Study of Ribbon Separation and Magnetic Reconnection Rates in a Two-Ribbon Flare

In this paper, we present a study of the correlation between the speed of flare ribbon separation and the magnetic flux density during the 10 April 2001 solar flare. The study includes the section of the neutral line containing the flare core and its peripheral area. This event shows clear two-ribbon structure and inhomogeneous magnetic fields along the ribbons, so the spatial correlation and distribution of the flare and magnetic parameters can be studied. A weak negative correlation is found between the ribbon separation speed (V _r) and the longitudinal magnetic flux density (B _z ). This correlation is the weakest around the peak of the flare. Spatially, the correlation is also weakest at the positions of the hard X-ray (HXR) sources. In addition, we estimate the magnetic reconnection rate (electric field strength in the reconnection region E _rec) by combining the speed of flare ribbons and the longitudinal magnetic flux density. During flare evolution, the time profiles of the magnetic reconnection rate are similar to that of the ribbon separation speed, and the speeds of ribbon separation are relatively slow in the strong magnetic fields (i.e., V _r is negatively correlated with B _z ). However, along the flare ribbons, E _rec fluctuates in a small range except near the HXR source. A localized enhancement of the reconnection rate corresponds to the position of the HXR source.     

The Precursor Phase of an X-class Flare:Magnetic Reconnection,Powering and Non-thermal Electrons

In this paper,we report three interesting phenomena that occurred during the precursor phase of the X1.6 class flare on 2014 September 10.(1)The magnetic reconn...     

A Reconnection Model for the Distribution of Flare Energies

A physically based explanation is given for the distribution of flare energies N(E)E ^– where 1.5. In contrast to previous approaches, the present treatment is based on a physical theory of the flare reconnection site. The central assumption is that topological flare energy, although released explosively, is slowly accumulated over several hundred Alfvén timescales. When coupled to the geometric properties of the reconnective flare source, this assumption is shown to lead naturally to a deduction of the flare energy distribution. Current sheet models yield the exponent whereas more compact current structures imply steeper spectra .     

Statistical Assessment of Photospheric Magnetic Features in Imminent Solar Flare Predictions

In this study we use the ordinal logistic regression method to establish a prediction model, which estimates the probability for each solar active region to produce X-, M-, or C-class flares during the next 1-day time period. The three predictive parameters are (1) the total unsigned magnetic flux T _flux, which is a measure of an active region’s size, (2) the length of the strong-gradient neutral line L _gnl, which describes the global nonpotentiality of an active region, and (3) the total magnetic dissipation E _diss, which is another proxy of an active region’s nonpotentiality. These parameters are all derived from SOHO MDI magnetograms. The ordinal response variable is the different level of solar flare magnitude. By analyzing 174 active regions, L _gnl is proven to be the most powerful predictor, if only one predictor is chosen. Compared with the current prediction methods used by the Solar Monitor at the Solar Data Analysis Center (SDAC) and NOAA’s Space Weather Prediction Center (SWPC), the ordinal logistic model using L _gnl, T _flux, and E _diss as predictors demonstrated its automatic functionality, simplicity, and fairly high prediction accuracy. To our knowledge, this is the first time the ordinal logistic regression model has been used in solar physics to predict solar flares.     

Statistical Models for the Solar Flare Interval Distribution in Individual Active Regions

This article discusses statistical models for the solar flare interval distribution in individual active regions. We analyzed solar flare data in 55 active regions that are listed in the Geosynchronous Operational Environmental Satellite (GOES) soft X-ray flare catalog for the years from 1981 to 2005. We discuss some problems with a conventional procedure to derive probability density functions from any data set and propose a new procedure, which uses the maximum likelihood method and Akaike Information Criterion (AIC) to objectively compare some competing probability density functions. Previous studies of the solar flare interval distribution in individual active regions only dealt with constant or time-dependent Poisson process models, and no other models were discussed. We examine three models – exponential, lognormal, and inverse Gaussian – as competing models for probability density functions in this study. We found that lognormal and inverse Gaussian models are more likely models than the exponential model for the solar flare interval distribution in individual active regions. The possible solar flare mechanisms for the distribution models are briefly mentioned. We also briefly investigated the time dependence of probability density functions of the solar flare interval distribution and found that some active regions show time dependence for lognormal and inverse Gaussian distribution functions. The results suggest that solar flares do not occur randomly in time; rather, solar flare intervals appear to be regulated by solar flare mechanisms. Determining a solar flare interval distribution is an essential step in probabilistic solar flare forecasting methods in space weather research. We briefly mention a probabilistic solar flare forecasting method as an application of a solar flare interval distribution analysis. The application of our distribution analysis to a probabilistic solar flare forecasting method is one of the main objectives of this study.     

The X10 Flare on 29 October 2003: Was It Triggered by Magnetic Reconnection between Counter-Helical Fluxes?

