共查询到20条相似文献，搜索用时 31 毫秒
1.
Numerical simulations of the helical (m=1) kink instability of an arched, line-tied flux rope demonstrate that the helical deformation enforces reconnection between
the legs of the rope if modes with two helical turns are dominant as a result of high initial twist in the range Φ≳6π. Such a reconnection is complex, involving also the ambient field. In addition to breaking up the original rope, it can form
a new, low-lying, less twisted flux rope. The new flux rope is pushed downward by the reconnection outflow, which typically
forces it to break as well by reconnecting with the ambient field. The top part of the original rope, largely rooted in the
sources of the ambient flux after the break-up, can fully erupt or be halted at low heights, producing a “failed eruption.”
The helical current sheet associated with the instability is squeezed between the approaching legs, temporarily forming a
double current sheet. The leg – leg reconnection proceeds at a high rate, producing sufficiently strong electric fields that
it would be able to accelerate particles. It may also form plasmoids, or plasmoid-like structures, which trap energetic particles
and propagate out of the reconnection region up to the top of the erupting flux rope along the helical current sheet. The
kinking of a highly twisted flux rope involving leg – leg reconnection can explain key features of an eruptive but partially
occulted solar flare on 18 April 2001, which ejected a relatively compact hard X-ray and microwave source and was associated
with a fast coronal mass ejection. 相似文献
2.
A local current sheet and a subsequent small interplanetary magnetic-flux rope were observed on 1 April 2003 by Wind and the Advanced Composition Explorer (ACE). A Petschek reconnection-like exhaust crossing of the local current sheet was identified using the Walén test. The Wind spacecraft re-entered the reconnection exhaust after the main exhaust encounter, and the reentry may be due to a spatial fold of the current-sheet surface itself. The absence of parallel strahls and the presence of antiparallel strahls on either side of the current sheet suggest that the magnetic-field lines before the exhaust and in the subsequent small flux rope are all open. The \(180^{\circ}\) pitch-angle strahls were clearly absent, and halo-suprathermal electron pitch-angle distributions were observed in the exhaust. This finding means that the open field lines of the magnetic-flux rope were reconnecting to the adjacent open field lines to produce U-shaped field lines disconnected from the Sun. These observations provide direct evidence that the magnetic fields of the interplanetary small magnetic-flux rope were disconnecting from the Sun through magnetic reconnection. This type of disconnected event potentially has important implications for the magnetic-flux budget of the heliosphere. 相似文献
3.
M. J. Owens 《Solar physics》2009,260(1):207-217
Magnetic clouds are a class of interplanetary coronal mass ejections (CME) predominantly characterised by a smooth rotation
in the magnetic field direction, indicative of a magnetic flux rope structure. Many magnetic clouds, however, also contain
sharp discontinuities within the smoothly varying magnetic field, suggestive of narrow current sheets. In this study we present
observations and modelling of magnetic clouds with strong current sheet signatures close to the centre of the apparent flux
rope structure. Using an analytical magnetic flux rope model, we demonstrate how such current sheets can form as a result
of a cloud’s kinematic propagation from the Sun to the Earth, without any external forces or influences. This model is shown
to match observations of four particular magnetic clouds remarkably well. The model predicts that current sheet intensity
increases for increasing CME angular extent and decreasing CME radial expansion speed. Assuming such current sheets facilitate
magnetic reconnection, the process of current sheet formation could ultimately lead a single flux rope becoming fragmented
into multiple flux ropes. This change in topology has consequences for magnetic clouds as barriers to energetic particle propagation. 相似文献
4.
