太阳大气中磁重联的MHD数值模拟

回顾了近３０年太阳大气中磁重联过程的ＭＨＤ数值模拟工作取得的进展。着重描述了在验证理论模型,解释观测现象,以及研究各种因素对重联的影响三个方面的成果,如快速磁重联,太阳耀斑机制及色球,日冕中的各种爆发现象等。指出了在数值模拟中应注意的几个问题,并对该领域今后的发展作了简要的展望。     

Magnetic Reconnection and Extraplanar Diffuse Ionized Gas

Photoionization by stars cannot be the only cause for the diffuseionized gas component in the distant halos of galaxies.Quasistationary localized magnetic reconnection can efficientlycontribute to the ionization of the extraplanar halo gas. The gas isheated and re-ionized in numerous current filaments fast enough tocompensate recombination. The necessary magnetic energy is provided bythe disk activity.     

Self-Generation of Magnetic Fields in Weakly Ionized Astrophysical Plasmas

In weakly ionized astrophysical plasmas, shear flow induced plasma - neutral gas friction yields self-generated magnetic fields of seed-field order. This process is of cosmological importance and relevant for protogalactic systems like Lyα-clouds. In our contribution we illustrate this mechanism by the help of 3-dimensional 2-fluid simulations of primordial rotating gas clumps in Lyα-clouds showing that plasma - neutral gas interactions cause large scale magnetic fields of the order of 10⁻¹⁵G on time scales of the order of 10⁶yrs. This revised version was published online in July 2006 with corrections to the Cover Date.     

Heating the Solar Corona by Magnetic Reconnection

Here I review briefly the theory of magnetohydrodynamic reconnection and ask what observational evidence is there that it is heating the corona. In particular, the new directions in which three-dimensional theory for reconnection is heading are outlined. Part of the coronal heating problem has been solved with the identification of reconnection driven by converging flux motions as the key for x-ray bright points. Furthermore, it has been shown that the large-scale diffuse corona is heated rather uniformly, so that turbulent reconnection by braiding or ion-cyclotron waves driven by network micro-flares are prime candidates. Finally, reconnection is the natural explanation for a wide variety of phenomena discovered by SOHO including explosive events, blinkers, the magnetic carpet and even possibly tornadoes. This revised version was published online in July 2006 with corrections to the Cover Date.     

太阳大气中的光球亮点

首先是对太阳光球亮点近年来研究工作的总结。光球亮点是一种发生在太阳光球上宁静区域的的小尺度和短寿命增亮现象,平均直径在100～300knm之间,平均寿命约为几分钟。光球亮点的研究对于光球辐射和磁场性质的认识具有重要意义。过去的观测显示,绝大多数光球亮点的产生和演化与磁场,特别是光球上的小尺度磁场的演化密切相关,比如,两个同极性磁场的合并,或者反极性磁场的对消,或者一个同极性磁场的分裂,均可以促使光球亮点产生或消失。基于这样的观测结果,统计研究了2722个光球亮点(1600A)与光球上偶极磁元的关系,发现大约有1/3的光球亮点出现在偶极磁元中心附近。     

Observed Effect of Magnetic Fields on the Propagation of Magnetoacoustic Waves in the Lower Solar Atmosphere

We study Hinode/SOT-FG observations of intensity fluctuations in Ca ii H-line and G-band image sequences and their relation to simultaneous and co-spatial magnetic field measurements. We explore the G-band and H-line intensity oscillation spectra both separately and comparatively via their relative phase differences, time delays and cross-coherences. In the non-magnetic situations, both sets of fluctuations show strong oscillatory power in the 3??C?7 mHz band centered at 4.5 mHz, but this is suppressed as magnetic field increases. A relative phase analysis gives a time delay of H-line after G-band of 20±1?s in non-magnetic situations implying a mean effective height difference of 140 km. The maximum coherence is at 4??C?7 mHz. Under strong magnetic influence the measured delay time shrinks to 11 s with the peak coherence near 4 mHz. A second coherence maximum appears between 7.5??C?10 mHz. Investigation of the locations of this doubled-frequency coherence locates it in diffuse rings outside photospheric magnetic structures. Some possible interpretations of these results are offered.     

