等离子体团型日冕物质抛射的形成机制

本文在球坐标二维磁静力平衡基态下，数值研究了电阻撕裂模不稳定性引起日冕电流片中发生磁场重联的过程，结果表明发生了具有两个Ｘ线的磁场重联，形成了磁岛和高温高密度的等离子体团，等离子体团在向上运动过程中有着明显的膨胀，其上升速度和膨胀过程与等离子体β值有关，这些结果可用于解释等离子体团型日冕物质抛射的形成。     

耀斑大尺度电流片的湍流耗散和能谱分析

磁重联在宇宙的许多动力学现象中都是非常核心的过程.磁流体动力学(MHD)数值模拟是研究磁重联过程以及相应物理图像的一种很有效的手段.通过不同的参数组合,来研究MHD数值模拟中磁雷诺数和空间分辨率对磁重联率、数值耗散和能谱分布的影响.对得到的数据进行分析后,发现磁雷诺数对磁重联率和能谱分布有一定的影响.磁雷诺数越大,磁重联过程进入非线性阶段所需的特征时间越短,磁重联率就越早发生跃升.磁雷诺数R_m对耗散开始发挥作用的Kolmogorov微观尺度l_ko有明显影响:R_m越大,l_ko就越小.研究了磁重联过程中包括数值耗散在内的额外耗散对重联过程的影响.结果表明,撕裂模不稳定性开始之前的额外耗散以纯数值耗散为主,撕裂模不稳定性出现之后,额外耗散出现同步跃升,说明不稳定性导致的湍流明显增强了耗散的效果,相当于在局部湍流区引入了超电阻.能谱分析进一步表明,大尺度电流片的l_ko完全可能出现在宏观的MHD尺度上.     

太阳大气中自发磁重联的数值模拟：（Ⅱ）重力,热传导下二维三分量的 …

在无力场中引入反常电阻,数值模拟电阻撕裂模不稳定性引起的均匀重力场中的二维三分量磁重联过程。首先研究了Ｖｃ和β０对磁重联的影响,结果表明：（１）在Ｖｃ大的情况下,重联率开始很小,但最终出现爆发现象。另外,温度的增加不如Ｖｃ小的情况明显;（２）Ｖｃ相同而β０不同的情况具有相似的演化特征,但β０越小,动力学过程越刷烈;（３）Ｖｃ大且β０也大的情况不能产生爆发现象。还研究了各向同性热传导对磁重联过程及扩     

1986年2月4日耀斑与电流环合并不稳定性

本文收集了１９８６年２月４日大耀斑的Ｈα、微波、Ｘ射线和γ射线全波段的观测资料。利用暗条电流环模型分析了该耀斑的物理过程，测量了活动暗条的上升运动，求解了动量方程和能量方程。结果表明：（１）１９８６年２月４日的３Ｂ／Ｘ３耀斑可能是由暗条电流环之间的合并不稳定性所致；（２）电阻撕裂摸不稳定性是一种有效的耀斑前预热机制；（３）耀斑的高能观测资料进一步表明了电流环合并不稳定性是引起该大耀斑期间所有高能粒     

太阳大气中自发磁重联的数值模拟(Ⅱ)重力、热传导下二维三分量的计算结果

在无力场中引入反常电阻,数值模拟电阻撕裂模不稳定性引起的均匀重力场中的二维三分量磁重联过程．首先研究了Vc（反常电阻模型中的临界漂移速度）和β0（气压磁压比）对磁重联的影响,结果表明：（1）在Vc大的情况下,重联率开始很小,但最终出现爆发现象．另外,温度的增加不如Vc小的情况明显。（2）Vc相同而β0不同的情况具有相似的演化特征,但β0越小,动力学过程越剧烈。（3）Vc大巨β0也大的情况不能产生爆发现象．还研究了各向同性热传导对磁重联过程及扩散区结构的影响．结果表明热传导加快磁重联速率．在无热传导的情况中,开始时温度增加迅速,重联率较低,但最终出现磁重联率迅速上升的现象．对上述结果及其在解释耀斑、日冕物质抛射及冕流等太阳大气现象方面进行了讨论．     

非对称四极无力场的磁能释放

本文采用二维三分量耗散ＭＨＤ模型，对带高电流层的局地非对称的四极无力场的磁能释放过程进行数值模拟，结果表明，磁能释放过程大体可以分为两个阶段：高电流层引起的异常电阻耗散使该层等离子体加热至３×１０６Ｋ的高温，形成一高温环；在高电流层耗散的触发下，磁分隔面的电流急剧增长并爆发异常电阻耗散和磁场重联，导致耀斑发生，主要的能量释放发生在磁分隔线和高剪切无力场中的磁分隔面上，等离子体温度可以达到１．９×１０７Ｋ。上述无力场的触发释能过程可能是太阳耀斑的一种重要的释能机制。     