A major solar active event called Bastille Day Event occurred in AR 9077 on July 14, 2000. Simultaneous occurrence of a filament
eruption, a flare and a coronal mass ejection was observed in this event. Previous analyses of this event show that before
the event, there existed an activation and eruption of a huge trans-equatorial filament, which might play a crucial role in
triggering the Bastille Day event. This implies that independent flux systems are closely related to and affect each other,
which has encouraged us to investigate the catastrophic behavior of a multiple coronal flux rope system with the use of a
2.5-D time-dependent MHD model. A force-free field that contains three separate coronal flux ropes is taken to be the initial
state. Starting from this state, we increase either the annular or the axial flux of a certain flux rope to examine the catastrophic
behavior of the system in two regimes, the ideal MHD regime and the resistive MHD regime. It is found that a catastrophe occurs
if the flux exceeds a certain critical value, or the magnetic energy of the system exceeds a certain threshold: the rope of
interest breaks away from the base and escapes to infinity, leaving a current sheet below. Moreover, the destiny of the remainder
flux ropes relies on whether reconnection takes place across the current sheet. In the ideal MHD regime, i.e., in the absence of reconnection, these ropes remain to be attached to the base in equilibrium, whereas in the resistive MHD
regime they abruptly erupt upward during reconnection and escape to infinity. Reconnection causes the field lines to close
back to the base and thus changes the background field outside the attached flux ropes in such a way that the constraint on
these ropes is substantially relaxed and the corresponding catastrophic energy threshold is reduced accordingly, leading to
a catastrophic eruption of these ropes. Since magnetic reconnection is generally inevitable when a current sheet forms and
develops through an eruption of one flux rope, the eruption of this flux rope must lead to an eruption of the others. This
provides an example to demonstrate the interaction between several independent magnetic flux systems in different regions,
as implied by the Bastille Day event, and may serve as a possible mechanism for sympathetic events occurring on the Sun. 相似文献
5.
Govind Dubey Bart van der Holst Stefaan Poedts 《Journal of Astrophysics and Astronomy》2006,27(2-3):159-166
The initiation of solar Coronal Mass Ejections (CMEs) is studied in the framework of numerical magnetohydrodynamics (MHD).
The initial CME model includes a magnetic flux rope in spherical, axi-symmetric geometry. The initial configuration consists
of a magnetic flux rope embedded in a gravitationally stratified solar atmosphere with a background dipole magnetic field.
The flux rope is in equilibrium due to an image current below the photosphere. An emerging flux triggering mechanism is used
to make this equilibrium system unstable. When the magnetic flux emerges within the filament below the flux rope, this results
in a catastrophic behavior similar to previous models. As a result, the flux rope rises and a current sheet forms below it.
It is shown that the magnetic reconnection in the current sheet below the flux rope in combination with the outward curvature
forces results in a fast ejection of the flux rope as observed for solar CMEs. We have done a parametric study of the emerging
flux rate. 相似文献
6.
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. 相似文献
7.
We determine the photospheric boundary conditions which maximize the magnetic energy released by a loss of ideal-MHD equilibrium
in two-dimensional flux-rope models. In these models a loss of equilibrium causes a transition of the flux rope to a lower
magnetic energy state at a higher altitude. During the transition a vertical current sheet forms below the flux rope, and
reconnection in this current sheet releases additional energy. Here we compute how much energy is released by the loss of
equilibrium relative to the total energy release. When the flux-rope radius is small compared to its height, it is possible
to obtain general solutions of the Grad-Shafranov equation for a wide range of boundary conditions. Variational principles
can then be used to find the particular boundary condition which maximizes the magnetic energy released for a given class
of conditions. We apply this procedure to a class of models known as cusp-type catastrophes, and we find that the maximum
energy released by the loss of equilibrium is 20.8% of the total energy release for any model in this class. If the additional
restriction is imposed that the photospheric magnetic field forms a simple arcade in the absence of coronal currents, then
the maximum energy release reduces to 8.6%. 相似文献
8.
We describe the evolution of morphological features of the magnetic configuration of CME according to the catastrophe model developed previously. For the parameters chosen for the present work, roughly half of the total mass is nominally contained in the initial flux rope, while the remaining plasma is brought by magnetic reconnection from the corona into the current sheet and from there into the CME bubble. The physical attributes of the difference in the observable features between CME bubble and flare loop system were studied. We tentatively identified distinguishable evolutionary features like the outer shell, the expanding bubble and the flux rope with the leading edge, void and core of the 3-component CME structure. The role of magnetic reconnection is discussed as a possible mechanism for the heating of the prominence material during eruptions. Several aspects of this explanation that need improvement are outlined. 相似文献
9.