Vector magnetograms taken at Huairou Solar Observing Station (HSOS) and Mees Solar Observatory (MSO) reveal that the super active region (AR) NOAA 10486 was a complex region containing current helicity flux of opposite signs. The main positive sunspots were dominated by negative helicity fields, while positive helicity patches persisted both inside and around the main positive sunspots. Based on a comparison of two days of deduced current helicity density, pronounced changes associated with the occurrence of an X10 flare that peaked at 20:49 UT on 29 October 2003 were noticed. The average current helicity density (negative) of the main sunspots decreased significantly by about 50%. Accordingly, the helicity densities of counter-helical patches (positive) were also found to decay by the same proportion or more. In addition, two hard X-ray (HXR) “footpoints” were observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) during the flare in the 50 – 100 keV energy range. The cores of these two HXR footpoints were adjacent to the positions of two patches with positive current helicity that disappeared after the flare. This strongly suggested that the X10 flare on 29 October 2003 resulted from reconnection between magnetic flux tubes having opposite current helicity. Finally, the global decrease of current helicity in AR 10486 by ∼50% can be understood as the helicity launched away by the halo coronal mass ejection (CME) associated with the X10 flare. Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users.     

Evidence for Tether-Cutting Reconnection in a Quadrupole Magnetic Configuration in the April 9, 2001, M7.9 Flare

We studied the M7.9 flare on April 9, 2001 that occurred within a δ-sunspot of active region NOAA 9415. We used a multi-wavelength data set, which includes Yohkoh, TRACE, SOHO, and ACE spacecraft observations, Potsdam and Ondřejov radio data and Big Bear Solar Observatory (BBSO) images in order to study the large-scale structure of this two-ribbon flare that was accompanied by a very fast coronal mass ejection (CME). We analyzed light curves of the flare emission as well as the structure of the radio emission and report the following: the timing of the event, i.e., the fact that the initial brightenings, associated with the core magnetic field, occurred earlier than the remote brightening (RB), argue against the break-out model in the early phase of this event. We thus conclude that the M7.9 flare and the CME were triggered by a tether-cutting reconnection deep in the core field connecting the δ-spot and this reconnection formed an unstable flux rope. Further evolution of the erupted flux rope could be described either by the “standard“ flare model or a break-out type of the reconnection. The complex structure of flare emission in visible, X-ray, and radio spectral ranges point toward a scenario which involves multiple reconnection processes between extended closed magnetic structures.     

Plasma Heating by Forced Magnetic Reconnection

A model of forced magnetic reconnection in a force-free magnetic field is considered, which allows calculation of the magnetic energy release during the current sheet reconnection. The dependence of this energy on characteristics of the magnetic configuration has been studied, and it was found that the released energy becomes very large when the field is near the marginal tearing stability. A persistent plasma heating provided by ongoing external driving and internal reconnection is also derived. It shows a typical relaxation-type dependence on the driving frequency, with dissipation becoming most efficient when the time-scales of the driving and reconnection are comparable. Possible implications of the obtained results for the problem of solar coronal heating are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

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。     

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.     

A Statistical Study of Microwave Flare Morphologies

This study has been motivated by the detection of a small number of optically thin microwave bursts with maximum emission near the loop top, which is contrary to the prediction of isotropic gyrosynchrotron models. Using Nobeyama Radioheliograph (NoRH) high-spatial-resolution images at 17 and 34 GHz, we study the morphology at the radio peak of 104 flares that occurred relatively close to the limb. Using data from the Nobeyama Polarimeter we were able to determine whether the 17- and 34-GHz emissions came from optically thin or thick sources. We identified single-loop events, taking into account supplementary information from EUV and soft X-ray (SXR) images. We found optically thin emission from the top of the loop in 36% of single-loop events. In agreement with standard models, in this sample 46% and 18% of the events showed optically thin emission from the footpoints and optically thick emission from the entire loop, respectively. The derived percentage of events with gyrosynchrotron emission from isotropic populations of energetic electrons is possibly an upper limit. This point is illustrated by the analysis of an optically thin event that shows footpoint emission during the rise phase and loop-top emission during the decay phase. A model that takes into account both anisotropies in the distribution function of nonthermal electrons and time evolution can reproduce the observed transition from footpoint to loop-top morphology, if electrons with pitch-angle anisotropy are injected near one of the footpoints.     

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.     

Preliminary Results of Laboratory Simulation of Magnetic Reconnection

In the Linear Magnetized Plasma (LMP) device of University of Science and Technology of China and by exerting parallel currents on two parallel copper plates, we have realized the magnetic reconnection in laboratory plasma. With the emissive probes, we have measured the parallel (along the axial direction) electric field in the process of reconnection, and verified the dependence of reconnection current on passing particles. Using the magnetic probe, we have measured the time evolution of magnetic flux, and the measured result shows no pileup of magnetic flux, in consistence with the result of numerical simulation.     

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.     

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.     

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.     

耀斑软X射线流量的统计性质

为了更定量地研究太阳耀斑软X射线辐射的统计性质,发展了一套对于给定峰值流量区间的耀斑的自动识别程序,并用它分析了从1980年到2013年GOES(Geostationary Operational Environmental Satellite)在两个软X射线波段上对太阳耀斑的观测.研究发现耀斑软X射线流量在峰值附近变化的统计特征和耀斑流量峰值的绝对大小无关:平均而言耀斑流量的上升时间约是下降时间的一半,而且高能量通道的上升和下降时间比相应的低能量通道时间要短,但是这些时间还是会随着耀斑流量变化幅度的增加而增加.     

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。