Evidence of Magnetic Reconnection in Solar Flares and a Unified Model of Flares

The solar X-ray observing satellite Yohkoh has discovered various new dynamic features in solar flares and corona, e.g., cusp-shaped flare loops, above-the-loop-top hard X-ray sources, X-ray plasmoid ejections from impulsive flares, transient brightenings (spatially resolved microflares), X-ray jets, large scale arcade formation associated with filament eruption or coronal mass ejections, and so on. It has soon become clear that many of these features are closely related to magnetic reconnection. We can now say that Yohkoh established (at least phenomenologically) the magnetic reconnection model of flares. In this paper, we review various evidence of magnetic reconnection in solar flares and corona, and present unified model of flares on the basis of these new Yohkoh observations. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the Magnetic Energy Avalanche in the Solar Atmosphere

We discuss a mechanism of energy production associated with the acoustic and MHD shocks produced by colliding and reconnecting flux tubes and present the observational results showing a connection between the dynamic changes in the photosphere and the high velocity and heating events in the transition region. We suggest that these processes may provide a constant energy supply for the origin of the fast wind and help to advance studies of coronal dynamics. This revised version was published online in July 2006 with corrections to the Cover Date.     

太阳大气磁精细结构的射电探讨

太阳大气磁场的研究对于太阳大气物理及太阳活动研究是十分重要的。目前探测光球以外的日够以球，过渡区磁场的几乎唯一办法，是在紧密联系其他频说段取得的信息基础上使用射电观测。根据在微波，米波段有关辐射机制和传播过程，介绍了推导磁场讯息的基本射电方法。     

磁场重联和日冕物质抛射

Basic processes of magnetic reconnection and observations of coronal mass ejection are introduced. A possible mechanism of CME caused by magnetic rcconnection in the current sheet of solar corona is suggested.     

Detection of Departure from Thermal Dynamic Equilibrium in the Solar Middle and Lower Atmosphere UsingTotal Solar Eclipse Data

There are abundant emission and absorption lines superimposed on the continuum spectra of the different solar atmospheric layers. The chemical composition and physical state of the solar atmosphere can be probed by the inversion of the profiles of these spectral lines. Due to the low density and large temperature difference in the chromosphere and transition region of the Sun, it is hard to establish the thermal dynamic equilibrium. It is necessary to adopt the theory of Non-Local Thermodynamic Equilibrium (N-LTE) to construct the corresponding atmospheric model. In this paper, the departure from the Local Thermodynamic Equilibrium (LTE) in the middle and lower atmosphere of the sun is investigated with the well-defined relative departure factor and the relevant calculations. We first make an inversion of the spectral lines formed at the different heights in the chromosphere and transition region during a total solar eclipse, to obtain the parameters of the observed spectral lines, such as the continuum source function, line source function, Doppler width, and thus the equivalent kinetic temperature. According to these line parameters obtained by the inversion, we calculate the quantitative results about the departure from the LTE at each space sampling point in the 2D field of view. Secondly, we reconstruct the 2D distributions of the radiation intensity, equivalent kinetic temperature, and relative departure factor according to the alignment of the optical fiber array in the integrated field unit (IFU) used by the telescope. The results show that there is a stronger correlation in the distributions of the temperature and relative departure factor existed in local small regions, but without evident correlation with the distribution of radiation intensity. There is an obvious difference between the distributions of the equivalent temperature and relative departure factor derived from two spectral lines, which shows a strong structurization and complexity existed in some local small regions of the solar atmosphere, and provides a new perspective for us to further understand the physics of the middle and lower atmosphere of the Sun.     

