摘要
为了解释太阳观测中磁流浮现形成活动区的主要特征——AFS弧顶的上升运动和弧脚的下沉运动,谱斑,Ellerman炸弹等,我们采用了MHD-Lagrangian数值模拟法计算了重力分层(压力密度指数递减),太阳大气中同基底磁场反向和同向的磁偶极子从光球下浮出时产生的动力学演化过程.计算证明:反向的偶极子会导致流体产生会聚和下降运动,为了能让反向偶极子浮出必须适当增压,演化的结果形成电流片,因而有可能引起如Ellerman炸弹等小型爆发现象.还证明了:在浮现处一定产生加热增温,即导致谱斑发亮现象.在弧顶气体是上升运动,速度小;弧脚处气体作下降运动,速度较大,这主要是重力分层气体压力密度非均匀引起的,和Brunt-Vaisala振动有同一的起因.我们还模拟了通常活动区形成时的初期现象:1500高斯强磁偶极子浮进100高斯的弱光球色球基底场时产生的动力学现象.计算表明:为了不致产生激波必须减压浮出,即类似于黑子降温式的浮现,浮现时间应是小时的量级,浮现后确实在色球里导致强的电流片,当这电流片升至低日冕后,会成为Heyvaerts,Priest等人期望的耀斑发生源,因而浮现过程有可能是耀斑储能和触发机理之一.另外我们利用计算结果试解释了北京天文台怀柔站的磁流浮现资料,即文[1]和文[2]的一些分析结果。
Numerical simulation method in MHD Lagrangian scheme is adopted to compute the dynamic evolution of solar magnetic bipolar-field imbedded in the gravity-stratified atmosphere when a parallel or an opposite magnetic dipolc emerges from the subphotosphere. It is shown that the emergence of an opposite dipole would lead to converging and descending motion of plasma, and that an augmented pressure has to be used in order to let this dipole float. The floating evolution process results in the formation of a current sheet in the interface between the old and the new emerging fields. Such sheet may bring about some explosive phenomena such as Ellerman bombs etc. The computation reveals that the gas at the arch's top is rising with a small velocity while the gases at the arch's legs are falling down with a large speed. Similar to Brunt-Vaisala oscillation, this peculiar property stems from the nonhomogeneity of density (pressure and temperature) in the gravity-stratified atmosphere, Furthermore, we simulate the formation of an usual solar active region by making a strong magnetic dipole (1500 Gauss) float into a weak background field (100 Gauss) in solar photosphere and chromosphere. It is indicated that a diminished pressure should be added on the boundary in order to avoide the occurrence of strong shock. The emerging process should be going on for at least several hours. After emergence a strong current sheet occurs in chromosphere, and when arriving at lower corona, it will become potential origin of solar flare as that expected bp Heyvaerts and Priest. Finally, the computation results are applied to explain magnetic emerging flux dara obtained at Huairou, Beijing Astronomical Observatory, as given in papers (1) and (2). Sun-atmosphere-MHD simulation