本文研究了磁流体力学与高频等离子体波（ 包括纵横模式） 之间的精巧的相互作用。研究表明,这些等离激元会在电流片内诱发一种阻抗不稳定,并最终导至磁重联,出现爆发性不稳定。在高涨的离声湍动情况下,高温电流片模型必须采用反常电导率,而非库仑电导率。理论估算的结果与观测相一致。因此这种计及等离激元有质动力作用的新磁重联理论,基本上能解释耀斑现象。 相似文献
10.
E. R. Priest 《Solar physics》1976,47(1):41-75
Current sheets have been suggested as the site for flare energy release because they can convert magnetic energy very rapidly into both heat and directed plasma energy. Also they contain electric fields with the potential of accelerating particles to high energies.The basic properties of current sheets are first reviewed. For instance, magnetic flux may be carried into a current sheet and annihilated. An exact solution for such a process in an infinitely long sheet has been found; it describes the annihilation of fields which are inclined at any angle, not just 180°. Moreover, field lines which are expelled from the ends of a current sheet can be described as having been reconnected. The only workable model for fast reconnection in the solar atmosphere, namely Petschek's mechanism, has recently been put on a firm foundation; it gives a reconnection rate which depends on the electrical conductivity but is typically a tenth or a hundredth of the Alfvén speed. A current sheet may be formed when the sources of an initially potential field start to move; a simple analytic technique for finding the position and shape of such a sheet in two dimensions now exists. Finally, a sheet with no transverse magnetic field component is subject to the tearing-mode instability, which rapidly produces a series of loops in the field.The main ways in which current sheets have been used for solar flare models is described. Syrovatskii's mechanism relies on the increase of the electric current density during the formation of a sheet, to a value in excess of the critical value j
* for the onset of microinstabilities. But Anzer has recently demonstrated that the critical value is most unlikely to be reached during the initial formation process. Sturrock, on the other hand, has advocated the occurrence of the tearing-mode instability in an open streamer-like configuration (which may result from the eruption of a force-free field). But recent observations do not point to that as the relevant configuration. Rather, they suggest that flares are triggered by the emergence of new magnetic flux from below the solar photosphere. This has led Heyvaerts, Priest, and Rust (1976) to propose a new emerging flux model, according to which, as more and more flux emerges, so reconnection occurs, producing some preflare heating. When the current sheet reaches such a height (around the transition region) that its current density exceeds j
*, then the impulsive phase of the flare is triggered. The main phase is caused by an enhanced level of magnetic energy conversion in a turbulent current sheet. The type of flare depends on the magnetic environment in which the emerging flux finds itself. A surge flare results if the flux appears near a strong unipolar region such as a simple sunspot, whereas a two ribbon flare may be produced by flux emergence near an active region filament, in which case the main phase energy is released from the field that surrounds the filament. 相似文献
11.
The subtle interactions between the magnetohydrodynamics (MHD) and transverse plasmons are investigated. It is shown that there is a resistive instability by the plasmon's soliton in a current sheet, which eventually turns into an eruptive instability at the magnetic field reconnection. In the case of ion-acoustic turbulence, the high temperature current sheet model must adopt the aromalous conductivity instead of the Coulomb conductivity. The numerical results are consistent with the observations obtained by Hanaoka (1994). Thus the flare caused by X-ray loop coalescence can be basically interpreted by this model of magnetic field reconnection driven by ponderomotive force. 相似文献
12.
Magnetic field enters the corona from the interior of the Sun through isolated magnetic features on the solar surface. These features correspond to the tops of submerged magnetic flux tubes, and coronal field lines often connect one flux tube to another, defining a pattern of inter-linkage. Using a model field, in which flux tubes are represented as point magnetic charges, it is possible to quantify this inter-linkage. If the coronal field were current-free then motions of the magnetic features would change the inter-linkage through implicit (vacuum) magnetic reconnection. Without reconnection the conductive corona develops currents to avoid changing the flux linkage. This current forms singular layers (ribbons) flowing along topologically significant field lines called separators. Current ribbons store magnetic energy as internal stress in the field: the amount of energy stored is a function of the flux tube displacement. To explore this process we develop a model called the minimum-current corona (MCC) which approximates the current arising on a separator in response to displacement of photospheric flux. This permits a model of the quasi-static evolution of the corona above a complex active region. We also introduce flaring to rapidly change the flux inter-linkage between magnetic features when the internal stress on a separator becomes too large. This eliminates the separator current and releases the energy stored by it. Implementation of the MCC in two examples reveals repeated flaring during the evolution of simple active regions, releasing anywhere from 1027–1029 ergs, at intervals of hours. Combining the energy and frequency gives a general expression for heat deposition due to flaring (i.e., reconnection). 相似文献
13.
