Observation of Two Slow Shocks Associated with Magnetic Reconnection Exhausts in the Interplanetary Space

In the Petschek magnetic reconnection model, two groups of slow shocks play an important role in the energy release. In the past half century, a large number of slow shocks were observed in the geomagnetic tail, and many slow shocks were associated with magnetic reconnection events in the geomagnetic tail. Slow shocks in the interplanetary space are rarer than in the geomagnetic tail. We investigated whether slow shocks associated with interplanetary reconnection exhausts are rare. We examined the boundaries of 50 reconnection exhausts reported by Phan, Gosling, and Davis (Geophys. Res. Lett. 36:L09108, 2009) in interplanetary space to identify slow shocks by fitting the Rankine–Hugoniot relations. Two slow shocks associated with magnetic reconnection exhausts were found and evaluated using observations from Wind and the Advanced Composition Explorer. The observed slow shocks associated with interplanetary reconnection exhausts are rarer than the observed slow shocks associated with geomagnetic tail reconnection exhausts.     

微波毫秒尖峰辐射的时间轮廓

对１９９０年７月３０日云南天文台４波段太阳射电同步观测系统所观测到的大量毫秒级尖峰辐射（ｍｓ－ｓｐｉｋｅｓ）的时间轮廓（包括各种定义的持续时间,上升时间,衰减时间,强度等）作了详细的统计分析,并与米波－长厘米波段的ｓｐｉｋｅ的时间轮廓作了一些比较,以求对微波段的ｓｐｉｋｅ的时间轮廓的特征有一个深入的了解,进而对研究其辐射机制提供有益的帮助     

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.     

Numerical Study for the Bursty Nature of Spontaneous Fast Reconnection

In this paper, spontaneous fast reconnection in a neutral current sheet, which is initially perturbed by a localized resistivity, is studied by the newly developed Space-Time Conservation Element and Solution Element (CESE) method. After the initial perturbation is switched off, an anomalous resistivity is allowed to occur if a threshold of the local electron-ion drift velocity is exceeded. For a given threshold value, the amount of the reconnected magnetic flux introduced by the initial perturbation is very crucial for the onset of the anomalous resistivity. The numerical results indicate that fast reconnection can develop self-consistently with slow shocks extending between the diffusion region and a large-scale plasmoid-like structure, which is pushed forward by the reconnection outflow. A Petschek-like configuration is then built up, but it can not be sustained as a quasi-steady state. In fact, during the reconnection evolution, the diffusion region undergoes an elongation process so that after the dynamic process is nonlinearly saturated secondary tearing is subject to occur at the center of the system. This leads to enhanced and time-dependent reconnection. The reconnection evolution is further studied in various physical situations, also confirming the bursty nature of the spontaneous fast reconnection mechanism.     

Microwave Type III-Like Bursts as Possible Signatures of Magnetic Reconnection

Magnetic reconnection is commonly accepted to play a key role in flare energy release, but only poor information about the main characteristics of this process is available so far. An intrinsic feature of reconnection is plasma density enhancement in current sheets. A unique method to detect this effect is provided by analysis of drifting bursts, whose emission frequency is close to the local Langmuir frequency or its harmonics. With this purpose, we analyze a series of several tens of drifting microwave bursts during the 30 March 2001 flare. The burst drift rates range from −10 to 20 GHz s⁻¹. Using one-dimensional scans recorded with the SSRT interferometer at two different frequencies near 5.7 GHz, we have measured relative positions of burst sources and their velocities along a flare loop revealed from soft X-ray and extreme-ultraviolet images. It is argued that the contribution of the increasing density effect into the observed frequency drift rates is about 6 GHz s⁻¹, which is shown to be consistent with theoretical models of magnetic reconnection with reasonable boundary conditions.     

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 .     

一个太阳微波射电爆发中的快速脉动现象

１９９０年７月３０日观测到一个射电大爆发,其中在２８４０ＭＨｚ射电爆发的峰值附近,发现了周期约为３０ｍｓ的快速脉动现象,脉动是窄带的,调制度约为５０％。     

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.     

