在磁绳的顶部形成一个弓激波,并且紧贴着磁绳的上部产生一个慢激波,以及在磁绳的两个侧后方的速度漩涡。随着演化时间不断地向前推进,所有这些扰动都逐渐地向外传播。快模激波和慢激波及速度漩涡传播的高度层次不同,快模波传播到达了日面,而慢模波及速度漩涡的传播则只能到达日面以上日冕当中的某个层次。另外,我们还对它们的速度大小以及当地的阿尔芬速度大小做了进一步的研究。通过这些数值结果,我们认为:快模波(即上面提到的弓激波)传播到色球层时,就会产生莫尔顿波,而慢模波及速度漩涡的相互耦合对应于日冕中的EIT波,并且弓激波也是产生Ⅱ型射电暴的源。同时,在我们的模拟结果中还发现在CME上升时,还伴有暗区出现,通过我们进一步的模拟证实了暗区是由于密度的损失引起的。另外,我们还数值研究了,在不同密度分布的背景场中,磁通量绳的动力学演化过程。并对这两种情况下的磁通量绳动力学演化过程做了进一步的比较和分析。而且把我们所取得的数值结果与已有的理论结果做了比较。在本文的最后,我们对曾经调研的另一个天体物理磁流体动力学程序(CANS:Coordinated Astronomical Numerical Software)中所涉及的一个算法做了进一步的考察,并对其基本形式的算法做了修正和数值实验,得到了较理想的结果。更多还原

【Abstract】 Solar coronal mass ejections (CMEs) are the largest known solar eruptive activity in spatial scale and also in energetics and geoeffectiveness. In such a process, a great deal of magnetized plasma and energy can be ejected into the outermost corona and interplanetary space within a short time, which can cause the strong interplanetary disturbances and effect satellite operation, aviation power, human space exploration, communication, navigation and so on. So CMEs are well related to human’s lives and production activities. Nonequilibria of the large-scale magnetic fields can cause CMEs. CMEs are associated with many dynamical phenomena, e.g., Moreton waves, EIT waves, dimming and type II radio bursts. These phenomena can help us to study the properties of CMEs.Since some dynamical phenomena can help researchers investigate CMEs, these phenomena associated with CMEs attract our interests. In this thesis, MHD processes of CMEs are numerically simulated by using ZEUS code. We adopt MHD numerical experiment to study qualitatively some dynamical phenomena associated with the lift-off of flux rope, such as the Moreton waves, the EIT waves and the dimming in two different backgrounds which have the different plasma density distribution. We use the velocity divergence and vorticity to further discuss and investigate the Moreton waves and the EIT waves. We found that as the flux rope rises rapidly, a fast shock is formed in front of it like a piston when its velocity exceeds the local magnetoacoustic speed, and the slow shock is also formed, which draws near the top of the flux rope, at the same time, the velocity vortices are developed at the backside of the flux rope. The proga-tion of these two shocks arrive the different height. The fast-mode shock arrives the solar surface, while the slow-mode shock and velocity vortices arrive some height above the surface. We also investigate the velocity of these two shocks and the local Alfvénic wave. Analyses show that the joint impact of the slow-mode shock and the vortices is quite likely to account for the EIT waves, when the fast-mode shock sweeps the chromosphere, it produces the Moreton waves. The piston-driven shock is the type II radio bursts. From our results, we also found the dimming with the lift-off flux rope. Our simulation shows the dimming is more likely caused by a loss in density.In addition, we investigated the evolutionary features of the magnetic configuration that includes a current-carrying flux rope, which used to model the filament in two different backgrounds. We discussed our results and the reported theoretical results.At last, we investigate the algorithm in another code-CANS(Coordinated Astronomical Numerical Software). We modify this algorithm, and provide the new preconditioned methods. From the theoretical proof and numerical experiments, we can conclude that our new preconditioners are more effective to accelerate convergence than the previous methods.更多还原