摘要
日珥的观测显示存在着强的磁场剪切分量,本文研究了剪切在日珥形成中的积极作用。首先计算一个二维偶极势场当脚根受对称剪切后产生怎样的物理现象。作了三种情形的数值模拟。计算表明产生蘑菇状流动是个普遍规律,在某些条件下将形成弧顶凹陷。为了说明蘑菇流的物理起因,我们探讨了演化初期的线性渐近解。保留一阶量下导出了剪切速度W_z和磁场剪切分量B_x的解析解,对二维流动V_x,V_y求出了近似解:流的振幅随时间指数增长。对于弧顶凹陷,给于了定性的说明:磁浮力-(1/c)J_xB_x和形变阻尼力(1/c)J_zB_x之间的相互作用导致弧预变平凹陷,并把剪切区磁力线打开成为开场。弧顶凹陷是形成日珥的重要条件。冕弧加热量的略微减小,产生初始的热不稳定性,在凹陷处物质凝聚温度降低,弧脚根处因色球受蒸发而流入的物质沿管上升进入弧顶陷阱,加剧了热不稳定性,物质进一步变密,温度再降,最终形成了日珥。
From the observations of quiescent prominences, it is suggested to investigate the active role of the magnetic field shear component in the formation of prominences. We study what dynamical phenomena may happen in a two dimensional bipolar potential field embedded in solar chromosphere-corona, when the feet of the field lines are subject to a symmetrical shear motion. Three numerical simulations are made. It is found that there might be a general rule that a mushroom-liked flow, mainly ascending motion, occurs in the shearing process. Under some circumstances, the magnetic arches will collapse and flat,or even form a sag on the apex of the arches in the shearing area. In order to realize the physical origin of the mushroom-liked flow, some asymptotic solutions are constructed during the preliminary stage of the development period. Under the condition of preserving the first-order tiny quantities, some analytical solutions are found for the shear velocity Wz and the magnetic shear component Bz. We obtain asymptotic solutions for the flow velocities Vx'Vy' whose amplitudes grow with time exponentially. A qualitative explanation is given forthe sag on the arch apex: there is an interaction between the magnetic buoyancy-1/cJxBz and the damping force 1/cJzBx which counteracts the field deformation. It isthis interaction which leads to formation of plain and pit on the arch apex, and leads to making the field lines open in shearing region as well. Furthermore, we show that the occurrence of apex sag is an important condition for the formation of quiescent prominences: the slight diminution of heat amount throughout the magnetic arch leads to a thermal instability, and, as a result, mass condensation and cooling occur in the apex sag. Subsequently, due to chromospheric vaporization, the chromos-pheric mass continuously enters the magnetic arch from its feet and reaches the apex sag where the enhanced thermal instability makes mass denser and denser and temperature lower and lower untill a full prominence eventually form
关键词
太阳
日珥形成
磁弧剪切
弧顶凹陷
Sun(the)-MHD shearing-Formation of solar prominences