磁场重联实验室模拟进展

磁场重联是空间等离子体和实验室等离子体中的常见现象,被认为是太阳耀斑和磁层亚暴的重要机制。实验室磁场重联的模拟研究已经有二十余年的历史,并且取得了一系列重要的结果。对几个主要的磁场重联实验装置进行了介绍,给出了各个装置的等离子体参数以及产生重联的方法,回顾了实验室研究中和太阳射电密切相关的几个问题。另外,以中国科技大学的线性磁化等离子体装置为基础,建立了国内首个磁场重联的实验装置,研制了实验中需要的诊断工具,并开展了初步的磁场重联实验。     

无碰撞重联区中的混沌感应电阻

无碰撞磁场重联作为一种将磁能有效转化为等离子体动能和热能的机制,已经被广泛应用于解释太阳耀斑、地球磁暴等各类等离子体的爆发活动.然而,在无碰撞重联区中反常电阻的微观物理机制仍然是尚未解决的基本问题.在众多反常电阻的形成机制中,基于磁零点附近粒子轨道混沌性产生的混沌感应电阻,虽然不是最普遍流行的形成机制,但它的微观物理图像却是最为清晰的.回顾了无碰撞重联区中混沌感应电阻的早期研究和基本理论模型,介绍了关于混沌感应电阻研究的新进展并阐述了混沌感应电阻未来的研究方向.     

Analytic Magnetic Reconnection Solutions in Cylindrical Geometry

In this paper we present a new class of exact reconnection solutions in cylindrical geometry. We point out that in the case of planar reconnection there is a natural cylindrical analog to the Cartesian Dawson function model for the magnetic field. Although the resistive energy release scalings of these solutions mimic the Cartesian models an important new feature is the presence of curvature in the current sheet. We go on to show that these solutions can be generalized to three dimensions.     

Formation of Intensive Magnetic Flux Tubes in a Converging Flow of Partially Ionized Solar Photospheric Plasma

Theoretical model, explaining a phenomenon of formation of Intensive Magnetic Flux Tube (IMFT) in a converging flow of partially ionized solar photospheric plasma is considered. Special attention is paid to the fact of weak ionization (n/n _n ∼ 10^-4) of plasma in the photosphere. The cases of 2D magnetic slab and cylindric magnetic tube are considered. It was shown that in a converging flow of photospheric plasma thin magnetic tubes, or slabs with the characteristic scale L ₀ ∼ (1 ÷ 5) ċ 10⁷ cm and magnetic field 1000 ÷ 2000 G can be generated. By this 2D magnetic slabs could be unstable with respect to an exchange instability and appear as an intermediate step during IMFT formation on the boundary of two supergranulation cells. Formation of compact strong magnetic field structures, and their energy balance are discussed. Stationary Joule energy dissipation taking place on the photospheric levels in the models of magnetic slab or IMFT under consideration increases towards the periphery of these objects and can exceed radiation looses. This can cause the occurrence of magnetic tubes with hot external envelopes, and modification of plasma temperature and density distribution, with respect to ones in a quiet atmosphere. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Statistical Study on the Reconnection in Boundary Layers of Small-scale Magnetic Flux Tubes in Solar Winds

With the data from WIND satellite in 1995—2005, the small-scale magnetic ?ux tubes determined with two methods, i.e., the arti?cial recognition and program selection respectively, are compared. It is found that there are magnetic reconnections in the boundary layers of 41% small-scale magnetic ?ux tubes determined with the program selection method, which is similar to the re- sult of small-scale magnetic ?ux tubes determined with the method of arti?cial recognition. The features of magnetic reconnections, such as the magnetic shear angle, magnetic ?eld strength, and duration of the dissipation region of recon- nection at the small-scale ?ux tubes’ boundaries determined from both methods have the same statistical tendency. This shows that there is no essential differ- ence in the properties of reconnections in the boundary layers of the small-scale magnetic ?ux tubes determined with the two methods. Hence the data yielded by both methods can be used as the samples for statistically studying the events of reconnection in the front and back boundary layers of small-scale ?ux ropes. There are totally 71 magnetic reconnection events selected in this paper. Our statistical result shows that in 50 events (70%) the decrease of magnetic ?eld strength in the dissipation region of reconnection is larger than 20%, and in 47 events (66%) the magnetic shear angle is larger than 90 degrees. These indicate that the magnetic reconnections in boundaries of the small-scale magnetic ?ux tubes are more likely to be anti-parallel. The statistics has been performed sep- arately in the reconnections of the front and back boundary layers of small-scale magnetic ?ux tubes. The results show that the features of reconnections in the front and back boundaries are similar to each other, which is different from the various properties in the front and back boundaries of the magnetic clouds, and this means that the expansion in the large-scale magnetic ?ux tubes, such as magnetic clouds, does not happen in the small-scale magnetic ?ux tubes.     

