Anomalous-plasmoid-ejection-induced secondary magnetic reconnection: modeling solar flares and coronal mass ejections by laser–plasma experiments

The driving mechanism of solar flares and coronal mass ejections is a topic of ongoing debate, apart from the consensus that magnetic reconnection plays a key role during the impulsive process. While present solar research mostly depends on observations and theoretical models, laboratory experiments based on high-energy density facilities provide the third method for quantitatively comparing astrophysical observations and models with data achieved in experimental settings.In this article, we show laboratory modeling of solar flares and coronal mass ejections by constructing the magnetic reconnection system with two mutually approaching laser-produced plasmas circumfused of self-generated megagauss magnetic fields. Due to the Euler similarity between the laboratory and solar plasma systems, the present experiments demonstrate the morphological reproduction of flares and coronal mass ejections in solar observations in a scaled sense,and confirm the theory and model predictions about the current-sheet-born anomalous plasmoid as the initial stage of coronal mass ejections, and the behavior of moving-away plasmoid stretching the primary reconnected field lines into a secondary current sheet conjoined with two bright ridges identified as solar flares.

Application of Galerkin spectral method for tearing mode instability

Magnetic reconnection and tearing mode instability play a critical role in many physical processes.The application of Galerkin spectral method for tearing mode instability in two-dimensional geometry is investigated in this paper.A resistive magnetohydrodynamic code is developed,by the Galerkin spectral method both in the periodic and aperiodic directions.Spectral schemes are provided for global modes and local modes.Mode structures,resistivity scaling,convergence and stability of tearing modes are discussed.The effectiveness of the code is demonstrated,and the computational results are compared with the results using Galerkin spectral method only in the periodic direction.The numerical results show that the code using Galerkin spectral method individually allows larger time step in global and local modes simulations,and has better convergence in global modes simulations.

How Does the Diffusion Dynamics Reconnect the Magnetic Field？

We clarify how magnetic reconnection can be derived from magnetohydrodynamics （MHD） equations in a way that is easily understandable to university students. The essential mechanism governing the time evolution of the magnetic field is diffusion dynamics. The magnetic field is represented by two components. It is clarified that the diffusion of a component causes agene ration of another component that is initially zero and, accordingly, that the magnetic force lines are reconnected. For this reconnection to occur correctly, the initial magnetic field must be directed oppositely in the two regions, e.g., y 〉 0 and y 〈 O; must be concave （convex） for y 〉 0 （y 〈 0）; and must be saturated foryfar from the x axis, which would indicate the existence of the current sheet. It will be clear that our comprehension based on diffusion runs parallel to the common qualitative explanation about the magnetic reconnection.

High Energy Emission from Magnetar Due to Giant Flare

We propose a theoretical model for magnetar giant flare to explain the flaring activity on 2004 December 27 from SGR1806-20 comprehensively. A global rearrangement is expected by the magnetic reconnection that requires explaining the giant SGR flares. In this paper we propose two regions of flares: preflare on the surface of magnetar and main burst at a distance of light cylinder radius. Acquiring the maximum potential drop on the magnetar surface, adopting space charge limited flow model, and using magnetic field B ≈ 1015 G, the luminosities of flare energies release for the preflare phase and main burst phase are found to be in the order of 1041 erg·sˉ1 and 1044 erg·sˉ1 respectively, conforming to magnetar burst energy and flare temperature is determined by considering black body radiation.

引力场中磁场重联的数值研究──（Ⅰ）磁岛的形成和合并

数值研究了引力场中电阻撕裂模不稳定性所引起的磁场重联，结果表明，在电流片长度Ｌ＝１Ｈ、半宽度δ＝Ｈ／２４的情况下，电流片两端附近将出现两个Ｘ线的磁场重联，并形成磁岛和高温高密度的等离子体团．磁岛宽度随着时间而增长，在ｔ≈５５τＡ时达到饱和，最大饱和岛宽约为４δ．同时从ｔ≈３７τＡ开始，磁岛中心位置逐渐下降；在ｔ＝５７τＡ时发生结合不稳定性，磁岛与底部附近的闭合磁场区合并，导致磁场湮灭和磁能的快速释放．另一方面，由于引力场和等离子体非均匀性的影响，顶部附近随时间而增长的等离子体外流速度达１．１４ＶＡ∞；磁岛中等离子体向下运动的最大速度达１．４１ＶＡ∞，且大于局地声速；在磁岛前方可形成快激波．这些结果可用于解释双带耀斑中后随耀斑环的形成、磁场湮灭和磁能释放、以及Ｄｏｐｐｌｅｒ速度图上观测到的红移现象．

无力场的触发重联和太阳耀斑

采用多步隐格式,对在瞬间形成的电流片的触发下的高剪切无力场的磁重联过程进行了数值模拟。磁重联首先在交界面处的非中性电流片区出现,然后向无力场区蔓延。在磁重联过程中,在无力场区形成一高温环状结构,物质向光球层流动。在高温环内侧的新喷发场区,物质向上流动。磁重联主要集中在初始电流片外侧的高剪切无力场区,高温环顶部的温度最高,位置基本固定。在磁重联的过程中,剪切磁场分量的空间梯度减小,无力场因子下降。

