Magnetic flux expulsion as an acceleration mechanism for stellar winds

Observations of the Sun show that magnetic flux is emerging through the surface in small scales in rather copious amounts. In order to maintain a steady state field strength, this flux must either be locally dissipated or explelled or both. We believe that magnetic reconnection and subsequent flux explusion is the most effective manner in which to achieve this. If new flux emerges into an already preexisting coronal magnetic field, the ambient field must be pushed aside to allow room for the new flux. If the ambient field strength decreases outward with radial distance as is expected for all stars, it may pinch off the emerging flux through magnetic reconnection and expell it outward. The net force on an isolated diamagnetic plasmoid produced by this process is shown to assume a particularly simple form, depending only on the plasmoid's mass, its temperature, and the radial gradient of the logarithm of the undisturbed magnetic pressure. If a sufficient number of these magnetic elements are produced per unit time, this process translates to a net outward magnetic force on the coronal plasma which can be greater that the gas pressure force. Thus, a stellar wind can be produced by magnetic forces alone without the need for a high coronal gas pressure — a mechanism which could be effective in explaining why stars, such as the late-type giants, which possess cool coronae nevertheless exhibit vigorous coronal expansions.     

Reconnection in multiple current sheet configurations in the solar corona

We investigate magnetic reconnection in a multiple current sheet configuration by means of three-dimensional resistive MHD simulations. This configuration might be of interest in the solar corona context, e.g. for coronal helmet streamers. We present results of our simulations of the linear and nonlinear development of the tearing mode instability. In particular, we highlight the changes in magnetic topology and the resulting plasma dynamics. Our results indicate that reconnection in complex coronal neighboring magnetic flux systems efficiently converts magnetic field energy into thermal energy and leads to small-scale tongue outflows rather than large-scale coronal mass ejections.     

Numerical simulation of reconnection in an emerging magnetic flux region

The resistive MHD equations are numerically solved in two dimensions for an initial-boundary-value problem which simulates reconnection between an emerging magnetic flux region and an overlying coronal magnetic field. The solution involves both ideal-MHD and resistive-MHD processes, and the solution shows an evolution which is remarkably suggestive of the preflare, impulsive, and main phases of the flare-cycle.     

3-D reconnection related to new emerging flux

We present evidences that emergence of new flux in the lower atmosphere leads to magnetic reconnection of field lines. In a first phase the phenomenon is observed in the chromosphere by the formation of dark filaments (arch filament system) which are overlaid by bright loops visible in soft X-rays. Different types of event appear according to the magnetic field configuration and the amount of energy involved. 3-D modelling of the photospheric magnetic field provides a new tool for understanding reconnection in real configurations. The observed chromospheric and coronal loops are good diagnostics for the modelling. We document our statement by examples obtained during coordinated campaigns with the H Multichannel Subtractive Double Pass spectrographs-MSDP (Pic du Midi and Tenerife) and the Yohkoh instruments.     

基于能量色散特征计算磁层顶重联位置的方法研究

磁层顶磁场重联是太阳风向磁层输入能量的主要方式.重联如何触发一直是空间物理研究的难点,其机制仍然有待深入研究.由于卫星穿越磁层顶时,很难恰好穿越重联发生的区域,因此难以观测到重联的触发条件.本文利用THEMIS卫星观测,确立了反演磁层顶重联点的方法.当重联刚开始发生时,卫星能够观测到离子的能量色散特征,可利用其计算卫星到重联发生位置的距离.沿着磁力线模型追踪该距离即可反演出磁层顶发生重联的位置.与其他方法进行了对比分析,结果显示本文方法比其他方法具有更高的精度.      