Manuela Temmer Julia K. Thalmann Karin Dissauer Astrid M. Veronig Johannes Tschernitz Jürgen Hinterreiter Luciano Rodriguez 《Solar physics》2017,292(7):93
We analyze the well-observed flare and coronal mass ejection (CME) from 1 October 2011 (SOL2011-10-01T09:18) covering the complete chain of effects – from Sun to Earth – to better understand the dynamic evolution of the CME and its embedded magnetic field. We study in detail the solar surface and atmosphere associated with the flare and CME using the Solar Dynamics Observatory (SDO) and ground-based instruments. We also track the CME signature off-limb with combined extreme ultraviolet (EUV) and white-light data from the Solar Terrestrial Relations Observatory (STEREO). By applying the graduated cylindrical shell (GCS) reconstruction method and total mass to stereoscopic STEREO-SOHO (Solar and Heliospheric Observatory) coronagraph data, we track the temporal and spatial evolution of the CME in the interplanetary space and derive its geometry and 3D mass. We combine the GCS and Lundquist model results to derive the axial flux and helicity of the magnetic cloud (MC) from in situ measurements from Wind. This is compared to nonlinear force-free (NLFF) model results, as well as to the reconnected magnetic flux derived from the flare ribbons (flare reconnection flux) and the magnetic flux encompassed by the associated dimming (dimming flux). We find that magnetic reconnection processes were already ongoing before the start of the impulsive flare phase, adding magnetic flux to the flux rope before its final eruption. The dimming flux increases by more than 25% after the end of the flare, indicating that magnetic flux is still added to the flux rope after eruption. Hence, the derived flare reconnection flux is most probably a lower limit for estimating the magnetic flux within the flux rope. We find that the magnetic helicity and axial magnetic flux are lower in the interplanetary space by ～?50% and 75%, respectively, possibly indicating an erosion process. A CME mass increase of 10% is observed over a range of \({\sim}\,4\,\mbox{--}\,20~\mathrm{R}_{\odot }\). The temporal evolution of the CME-associated core-dimming regions supports the scenario that fast outflows might supply additional mass to the rear part of the CME. 相似文献
14.
磁云因其独特的磁场结构经常是重大灾害性空间天气的驱动源. 近来从磁云的边界层结构、环向通量、大尺度结构等方面关于磁云传播的动力学演化过程的研究取得了一些进展. 在磁云边界存在一个由于磁场重联而形成的边界层结构. 在磁云传播过程中, 这种发生在边界处的磁场重联可能会把磁云的磁场剥蚀掉, 进而引起其磁通量绳结构环向通量的减少以及不对称. 在磁云内部, 经常会观测到多个子通量绳结构. 这些特性各异的子通量绳可以通过磁场重联而合并, 进而引起磁云磁结构的改变. 关于磁云大尺度磁场拓扑位形的演化机制, 除了较早提出的交换重联外, 目前的研究表明在行星际空间中, 磁云边界处的重联过程也可以将磁云闭合或半开放的磁场线打开或断开. 尽管在相关研究中已经取得了较大进展, 但关于磁云传播的动力学演化过程还有许多问题尚不清楚. 在行星际小尺度磁通量绳边界也发现了边界层结构, 那么磁云是否会因剥蚀而成为小尺度通量绳? 磁云内子通量绳结构在相互作用中会不会引起某些不稳定性而导致整个通量绳系统的崩溃? 这些问题的解决还有待于进一步的理论、观测和数值模拟研究. 相似文献
15.
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. 相似文献
16.