三维非线性撕裂模耦合和快速磁场重联

基于三维不可压缩电阻性ＭＨＤ方程，在长柱形位型下，数值研究了电阻撕裂模不稳定性所引起的磁场重联过程．研究结果表明，撕裂模的非线性相互作用和耦合，将产生许多模式的强烈解稳和导致快速的磁场重联．这一过程的进一步发展，导致具有螺旋形结构、不同模式的磁岛磁力线重叠，从而引起较大区域内的磁场各态经历和磁力线随机走向，形成撕裂模湍动；该湍动将会产生较大的反常电阻，反过来可以进一步加速撕裂模的非线性发展．这种自激过程，有助于理解日冕中的快速磁重联，并可作为一种可能的太阳耀斑机制．

太阳耀斑磁重联的数值模拟

数值模拟了太阳耀斑中二维磁重联过程。结果表明，当重联Ｘ 点比较高时，演化过程能再现双带耀斑中耀斑环的运动等主要特征；当重联Ｘ 点比较低时，可解释致密耀斑的观测特征。结果还表明，耀斑环上升和重联点上升之间没有直接的联系。

耀斑环动力学的数值研究

数值模拟了太阳耀斑环动力学演化的二维磁重联过程。结果表明，在重联Ｘ 点比较高时，演化过程能再现双带耀斑中的耀斑环活动；而重联Ｘ 点比较低时。

Radio observations of the fine structure inside a post-CME current sheet

A solar radio burst was observed in a coronal mass ejection/flare event by the Solar Broadband Radio Spectrometer at the Huairou Solar Observing Station on2004 December 1. The data exhibited various patterns of plasma motions, suggestive of the interaction between sunward moving plasmoids and the flare loop system during the impulsive phase of the event. In addition to the radio data, the associated whitelight, Hα, extreme ultraviolet light, and soft and hard X-rays were also studied.

多层电流片中撕裂模不稳定性的数值研究

应用二维磁流体动力学模拟方法数值研究了三层电流片中电阻撕裂模不稳定性的特征及磁场重联过程．结果表明，这是一种复杂的非稳态磁场重联．在初期阶段，三个电流片中分别由撕裂模不稳定性引起磁场重联，形成薄而长的磁岛．随着撕裂模不稳定性的非线性发展，每个磁岛的宽度都逐步增大，以至导致新的磁场重联发生．同时，三个电流片的强度都逐渐减弱，且原中心反向电流区最终消失．部分磁能不断地转化为等离子体的热能和动能，引起等离子体的加热和加速．多层电流片中撕裂模不稳定性引起的自发重联，可能对太阳耀斑、日冕加热、太阳风与磁层耦合等有重要影响．

粒子云网格方法在大尺度湍流磁重联研究中的应用和进展

作为汪景琇院士主持的中国科学院数理学部"天体辐射磁流体力学"战略研究项目组成部分^([1]),从等离子体的动力学属性出发,介绍了用于研究等离子体与周围磁场结构相互作用的粒子云网格方法,及其在研究具有复杂等离子体和磁场结构的磁重联过程中的应用。这里提到的磁场与等离子体的复杂性包括变化的等离子体β值,磁重联电流片中包含有多种尺度的结构,以及这些结构之间的相互作用,而且这些结构的尺度变化范围很大(从105km变化到102km)。进一步探讨了该方法在太阳物理领域,特别是在太阳爆发磁重联过程方面的可能应用以及未来的发展;并为项目研究中其他部分在数值方法^([2–5])和太阳物理^([6,7])方面提供借鉴和补充。

Research progress based on observations of the New Vacuum Solar Telescope

The purpose of this paper is to introduce the main scientific results made by the one-meter New Vacuum Solar Telescope(NVST),which was put into commission on 2010. NVST is one of the large aperture solar telescopes in the world, located on the shore of Fuxian lake of Yunnan province in China, aiming at serving solar physicists by providing them with high resolution photospheric and chromospheric observational data. Based on the data from NVST and complementary observations from space(e.g.,Hinode, SDO and IRIS, etc), dozens of scientific papers have been published with a wide range of topics concentrating mainly on dynamics and activities of fine-scale magnetic structures and their roles in the eruptions of active-region filaments and flares.The achievements include dynamic characteristics of photospheric bright points, umbral dots, penumbral waves, and sunspot/light bridge oscillation, observational evidence of small-scale magnetic reconnection, and fine-scale dynamic structure of prominences.All these new results will shed light on the better understanding of solar eruptive activities. Data release, observation proposals,and future research subjects are introduced and discussed.

