日珥形成的数值模拟研究

发布时间：2018-07-14 09:23

【摘要】：日珥是太阳大气中美丽而复杂的等离子体活动现象,它们由低温(～8000K)、稠密(1010—1011cm-3)的等离子体组成,动态地悬浮在光球磁场中性线上方高温稀薄的日冕中。日珥由许多向下伸出的足连接到光球。在日面上,日珥呈现为暗的条状结构,因此又被称为暗条。日珥的形成、维持和抛射等现象中的物理过程十分复杂,其中包含有辐射、热传导、加热、磁场重联、太阳重力等物理因素的非线性相互作用。对美丽而复杂的日珥现象进行解析的理论研究十分困难。因此,通常需要借助数值模拟才能进行定量的研究。本论文拟通过数值模拟对日珥的形成进行系统的研究。前人的研究发现对磁环足点的加热能够引起色球蒸发。高温等离子体充满冕环后在一定条件下触发辐射不稳定性,进而导致物质凝聚成日珥。在此基础上,我们首先利用包含自适应网格的辐射流体力学数值模拟,研究了在持续稳定或有限时间段的色球局部加热下,在一个固定位形的磁环中暗条形成的过程。与前人的研究不同,我们采用了具有浅磁凹陷结构的低平磁环位形来模仿活动区附近暗条的磁场结构,还考虑了更精确的辐射损失。我们首次证明了线性热不稳定性理论可以解释暗条形成时物质凝聚的发生。模拟结果显示在局部加热开始2个多小时后,日冕物质冷凝发生形成暗条物质。取不同的局部加热强度和衰减标高,暗条长度生长的速率范围为800—4000km hr-1。我们还发现了决定暗条物质最初形成时是一段还是两段的条件。在不对称局部加热下,当两段暗条物质形成后,它们汇聚、合并成一段,然后沿磁环运动掉落到磁环足点。在持续稳定的局部加热下,这一动态过程循环地发生,周期为几个小时。我们除了验证前人的发现之外,还发现持续的局部加热不是维持日冕物质冷凝的必要条件。暗条物质一旦形成,即使没有局部加热也能够由于虹吸作用缓慢生长。此外,我们发现暗条在光球p模波的扰动下能够稳定存在。在一维模型的基础上,我们发展了日珥形成的2.5维蒸发—凝聚模型。在包含色球、过渡区和日冕的剪切磁拱中,我们利用2.5维辐射磁流体力学模拟,得到了正常极性宁静日珥在日冕中的形成过程。集中于色球的加热引起色球物质蒸发,日冕物质密度升高、温度先上升后下降,接着热不稳定发生,导致日冕物质冷凝,最终在磁拱顶部形成了日珥。与前人的结果不同,我们的模拟展现了完整的日珥形成过程,其中包括热平衡的破坏、日冕物质冷凝、日珥在宽度和高度方向的生长、在日珥重力的作用下磁拱逐渐被压弯形成磁凹陷。我们得到的日珥形如帘状,悬浮在磁中性线上方,其中磁场方向与日珥长轴方向成一定的夹角。持续的色球加热把色球物质带入日冕,然后冷凝进入日珥,使得日珥稳定生长。我们首次证实了日珥形成的蒸发—凝聚模型在低气压磁压比的日冕中可以得到受力平衡的日珥,并且这一结果可以和解析的Kippenhahn-Schluter静态日珥模型相比较。在论文的最后,我们对日珥形成的数值模拟与观测研究进行了展望。
[Abstract]:Prominence is a beautiful and complex plasma activity in the solar atmosphere. They are composed of low temperature (~ 8000K), dense (1010 - 1011cm-3) plasma, which are dynamically suspended in the thin corona above the neutral line of the photospheric magnetic field. The prominence is connected to the ball of light from a number of downward legs. The prominence of the prominence is a dark strip on the surface. It is also known as a dark strip. The physical processes in the formation, maintenance and ejection of prominence are very complicated, including the nonlinear interaction of physical factors such as radiation, heat conduction, heating, reconnection of the magnetic field, and the solar gravity. It is very difficult to analyze the beautiful and complex prominence phenomena. Only by numerical simulation can we conduct quantitative research. This paper intends to systematically study the prominence formation through numerical simulation.
Previous studies have found that the heating of the foot of a magnetic loop can cause chromosphere evaporation. The high temperature plasma is filled with a coronal ring and triggers the radiation instability under a certain condition, which leads to the condensation of the material into a prominence. Different from previous studies, we adopt a low level magnetic ring position with a shallow magnetic depression to mimic the magnetic structure of a dark strip near the active area and consider more accurate radiation loss. We have first proved linear thermal instability. The theory of sex can explain the occurrence of material condensation at the formation of a dark strip. The simulation results show that after 2 hours of local heating, coronal mass condensation occurs to form a dark strip. Different local heating intensity and attenuation elevation are taken. The rate range of the growth of the length of the dark strip is 800 - 4000km hr-1.. We also found that the dark strip was initially determined. Under asymmetric local heating, when the two sections of dark strips are formed, they converge and merge into one section, and then fall along the magnetic ring to the foot of the ring. This dynamic process circularly occurs under steady and stable local heating. The cycle is a few hours. We have verified the discoveries of the predecessors. In addition, it is found that continuous local heating is not a necessary condition for maintaining coronal condensation. Once dark strips are formed, even without local heating, the growth of the siphon can be slowed down. In addition, we find that the dark strips can be stable under the perturbation of the P mode wave of the photosphere.
On the basis of one dimensional model, we developed a 2.5 dimensional evaporation condensation model of prominence. In the shear magnetic arches containing chromosphere, transition zone and corona, we use 2.5 dimensional radiation magnetohydrodynamic simulation to obtain the formation process of normal polar tranquil prominence in the corona. The coronal mass density rises, the temperature rises first and then decreases, then the thermal instability occurs, resulting in the condensation of the coronal matter and the formation of a solar prominence at the top of the magnetic arch. Unlike the previous results, our simulation shows a complete process of prominence, including the destruction of the heat balance, the condensation of the coronal matter, the birth of the prominence in width and height. Under the action of the prominence of the solar prominence, the magnetic arch is gradually formed to form a magnetic depression. The prominences we get are like a curtain like, suspended above the magnetic neutral line, in which the direction of the magnetic field is in a certain angle with the long axis of the prominence. The continuous chromosphere is heated to bring the chromosphere into the corona and then cold to enter the prominence, making the prominence growing steadily. For the first time, we have grown. It is confirmed that the evaporation and condensation model of prominence can obtain a force balance prominence in the corona of low pressure magnetic pressure ratio, and this result can be compared with the analytic Kippenhahn-Schluter static prominence model.
At the end of the paper, we prospected the numerical simulation and observational research of prominence formation.
【学位授予单位】：南京大学
【学位级别】：博士
【学位授予年份】：2012
【分类号】：P182

【共引文献】