用日全食光谱探测太阳中低层大气对局部热动平衡的偏离

在太阳大气不同层次连续光谱中叠加有丰富的发射线或吸收线,对这些谱线轮廓进行反演分析可以探测太阳大气的化学成分和物理状态.太阳大气的色球及过渡区由于其密度低难以建立热动平衡,建立相应的大气模型需要采用非局部热动平衡(Non-Local Thermodynamic Equilibrium,N-LTE)理论.根据相对偏离因子计算来研究太阳中低层大气偏离局部热动平衡(Local Thermodynamic Equilibrium,LTE)分布的情况.首先对日全食观测过程中得到色球和过渡区不同高度形成的两条光谱数据进行反演,得到确定观测谱线的参数信息,如连续谱源函数、谱线源函数、多普勒宽度和由此推出的等效动力学温度;根据这些反演出的谱线参量计算出二维视场内每个空间采样点偏离LTE状态的定量结果;其次,根据用于观测的积分视场单元光纤排布阵列重构出辐射强度、等效动力学温度和相对偏离因子二维分布.结果显示:在局部小区域,温度分布和相对偏离因子的分布存在较强相关性,而与辐射强度分布无明显相关.从两条谱线导出的等效温度和相对偏离因子分布存在明显的差异.这两种二维分布揭示出太阳大气某些小尺度区域具有较强的结构性和复杂性,为进一步理解太阳中低层大气物理提供了一种新的视角.     

日冕磁场和重联的射电信号

总结了近期用射电频谱仪（高时间和高频谱分辨）和野边山射电日像仪（高空间分辨）以及国内外其它空间和地面设备分析日冕磁场和重联的系列工作。主要结论可归纳为：1）在Dulk等人（1982）的近似下自恰计算射电爆发源区磁场的平行和垂直分量，并首次得到该磁场在日面的两雏分布。2）为了考虑非热电子低能截止的影响，必须采用更严格的回旋同步辐射理论来计算。结果表明：低能截止和日冕磁场对计算有明显的影响，而其它参数（包括背景温度、密度、高能截止和辐射方向）的影响均可忽略。因此，对低能截止和日冕磁场必须联立求解。3）射电爆发中的精细结构可能反映了射电爆发源比较靠近粒子加速（磁场重联）的区域，利用高时间和高频率分辨的频谱仪和高空间分辨的日像仪联合分析，可以确定精细结构的源区位置，从而确定粒子加速（磁场重联）的准确时间和地点。     

Magnetic Reconnection, Turbulence, and Collisionless Shock

A short summary of recent progress in measuring and understanding turbulence during magnetic reconnection in laboratory plasmas is given. Magnetic reconnection is considered as a primary process to dissipate magnetic energy in laboratory and astrophysical plasmas. A central question concerns why the observed reconnection rates are much faster than predictions made by classical theories, such as the Sweet–Parker model based on MHD with classical Spitzer resistivity. Often, the local resistivity is conjectured to be enhanced by turbulence to accelerate reconnection rates either in the context of the Sweet–Parker model or by facilitating setup of the Pestchek model. Measurements at a dedicated laboratory experiment, called MRX or Magnetic Reconnection Experiment, have indicated existence of strong electromagnetic turbulence in current sheets undergoing fast reconnection. The origin of the turbulence has been identified as right-hand polarized whistler waves, propagating obliquely to the reconnecting field, with a phase velocity comparable to the relative drift velocity. These waves are consistent with an obliquely propagating electromagnetic lower-hybrid drift instability driven by drift speeds large compared to the Alfven speed in high-beta plasmas. Interestingly, this instability may explain electromagnetic turbulence also observed in collisionless shocks, which are common in energetic astrophysical phenomena.