Plasma Heating Induced by Tadpole-like Downflows in the Flaring Solar Corona

As one of the most spectacular energy release events in the solar system,solar flares are generally powered by magnetic reconnection in the solar corona.As a result of the re-arrangement of magnetic field topology after the reconnection process,a series of new looplike magnetic structures are often formed and are known as flare loops.A hot diffuse region,consisting of around 5–10 MK plasma,is also observed above the loops and is called a supra-arcade fan.Often,dark,tadpole-like structures are seen to descend through the bright supra-arcade fans.It remains unclear what role these so-called supra-arcade downflows(SADs)play in heating the flaring coronal plasma.Here we show a unique flare observation,where many SADs collide with the flare loops and strongly heat the loops to a temperature of 10–20 MK.Several of these interactions generate clear signatures of quasi-periodic enhancement in the full-Sun-integrated soft X-ray emission,providing an alternative interpretation for quasi-periodic pulsations that are commonly observed during solar and stellar flares.

激光等离子体模拟研究日地磁场活动特征

激光等离子体磁重联实验再现了卫星观测到的日地磁场活动特征。一方面,实验再现了太阳冕区物质抛射及耀斑结构,包括明亮的尖屋顶状环、具有微细结构的磁化等离子体团以及二者之间因为磁场拉扯而产生的二阶电流片。另一方面,实验发现存在三个电子扩散区(EDR),这与欧洲空间局Cluster卫星先后在2003年和2005年发现的分别处于地磁尾重联区中间部位及两侧分形线位置的两类EDR结构相似。所不同的是,在激光等离子体磁重联实验中,两类EDR在一次重联过程中产生,但中心EDR出现时间晚于两侧EDR,且其发展速度更快,喷流速度接近或者超过迎流Alfven速度。通过对太阳耀斑附近、地磁尾重联区以及激光等离子体自生磁场重联区位置等离子体的参数比较,显示三者在一定程度上具有Euler-Alfven相似性,这表明可以通过激光等离子体自生磁场的重联过程来研究其他两种等离子体中的磁重联现象。

太阳耀斑演化的数值模拟与可视化

太阳耀斑是太阳大气中的剧烈活动之一,它常伴随日冕物质抛射、射电爆发及高能粒子辐射等,并可能影响日地空间环境。因此,太阳耀斑研究不仅在天体物理中有重要的意义,同时也有利于空间物理、航天技术等学科的发展。现在已普遍认为太阳耀斑是磁场重联产生的爆发活动,对其动力学演化的研究主要有2种途径:一种是研究磁重联释放能量后在磁流管中的响应过程,被称为耀斑动力学模型研究。它可以和光谱诊断相结合。

太阳耀斑区等离子体加热的一种新方式——“蝌蚪撞日”

太阳耀斑是太阳大气中的磁能通过磁重联快速释放后产生的一种活动现象.时至今日,耀斑区内等离子体的加热方式还有待厘清.最近,来自于乔治梅森大学和北京大学等高等院校和科研院所的研究人员发现耀斑过程中产生的一群暗黑色"蝌蚪"状下落流陆续撞击耀斑环顶端,并剧烈加热撞击处的日冕大气至一两千万摄氏度的高温.这一新发现也为太阳和恒星耀斑期间常见的准周期脉动现象提供了一种新解释.

太阳大气中不可忽视的爆发活动——喷流

人类第一次注意到太阳大气中发生的喷流现象,是在20世纪20年代的Mt.Wilson观测台站发现的Hα冲浪。尽管已经过去了接近一个世纪,由于喷流本身并没有表现出像太阳耀斑那样的耀眼增亮,也没有像日冕物质抛射那样能够直接影响到近地空间环境,太阳大气喷流一直没能引起人们足够的重视。然而,近些年的一些研究进展表明,喷流不仅在分析太阳磁场拓扑结构和演化、与磁流体力学波(如类阿尔芬波)的相互作用、加热局地日冕以及加速太阳风等方面可能起着十分重要的作用,也与耀斑、日冕物质抛射等太阳上的极端爆发活动有着密切的关系。文章首先简要回顾太阳大气喷流的理论和观测成果,然后介绍作者在喷流触发高速日冕物质抛射方面最新的研究成果。最后简述喷流研究中亟待解决的一些问题和未来的研究方向。

太阳爆发:源动力之谜

看似宁静的太阳上存在着诸多活动现象,其中的爆发活动,包括耀斑(solar flares)和日冕物质抛射(CMEs),是太阳系内最剧烈的能量释放,也是造成空间环境和空间天气变化的最主要因素。磁场重联一直被认为是其背后的驱动力,然而确凿的证据一直没有找到。现在,新一代观测卫星太阳动力学天文台(SDO)终于记录到了完整可信的日冕重联过程。文章中我们简要回顾太阳爆发的研究历史,介绍最新的研究成果,并讨论将来的研究重点。

A Mechanism for Formation of Coronal Mass Ejection

Coronal mass ejection (CME) is a frequent physical phenomenon associated closely with solar flare and eruptive prominence in the solar.atmosphere. Observations show that flare-associated CME has the highest speed, that reaches 500—600 km/s general-