日珥磁场结构的Rayleigh-Tayler不稳定性和无黑子双带耀斑

本文根据无黑子双带耀斑发生前暗条周围色球纤维形态的观测特征,讨论了这类耀斑的储能过程。文中还讨论了这类日珥磁场结构的不稳定性问题。当宁静日珥中磁场梯度和顶部磁力线曲率超过一定的阈值时,会引起日珥磁场结构中的Rayleigh-Tayler不稳定性的发生,导致暗条突然消失,发生这种不稳定性时,日珥支撑磁场中会产生中性电流片,并能引起两边的磁流向中心部分集中。磁力线的快速重连导致双带耀斑的产生。     

有流动情况下等离子体柱的扭折不稳定性

本文从磁流力学理论出发,研究了有流动情况下等离子体柱的扭折不稳定性线性理论,并以所导出的色散方程计算色散曲线,从而估计速度因子在线性不稳定性理论中所起的作用。本文发现流体的流动与磁场环向分量H_(eφ)一样是一个不稳定因子,随着流动速度的出现和增加,不稳定性增长率会提高几倍至几十倍,发生不稳定的波长范围会扩大.在速度较大的情况下,对短波扰动,m=0,m=1模几乎会同时出现。最后,用本文计算结果对太阳大气中出现的几个典型事例,进行了简单的定性解释。     

太阳黑子发展中的磁扩散不稳定性

根据太阳发电机理论中的ω-效应,在太阳对流层内将产生纬向磁场,它的磁浮力要促使流团上浮。在文[5]中讨论了在流团上浮过程中,流团表面的磁扩散率梯度将对纬向磁场产生扰动,这一扰动使纬向磁场集积在流团表面磁扩散率梯度大的地方,围绕流团表面形成了黑子磁环。 本文进一步从磁流力学方程组的小扰动方程出发探讨了太阳黑子磁环发展的不稳定性问题。结果表明:在扰动方程中存在着不稳定模式。这一不稳定性产生的原因是由于当温度(或者说磁扩散率)受到小扰动时,纬向磁场要集积在磁扩散率(或温度)梯度大的地方,而磁场的集积将导致磁压增强及气压减低。在绝热条件下,这将使温度减低,而温度的减低又加强了温度梯度的增大,这又进一步促使磁场在梯度大的地方集积。这种磁场与温度发展的相互促进关系可以称它为磁扩散不稳定性。本文认为太阳黑子磁环和它低温的形成正是由于这种不稳定性发展起来的。     

含三维磁场等温薄吸积盘的振荡不稳定性

本文从磁流体动力学方程组出发,用微扰法得出含三维磁场等温薄吸积盘的径向、轴向及环向不稳定性的色散方程．并详细讨论了磁场各分量对吸积盘不稳定性的影响．结果表明,磁场的径向和环向分量对磁声模和非轴对称模的不稳定性的增加起着主要的作用,且径向振荡大于轴向振荡．这一模型有利于解释ＦＵＯｒｉｏｎｉｓ和ＴＴａｕｒｉＳｔａｒｓ的周期光变现象     

Interaction between sheared flow and resistive tearing mode

We investigate the nonlinear evolution of resistive tearing mode in a current sheet with a sheared flow in a long, thin cylinder. The results show that a hyperbolic secant (sech) flow field will lead to instability of the resistive tearing mode, formation of magnetic islands and rapid release of magnetic energy. The coupling between sheared flow and the tearing mode and interaction between suprathermal instabilities change the degree of shear in the magnetic field (the electric current gradient) and drive the development of the instability. This process may be one of the mechanisms of solar flares.     

Radiative and reconnection instabilities: Compressible and viscous effects

Filaments and flares are prominent indicators of the magnetic fields of solar activity. These instability phenomena arise from the influence of weak transport effects (radiation and resistivity, respectively) on coronal magnetodynamics and energy flow. We have previously shown that the filament and flare (tearing or reconnection) mechanisms are resistively coupled in sheared magnetic fields of the kind existing in active regions. The present paper expands this treatment to include the effects of compressibility and viscosity, which are most prominent at short wavelengths. The results show that compressibility affects the radiative mode, including a modest increase of its growth rate, and that viscosity modifies the tearing mode, partially through a decrease of its growth rate. A comprehensive discussion of the mode structures and flows is presented. The strongest effect found is a reversal, at very long wavelengths, of the radiative cooling of the resistive interior layer of the tearing mode, caused by compressional heating.     