Plasmoid formation in eruptive flares

One phenomena Yohkoh has observed is plasmoid eruption in flares. Thus this is a key factor that must be explained in any flare mechanism. In order to understand the dynamics of a plasmoid, we performed a numerical MHD simulation and investigated the evolution of the coronal magnetic field, which is initially a force-free configuration. The main results are as follows. At first, small amount of dissipation, induced by the initial perturbation, occurs in the current sheet where the plasmoid forms. This plasmoid is slowly going upward by magnetic tension force of the reconnected magnetic fields produced by initial dissipation. The crucial point comes when the perpendicular magnetic fields are washed away from the reconnection point, after that the reconnection proceeds effectively so that the magnetic tension force of the reconnected fields becomes strong, which make the plasmoid be rapidly erupted upward. These are consistent with the observational results, which say that before the main energy release the plasmoid slowly rises and when the flare sets in it is rapidly accelerated upward. In this paper, we emphasize on the role that the perpendicular magnetic fields play in the evolution of flare.     

Dynamical magnetic reconnection in Parker''s coronal heating model

The excitation (flares, ejections, heating, …) of the corona can be understood in terms of the dynamics of the confectively driven magnetized plasma. In particular, anomalous ohmic heating may be a consequence of the formation and rapid dissipation of small-scale magnetic fields in the corona. We have performed numerical simulations of the loop heating model proposed by Parker (1972, 1994), and have studied its dynamics and global power balance in order to assess its viability as a coronal heating candidate, with promising results. Our results suggest the following view of the small-scale dynamics of coronal loops. First of all, photospheric granular motions quasi-statically twist the magnetic field of the corona in a random-walk fashion. In topologically closed structures, the perpendicular magnetic energy increases, causing magnetic shear to build up at the quasi-separatrices of the resulting close-packed magnetic flux tubes. At some point, the boundary driving causes this stressed configuration to cross the threshold of an ideal time-scale MHD instability (possibly magnetic coalescence or resistive tearing) or a point of nonequilibrium and the field lines pinch toward a small-scale sheared configuration. It then becomes energetically favorable for dynamic reconnection to occur, producing narrow current sheets and an Ohmic heating rate sufficient to balance the input Poynting flux.     

Signatures of magnetic reconnection in solar radio observations?

Magnetic reconnection occurs during eruptive processes (flares, CMEs) in the solar corona. This leads to a change of magnetic connectivity. Nonthermal electrons propagate along the coronal magnetic field thereby exciting dm- and m-wave radio burst emission after acceleration during reconnection or other energy release processes in heights of some Mm to ⩾700 Mm. We summarize the results of some case studies which can be interpreted as radio evidence of magnetic reconnection: under certain conditions, simple spectral structures (pulsation pulses, reverse drift bursts) are formed by simultaneously acting but widely spaced radio sources. Narrowband spikes are emitted as a side-effect during large-scale coronal loop collisions. In dynamic radio spectra, the lower fast mode shock formed in the reconnection outflow appears as type II burst-like but nondrifting emission lane. It has been several times observed at the harmonic mode of the local plasma frequency between 250 and 500 MHz and at heights of ≈200 Mm.     

Where will efficient energy release occur in 3-D magnetic configurations?

The energy needed to power flares is thought to be stored in the coronal magnetic field. However, the energy release is efficient only at very small scales. Magnetic configurations with a complex topology, i.e. with separatrices, are the most obvious configurations where current sheets can form, and then, reconnection can efficiently occur. This has been confirmed for several flares computing the coronal field and comparing the locations of the flare loops and ribbons to the deduced 3-D magnetic topology. However, this view is too restrictive taking into account the variety of observed solar flaring configurations. Indeed, “Quasi-Separatrix Layers” (QSLs), which are regions where there is a drastic change in field-line linkage, generalize the definition of separatrices. They let us understand where reconnection occurs in a broader variety of flares than separatrices do. The strongest electric field and current are generated at, or close to where the QSLs are thinnest. This defines the region where particle acceleration can efficiently occur. A new feature of 3-D reconnection is the natural presence of fast field-line slippage along the QSLs, a process called “slip-running reconnection”. This is a plausible origin for the motions of the X-ray sources along flare ribbons.     

