基于太阳爆发灾变模型的数值实验

摘要

在Isenberg等人发展的灾变模型基础上根据接近真实的日冕环境,通过数值实验,对磁通量绳的平衡高度对光球磁场变化的响应开展了研究.利用NIRVANA程序进行了计算.日冕的等离子密度分布采用了一个半经验的模型,模拟中包含了物理耗散.考察了:磁通量绳的平衡位置及其演化特征;参考半径的变化对磁通量绳平衡位置的影响;磁通量绳内部平衡的性质以及在磁通量绳失去平衡之后一段时间内的动力学与运动学特征.结果表明:数值实验中得到的磁通量绳的平衡态位置与Isenberg等人的理论结果有微小的偏离,但是演化特征基本一致,在临界点处系统迅速失去平衡,向爆发态演化;参考半径的变化对磁通量绳平衡位置的影响与灾变模型给出的结果基本一致;磁通量绳在随着宏观磁结构演化的同时,还通过自身的调节达到内部平衡,当磁通量绳的内部和外部平衡都实现之后,系统整体也就达到了平衡状态;在爆发态下,磁通量绳的运动特征与Lin-Forbes模型和观测给出的结果一致,并且在通量绳的前方有快模激波出现;由于数值实验中包括了耗散,爆发过程中的磁能向其他形式能量的转换非常明显.%On the basis of the catastrophe model developed by Isenberg et al., we use the NIRVANA code to perform the magnetohydrodynamics (MHD) numerical ex-periments to look into various behaviors of the coronal magnetic configuration that includes a current-carrying flux rope used to model the prominence levitating in the corona. These behaviors include the evolution in equilibrium heights of the flux rope versus the change in the background magnetic field, the corresponding internal equi-librium of the flux rope, dynamic properties of the flux rope after the system loses equilibrium, as well as the impact of the referential radius on the equilibrium heights of the flux rope. In our calculations, an empirical model of the coronal density distri-bution given by Sittler&Guhathakurta is used, and the physical diffusion is included. Our experiments show that the deviation of simulations in the equilibrium heights from the theoretical results exists, but is not apparent, and the evolutionary features of the two results are similar. If the flux rope is initially locate at the stable branch of the theoretical equilibrium curve, the flux rope will quickly reach the equilibrium position in the simulation after several rounds of oscillations as a result of the self-adjustment of the system;and the flux rope lose the equilibrium if the initial location of the flux rope is set at the critical point on the theoretical equilibrium curve. Correspondingly, the internal equilibrium of the flux rope can be reached as well, and the deviation from the theoretical results is somewhat apparent since the approximation of the small radius of the flux rope is lifted in our experiments, but such deviation does not affect the glob-al equilibrium in the system. The impact of the referential radius on the equilibrium heights of the flux rope is consistent with the prediction of the theory. Our calculations indicate that the motion of the flux rope after the loss of equilibrium is consistent with which is predicted by the Lin-Forbes model and observations. Formation of the fast mode shock ahead of the flux rope is observed in our experiments. Outward motions of the flux rope are smooth, and magnetic energy is continuously converted into the other types of energy because both the diffusions are considered in calculations, and magnetic reconnection is allowed to occur successively in the current sheet behind the flux rope.