The 4 February, 1986 flare and flare-filament current model

In this paper, the observational data in H, radio, soft X-ray, hard X-ray, and -ray emissions for the 3B/X3.0 solar flare on 4 February, 1986 are collected. This flare is studied in detail by using the flare-filament current model. The momentum equations and the energy equations of the filament current have been solved. The influence of the highly sheared background magnetic field on the motion of the filaments is studied through numerical calculation. The results show that the resistive tearing instability is a possible pre-heating mechanism in the preflare phase, and both the rotation of the spiral sunspots and the highly sheared background field are necessary for the energy storage of this flare. The high-energy data of the flare imply that the current-loop coalescence instability is a possible eruptive mechanism.     

The study of magnetic reconnection in solar spicules

This work is devoted to study the magnetic reconnection instability under solar spicule conditions. Numerical study of the resistive tearing instability in a current sheet is presented by considering the magnetohydrodynamic (MHD) framework. To investigate the effect of this instability in a stratified atmosphere of solar spicules, we solve linear and non-ideal MHD equations in the x?z plane. In the linear analysis it is assumed that resistivity is only important within the current sheet, and the exponential growth of energies takes place faster as plasma resistivity increases. We are interested to see the occurrence of magnetic reconnection during the lifetime of a typical solar spicule.     

MAGNETIC RECONNECTION IN MULTIPLE HELIOSPHERIC CURRENT SHEETS

Satellite observations of the heliospheric current sheet indicate that the internal structure of sector boundaries is a very complex structure with many directional discontinuities in the magnetic field. This implies that the heliospheric current sheet is not a single surface but a constantly changing layer with a varying number of current sheets. In this paper, we investigate magnetic reconnection caused by the resistive tearing mode instability in non-periodic multiple current sheets by using two-dimensional magnetohydrodynamic simulation. The results show that it is complex unsteady magnetic reconnection. Accompanying the nonlinear development of the tearing mode, the width of each magnetic island in multiple current sheets increases with time, and this leads to new magnetic reconnection. At the same time, the width of each current sheet increases, and the current intensity decreases gradually. Finally, the reverse current disappears, and a big magnetic island is formed in the central region. This process is faster when the separation between the current sheets is smaller. We suggest that the occurrence of multiple directional discontinuities observed at sector boundary crossings in the heliosphere may be associated with the magnetic islands and plasmoids caused by magnetic reconnection in multiple current sheets.     

Tearing mode instability in the atmosphere above a sunspot

We derived the energy balance equation in steady state and made numerical calculations for 37 sets of parameter values for three layers of the atmosphere. The main results are: 1) The energy density released by tearing mode instability rapidly falls with rising temperature. It is much greater in the colmnar pinch than in the thin plate pinch. 2) The critical temperature increases with the intensity of the sheared magnetic field and decreases with its width. It is higher in the columnar pinch, and the rise time is shorter, than in the thin plate pinch. 3) At the surface of the photosphere, the energy released by tearing mode instability raises the local temperature by less than 1 K. 4) When the sheared width of the magnetic field is below a certain threshold, the local temperature is raised sharply to over (+6) K. This threshold is an increasing function of the strength of the sheared field. Our calculated results fit the observations to within one order of magnitude.     

Magnetic reconnection in a high-temperature plasma of solar flares

Dispersion relations for the resistive tearing instability are analytically found in the hydromagnetic approximation for a current sheet with a small normal component of the magnetic field. A strong stabilizing influence of the normal component on the development of the tearing instability is shown to exist. These results are also obtained from physical considerations, and so a simple interpretation of the stabilization effect of the normal component is given. The results of the present paper are compared with those of previous works on the topic, and the previous negative results are explained.     

速度切变对撕裂模不稳定性影响的初步分析

在电流片两侧的基态速度场与磁场相互平行的位形下,讨论了速度切变对撕裂模不稳定性的影响.结果表明,只有当速度场的相对变化率与磁场的相对变化率符号相同(Sign(v_0/v_0)=sign(B_0~')),|v_0~'/v_0|≥|B_0'/B_0|时,速度切变有利于不稳定增长,否则不利于不稳定增长.     

Magnetic reconnection via tearing instability in a rotating viscous fluid

The resistive tearing instability of a sheet pinch, first investigated by Kuang & Roberts (1990) for the case of a rapidly rotating inviscid fluid, is studied for arbitrary rotation rate in a visco‐resistive fluid. Altogether there are three regimes of the resistive tearing instability which correspond to the particular parameter domain in the (Ω, P_m) plane. Here Ω is the angular velocity of the medium which is normalized to the Alfvén time and P_m is the magnetic Prandtl number. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)