The magnetic field topology associated with two M flares

On 27 October, 2003, two GOES M-class flares occurred in an interval of 3 h in active region NOAA 10486. The two flares were confined and their associated brightenings appeared at the same location, displaying a very similar shape both at the chromospheric and coronal levels. We focus on the analysis of magnetic field (SOHO/MDI), chromospheric (HASTA, Kanzelhöhe Solar Observatory, TRACE) and coronal (TRACE) observations. By combining our data analysis with a model of the coronal magnetic field, we compute the magnetic field topology associated with the two M flares. We find that both events can be explained in terms of a localized magnetic reconnection process occurring at a coronal magnetic null point. This null point is also present at the same location one day later, on 28 October, 2003. Magnetic energy release at this null point was proposed as the origin of a localized event that occurred independently with a large X17 flare on 28 October, 2003 [Mandrini, C.H., Démoulin, P., Schmieder, B., Deluca, E., Pariat, E., Uddin, W. Companion event and precursor of the X17 flare on 28 October, 2003. Solar Physics, 238, 293–312, 2006], at 11:01 UT. The three events, those on 27 October and the one on 28 October, are homologous. Our results show that coronal null points can be stable topological structures where energy release via magnetic reconnection can happen, as proposed by classical magnetic reconnection models.     

Magnetic reconnection in solar coronal loops

Simulations of the evolution of kink modes in line-tied coronal loops are presented which demonstrate the occurrence of magnetic reconnection in the non-linear stage of the instability. In loops which do not carry a net axial current (and are confined by a potential purely axial field) the reconnection is limited to the initial current-carrying channel and no overall loss of confinement is observed. In loops which carry a net current on the other hand, reconnection progressively involves field lines at greater and greater distances from the axis and even regions where the field was initially potential, leading to a total disruption of the magnetic field topology.     

Double null points and magnetic reconnection

2D reconnection is possible only in connection with the existence of a singularity in the magnetic field line topology, associated with a magnetic null point or a current sheet. Both of these provide an X-type structure of the magnetic field where fields of opposite polarity meet and reconnect. In 3D a similar topology is found in a null point pair, when the null points are connected by a separator line. The separator is defined as the intersection line of the two null-point fan planes. This paper reports on the topological evolution of this configuration with respect to different perturbations emerging from imposed boundary velocities, using a nonlinear numerical approach.     

磁层顶通量传输事件的经验重构

在地球磁层顶附近观测到的通量传输事件（Flux Transfer Event,FTE）一般被认为是瞬态局域磁重联的产物,是太阳风质量、动量和能量进入地球内磁层的重要通道.重构FTE的磁场结构可促进对其形成、演化过程及其与周围等离子体环境相互作用的理解.Grad-Shafranov重构法和磁通量绳拟合法等传统磁场重构方法适用于满足特定物理条件的磁场结构.基于平面线性插值原理,设计了一种不受具体物理条件限定的二维FTE磁场结构重构法.模型测试以及对THEMIS和Cluster卫星簇分别观测到的两个FTE的实际应用表明,在合适的多卫星位形条件下,该方法能快速有效重构出FTE的磁场空间分布,有助于推测FTE的磁场线位形,理解卫星测量数据的时间变化,以及分析等离子体物理量相对于FTE的磁场空间分布特征.      

Some statistical properties of transequatorial loops

Yohkoh soft X-ray telescope brought plenty of high quality images, it provides a good chance to research coronal loops, especially for transequatorial loops (TLs). In this paper, we focus on the statistical results of TLs including static properties and dynamic properties.

There are two types of classification about TLs: according to configuration and according to magnetic polarities of footpoints, respectively. The footpoints of TLs never root in sunspot, in a general way, they exist in moderately strong field. The mean separation value of TLs is close to 30° and the separation value varies with solar cycle. The helicity patterns of active regions connected by TLs are discussed, the mean twist value of TLs is close to zero. The formation of TLs is generally thought to be caused by magnetic reconnection, the relationship of TLs eruption with flare and CME is introduced.     