2 1/2-Dimensional Reconnection Model and Energy Release in Solar Flares

We employ a 2 1/2-dimensional reconnection model to analyse different aspects of the energy release in two-ribbon flares. In particular, we investigate in which way the systematic change of inflow region variables, associated with the vertical elongation of current sheet, affects the flare evolution. It is assumed that as the transversal magnetic field decreases, the ambient plasma-to-magnetic pressure ratio increases, and the reconnection rate diminishes. As the transversal field decreases due to the arcade stretching, the energy release enhances and the temperature rises. Furthermore, the magnetosonic Mach number of the reconnection outflow increases, providing the formation of fast mode standing shocks above the flare loops and below the erupting flux rope. Eventually, in the limit of a very small transversal field the reconnection becomes turbulent due to a highly non-linear response of the system to small fluctuations of the transversal field. The turbulence results in the energy release fragmentation which increases the release efficiency, and is likely to be responsible for the impulsive phase of the flare. On the other hand, as the current sheet stretches to larger heights, the ambient plasma-to-magnetic pressure ratio increases which causes a gradual decrease of the reconnection rate, energy release rate, and temperature in the late phase of flare. The described magnetohydrodynamical changes affect also the electron distribution function in space and time. At large reconnection rates (impulsive phase of the flare) the ratio of the inflow-to-outflow magnetic field strength is much smaller than at lower reconnection rates (late phase of the flare), i.e., the corresponding loss-cone angle becomes narrower. Consequently, in the impulsive phase a larger fraction of energized electrons can escape from the current sheet downwards to the chromosphere and upwards into the corona – the dominant flare features are the foot-point hard X-ray sources and type III radio bursts. On the other hand, at low reconnection rates, more particles stay trapped in the outflow region, and the thermal conduction flux becomes strongly reduced. As a result, a superhot loop-top, and above-the-loop plasma appears, as sometimes observed, to be a dominant feature of the gradual phase.     

Millisecond Radio Spikes in the Decimetric Band

We present the results of the analysis of thirteen events consisting of dm-spikes observed in Toruń between 15 March 2000 and 30 October 2001. The events were obtained with a very high time resolution (80 microseconds) radio spectrograph in the 1352 – 1490 MHz range. These data were complemented with observations from the radio spectrograph at Ondřejov in the 0.8 – 2.0 GHz band. We evaluated the basic characteristics of the individual spikes (duration, spectral width, and frequency drifts), as well as their groups and chains, the location of their emission sources, and the temporal correlations of the emissions with various phases of the associated solar flares. We found that the mean duration and spectral width of the radio spikes are equal to 0.036 s and 9.96 MHz, respectively. Distributions of the duration and spectral widths of the spikes have positive skewness for all investigated events. Each spike shows positive or negative frequency drift. The mean negative and positive drifts of the investigated spikes are equal to −776 MHz s⁻¹ and 1608 MHz s⁻¹, respectively. The emission sources of the dm-spikes are located mainly at disk center. We have noticed two kinds of chains, with and without frequency drifts. The mean durations of the chains vary between 0.067 s and 0.509 s, while their spectral widths vary between 7.2 MHz and 17.25 MHz. The mean duration of an individual spike observed in a chain was equal to 0.03 s. While we found some agreement between the global characteristics of the groups of spikes recorded with the two instruments located in Toruń and Ondřejov, we did not find any one-to-one relation between individual spikes.     

太阳微波脉冲事件中源区加热的证据

本文对具有一定空间分辨率（～４’）和高灵敏度（０．００４ｓｆｕ）特性的１３．７ｍ射电望远镜，在１．３５ｃｍ（２２ＧＨｚ）波段上观测到的两个脉冲事件前的周期性脉动和吸收现象进行了分析，并认为这种观测特征，可用源区局部区域的加热得到合理解释。     

Collisionless Magnetic Reconnection in the Presence of Initial Guide Field

By using the method of 2-dimensional, 3-component full particle simulation, collisionless magnetic reconnection in the presence of various initial guide fields and the Harris current sheet with 1-dimensional initial state are studied. The results show that strong guide fields with B_z0 > 0.5B₀ can evidently alter not only the trajectory of the particles, but also the structure of the electric and velocity fields in the vicinity of the reconnection region, thereby affecting the rate of reconnection and the acceleration of electrons. The generalized Ohm's law is employed to interpret the structural characteristics of the electric fields with various guide fields. Also, via the tracing of the electron beam near he diffusion region, it is revealed that in the 2-D model, for both strong and weak guide fields, the induced electric field perpendicular to the simulation plane at the center of the diffusion region plays the major role in the acceleration of electrons. The contribution of the planar electric field outside the diffusion region is very small.     