A model of ‘disparitions brusques’ (sudden disappearence of eruptive prominences) is discussed based on the Kippenhahn ans Schlüter configuration. It is shown that Kippenhahn and Schlüter's current sheet is very weakly unstable against magnetic reconnecting modes during the lifetime of quiescent prominences. Disturbances in the form of fast magnetosonic waves originating from nearby active regions or the changes of whole magnetic configuration due to newly emerged magnetic flux may trigger a rapid growing instability associated with magnetic field reconnection. This instability gives rise to disruptions of quiescent prominences and also generates high energy particles. 相似文献
17.
Flux pile-up magnetic merging solutions are discussed using the simple robust arguments of traditional steady-state reconnection theory. These arguments determine a unique scaling for the field strength and thickness of the current layer, namely B
s–1/3, l2/3, which are consistent with a variety of plasma inflow conditions. Next we demonstrate that flux pile-up merging can also be understood in terms of exact magnetic annihilation solutions. Although simple annihilation models cannot provide unique reconnection scalings, we show that the previous current sheet scalings derive from an optimized solution in which the peak dynamic and magnetic pressures balance in the reconnection region. The build-up of magnetic field in the current sheet implicit in flux pile-up solutions naturally leads to the idea of saturation. Hydromagnetic pressure effects limit the magnetic field in the sheet, yielding an upper limit on the reconnection rate for such solutions. This rate is still far superior to the Sweet–Parker merging rate, which can be derived by seeking solutions that avoid all forms of saturation. Finally we compare time dependent numerical simulations of the coalescence instability with the optimized flux pile-up models. This comparison suggests that merging driven by the relatively slow approach of large flux systems may be favored in practice. 相似文献
18.
Using a 2.5-D, time-dependent ideal MHD model in Cartesian coordinates, we carried out numerical simulations to investigate
the equilibrium and evolution properties of a magnetic configuration that consists of a coronal magnetic flux rope and a partly
open photospheric background field, which is equivalent to that produced by a two-patch magnetic source on the photospheric
surface. The axial and annular magnetic fluxes of the flux rope are given and fixed. The global magnetic configuration evolves
in response to three types of changes of the background field: decreasing of the distance between the two sources, shrinking
of the size of each source, and increasing of the shear in the closed component of the background field. As a result, the
geometrical parameters of the flux rope, i.e. the height of the rope axis, the half-width of the rope and the length of the
vertical current sheet below the rope, change due to the variation of the background field. It is shown that for a given coronal
magnetic flux rope in a partly open background field, the variation of the geometrical parameters of the flux rope displays
a catastrophic behavior, namely, there exists a critical point for each case, at which an infinitesimal change of the background
field leads to a loss of equilibrium, and thus a jump of the flux rope. The implication of such a catastrophe in solar active
phenomena is briefly discussed. 相似文献
19.
The kink instability may be responsible for compact loop flares since the instability is triggered once the twist in a coronal loop exceeds a critical value. During the non-linear evolution of the instability a large current builds up, reconnection can occur and the magnetic energy released due to reconnection may explain the rapid heating of the flare. However, there has been some debate over the nature of the current concentration and, in particular, whether the current saturates or whether it is a current sheet, and what influences these possible states. In this paper we consider two similar equilibria having a twist function which rises to a peak and then falls off. One is steeper than the other allowing us to investigate whether the steepness of the peak has any effect on the nature of the current. For each profile, we run the code on five different grid resolutions and see how the maximum of the current scales with grid resolution. We also look for behavior in the x-component of the velocity which might be similar to the step-function behavior associated with singularities in the linear kink instability. For both profiles we find that the current scales almost linearly with resolution and that v
x drops steeply at the position of the current concentration. This suggests that, for these particular profiles, there are indications of current sheet formation and that the steepness in the peak of the twist does not affect the nature of the current. 相似文献
20.
This work is devoted to study the magnetic reconnection instability under solar spicule conditions. Numerical study of the resistive tearing instability in a current sheet is presented by considering the magnetohydrodynamic (MHD) framework. To investigate the effect of this instability in a stratified atmosphere of solar spicules, we solve linear and non-ideal MHD equations in the x?z plane. In the linear analysis it is assumed that resistivity is only important within the current sheet, and the exponential growth of energies takes place faster as plasma resistivity increases. We are interested to see the occurrence of magnetic reconnection during the lifetime of a typical solar spicule. 相似文献