Two energy release processes for CMEs: MHD catastrophe and magnetic reconnection

It remains an open question how magnetic energy is rapidly released in the solar corona so as to create solar explosions such as solar flares and coronal mass ejections (CMEs). Recent studies have confirmed that a system consisting of a flux rope embedded in a background field exhibits a catastrophic behavior, and the energy threshold at the catastrophic point may exceed the associated open field energy. The accumulated free energy in the corona is abruptly released when the catastrophe takes place, and it probably serves as the main means of energy release for CMEs at least in the initial phase. Such a release proceeds via an ideal MHD process in contrast with nonideal ones such as magnetic reconnection. The catastrophe results in a sudden formation of electric current sheets, which naturally provide proper sites for fast magnetic reconnection. The reconnection may be identified with a solar flare associated with the CME on one hand, and produces a further acceleration of the CME on the other. On this basis, several preliminary suggestions are made for future observational investigations, especially with the proposed Kuafa satellites, on the roles of the MHD catastrophe and magnetic reconnection in the magnetic energy release associated with CMEs and flares.     

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。     

Coronal heating by reconnection

The outermost atmosphere of the Sun, called the corona, is some 200 times hotter than the surface of the Sun. The main source of energy for heating the corona is believed to be the magnetic field which dominates the corona. Magnetic reconnection is probably the most important mechanism for releasing magnetic energy and may, therefore, be important for coronal heating or micro-flaring. The best observational examples of reconnection in the corona are thought to be X-ray bright points, which are small-scale brightenings seen randomly throughout the whole corona. Theoretical models can not only explain the key observations relating to bright points, but they can also explain the complex three-dimensional structures often seen in bright points. In these models magnetic neutral points play a significant role as the centres for reconnection both in two and three dimensions.     

Multi-wavelength study of a coronal mass ejection: a flare event from AR#9393

The active region, AR#9393, produced a number of intense flares during March–April 2001. In this paper, we report the analysis of an X1.1 flare event of April 2, 2001 and its associated coronal mass ejection. The timing and location of the H_α eruption, radio burst activities, and the onset of mass ejection suggest an energy release that occurred close to the surface of the sun. At this region, as shown by the magnetogram, X-ray and EUV images, the field configuration was complex and the 3-D extrapolation revealed the presence of a magnetic null point. Results also suggest that the energy release is followed by the magnetic reconnection between the low-lying loops near the separator point and outlying loops. This study provides the support for the magnetic break-out process to trigger the energy release in eruptive flare event.     

What is the role of magnetic null points in large flares?

We have performed the analysis of the magnetic topology of active region NOAA 10486 before two large flares occurring on October 26 and 28, 2003. The 3D extrapolation of the photospheric magnetic field shows the existence of magnetic null points when using two different methods. We use TRACE 1600 Å and 195 Å brightenings as tracers of the energy release due to magnetic reconnections. We conclude on the three following points:

1. The small events observed before the flares are related to low lying null points. They are long lasting and associated with low energy release. They are not triggering the large flares.

2. On October 26, a high altitude null point is found. We look for bright patches that could correspond to the signatures of coronal reconnection at the null point in TRACE 1600 Å images. However, such bright patches are not observed before the main flare, they are only observed after it.

3. On October 28, four ribbons are observed in TRACE images before the X17 flare. We interpret them as due to a magnetic breakout reconnection in a quadrupolar configuration. There is no magnetic null point related to these four ribbons, and this reconnection rather occurs at quasi-separatrix layers (QSLs).

We conclude that the existence of a null point in the corona is neither a sufficient nor a necessary condition to give rise to large flares.     

日冕冲浪形成的磁流体动力学模拟

应用二维时变可压缩磁流体动力学模拟,数值研究了双极-单极磁场中电阻撕裂模不稳定性引起的磁场重联过程,用于模拟日冕冲浪的形成.结果表明,在包含有三区——双极场、电流片和单极场的磁静力平衡初态下,双极场和单极场中的磁力线将会直接重联,磁场演变成鞭状(whip)结构.由弯曲磁力线支撑的等离子体团向上运动到最高位置后,逐渐下落和弥散.等离子体团上升速度可达到0.10v_A(v_A为双极场中的Alfv'én速度).模拟结果证实日冕冲浪的形成可能与双极-单极场中的磁场重联密切相关.