日冕三重无力场电流片的磁场重联

采用二维三分量磁流体力学模型,对日冕三重无力场电流片的磁场重联进行了数值研究,揭示了重联过程的基本物理特征．这类重联过程将加热和加速日冕等离子体,并导致多个高温、高密度、高磁螺度的磁岛的形成和向上喷发．这表明,多重无力场电流片的重联可能在日冕磁能释放、上行等离子体团的形成和太阳磁场螺度向行星际空间的逃逸方面起重要的作用．     

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。     

太阳微波爆发FFS的基本单元

参加了Ｆｌａｒｅｓ２２和Ｍａｘ＇９１国际联合观测之后，我们处理了三个频率（１．４２，２．８４，３．６７ＧＨｚ）和四个频率（１．４２，２．００，２．８４，４．００ＧＨｚ）或（１．４２，２．１３，２．８４，４．２６ＧＨｚ）快速采样射电望远镜的观测资料。结果除了发现射电爆发源的局部区域中存在有射电辐射的第四种基本分量而外，还在微波爆发快速精细结构中发现了三种基本时间单元。其量级分别是：０．１秒＞τ１≥１毫秒；１秒＞τ２≥０．１秒；１００秒＞τ３≥１秒。尽管出现在各自基本时间单元内的ＦＦＳ事件的形态及特性各自不同，但是，叠加在射电爆发背景之上的特性，构成了它们的共同属性。三种基本时间单元的确认，对于研究微波快速活动的精细时间结构，划分ＦＦＳ事件的种类找到了根据。三种基本时间单元的研究，对于深入探讨产生ＦＦＳ源的ＥＣＭ理论，也具有重要的科学价值。     

太阳光球层磁重联的直接证据

本文首次给出了发生在太阳光球磁重联的一个直接的观测证据。 这一磁重联的观测特征是:(1)重联发生在一新浮现磁通量区的一极与极性相反的老磁通量之间;(2)重联前中性线附近磁剪切明显;(3)被重联两极为一对消磁结构,重联发生在稳定的磁通量损失数小时之后;(4)一个级别为C2.9的亚耀斑发生在重联之前。该耀斑以重联区为中心,双带离重联位置2～3万公里,直到耀斑极大相后14分钟,重联仍未发生;(5)重联后,磁对消速率呈增大趋势。     

Effective Acceleration Model for the Arrival Time of Interplanetary Shocks driven by Coronal Mass Ejections

In a previous work (Paouris and Mavromichalaki in Solar Phys. 292, 30, 2017), we presented a total of 266 interplanetary coronal mass ejections (ICMEs) with as much information as possible. We developed a new empirical model for estimating the acceleration of these events in the interplanetary medium from this analysis. In this work, we present a new approach on the effective acceleration model (EAM) for predicting the arrival time of the shock that preceds a CME, using data of a total of 214 ICMEs. For the first time, the projection effects of the linear speed of CMEs are taken into account in this empirical model, which significantly improves the prediction of the arrival time of the shock. In particular, the mean value of the time difference between the observed time of the shock and the predicted time was equal to +3.03 hours with a mean absolute error (MAE) of 18.58 hours and a root mean squared error (RMSE) of 22.47 hours. After the improvement of this model, the mean value of the time difference is decreased to ?0.28 hours with an MAE of 17.65 hours and an RMSE of 21.55 hours. This improved version was applied to a set of three recent Earth-directed CMEs reported in May, June, and July of 2017, and we compare our results with the values predicted by other related models.     

Radiative Shocks in Astrophysics and the Laboratory

This paper explores the variations in radiative shock behavior originating from the properties of the system containing the shock. Specifically, the optical depth of the upstream region and the downstream region both affect the behavior of radiative shocks. Optically thick systems such as stellar interiors or supernovae permit only limited shock-induced increases in density. At the other limit, the radiation and shock dynamics in optically thin systems permits the post-shock density to reach arbitrarily large values. The theory of the shock structure is summarized for systems in which the upstream region is optically thin, common to some astrophysical systems and a number of experiments.     

Observations of Microwave Ultra-Fast Absorption Phenomena Above Solar Active Region

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