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1.
We consider two force-free current loops, as proposed by Gold and Hoyle (1960), as the initial current loops, to investigate two types of the magnetic reconnection process, the partial and complete reconnections during coalescence of these loops, by using a 3-D resistive MHD code. It is shown that two plasmoids can be produced on both sides of the coalescence area by both types of the magnetic reconnection process during coalescence of two current loops. It is also shown that strong fast magnetosonic waves can be induced in the partial reconnection case of two-current-loop collision. When two current loops collide locally at two points, four plasmoids can be produced and two of these plasmoids merge into one. 相似文献
2.
Jun-Ichi Sakai 《Solar physics》1996,169(2):367-376
It is shown by using a 3-D resistive MHD simulation code, taking into account the recombination effect, that magnetic reconnection during collision of two current loops can be enhanced by recombination. It is also shown that the temperature in the thin current sheet formed between two loops increases from few to about thirty times larger than a case of no recombination, depending on both the plasma beta and the strength of recombination. The simulation results obtained here may be applicable for a mechanism of chromospheric heating and as an explanation of X-ray bright points as well as solar flares observed in the chromosphere.Dedicated to Cornelis de Jager 相似文献
3.
We present simulations of the non-linear evolution of the m=1 kink mode in line-tied coronal loops. We focus on the structure of the current concentrations which develop as a consequence of the instability in two different types of magnetic field configuration, one containing a net axial current and the other with a vanishing total axial current. In the first case, current sheets develop one third of the way from footpoint to loop apex (where the non-linear kink mode folds on itself) within the body of the current channel, while in the second case the current sheet develops at the loop apex at the interface between the current containing channel and the outer axial potential field. In both cases line-tying, while playing a stabilizing role in the linear theory, acts as a destabilizing agent for the non-linear resistive evolution. The unwrapping of magnetic field lines in the vanishing axial current model appears to be consistent with the geometry of compact recurrent loop flares. 相似文献
4.
Loop models of solar flares: Revisions and comparisons 总被引：1，自引：0，他引：1
D. S. Spicer 《Solar physics》1981,70(1):149-172
Due to developments in solar flare observations which appear to show that a particular class of solar flares result from instabilities occurring in magnetic loops we re-examine the Alfvén-Carlqvist flare model to show that it is workable and we update the Spicer loop model of a flare. It is noted that the Alfvén-Carlqvist model of necessity requires an external current driver which must maintain the current driven instability at marginal stability during the duration of the flare. In addition, it is argued that if the Alfvén-Carlqvist model is to work the current density must rise in a time shorter than an MHD or resistive tearing mode time scale. Otherwise, the dominant flare mechanism must be an ideal MHD or tearing type instability. Further, the distinctions between the two models are highlighted and a new hybrid model of the Alfvén-Carlqvist and Spicer models is introduced. 相似文献
5.
本文收集了１９８６年２月４日大耀斑的Ｈα、微波、Ｘ射线和γ射线全波段的观测资料。利用暗条电流环模型分析了该耀斑的物理过程，测量了活动暗条的上升运动，求解了动量方程和能量方程。结果表明：（１）１９８６年２月４日的３Ｂ／Ｘ３耀斑可能是由暗条电流环之间的合并不稳定性所致；（２）电阻撕裂摸不稳定性是一种有效的耀斑前预热机制；（３）耀斑的高能观测资料进一步表明了电流环合并不稳定性是引起该大耀斑期间所有高能粒 相似文献
6.
Using a two-dimensional, dissipative magnetohydrodynamic model, this paper presents a numerical simulation of the magnetic energy buildup in a quadrupolar field by photospheric shear motion. When electric current density is larger than a certain critical value, an anomalous resistivity is introduced in order to account for the dissipation caused by instabilities in high current regions. It is shown that like a bipolar field, a quadrupolar field can efficiently store magnetic free energy through photospheric shear motion. Electric current formed by shear concentrates on the separatrix and magnetic loops rooted in areas where the shear velocity gradient is large. The atmosphere is heated by anomalous resistive dissipation during the shear. Both magnetic and thermal energy increases nonlinearly with shearing displacement. When the anomalous resistivity increases or the critical current density decreases, the growth rate reduces for magnetic energy but goes up for thermal energy. 相似文献
7.
The nonlinear evolution of a reconnecting magnetic field configuration similar to that occurring just before the onset of ‘post’-flare loops in two-ribbon flares is determined. The evolution, which is obtained by numerically solving the resistive MHD equations, shows two new features that have not yet been incorporated into contemporary models of ‘post’-flare loops. The first of these new features is the formation of a nearly stationary fast-mode shock above the region corresponding to the top of the loops. This fast-mode shock occurs just below the magnetic neutral line and between the slow-mode shocks associated with fast magnetic reconnection at the neutral line. The second new feature is the creation and annihilation of large-scale magnetic islands in the current sheet above the loops. The annihilation of the islands occurs very rapidly and appears to be a manifestation of the coalescence instability. The creation and annihilation of magnetic islands could be important in understanding the energetics of ‘post’-flare loops since the coalescence instability can produce an intermittent energy release more than an order of magnitude faster than that predicted by steady-state reconnection theories. 相似文献
8.
To interpret the present-day satellite observations of the sequential brightening of coronal loops in solar flares, we have solved the problem of the stability of small longitudinal perturbations of a homogeneous reconnecting current layer (CL). Within the magnetohydrodynamic approximation we show that an efficient suppression of plasma heat conduction by amagnetic field perturbation inside the CL serves as an instability condition. The instability in the linear phase grows in the characteristic radiative plasma cooling time. A periodic structure of cold and hot filaments located across the direction of the electric current can be formed as a result of the instability in the CL. The proposed mechanism of the thermal instability of a reconnecting CL can be useful for explaining the sequential brightening (“ignition”) of flare loops in solar flares. 相似文献
9.
We present a model of solar flares triggered by collisions between current loops and plasmoids. We investigate a collision process between a force-free current loop and a plasmoid, by using 3-D resistive MHD code. It is shown that a current system can be induced in the front of a plasmoid, when it approaches a force-free current loop. This secondary current produced in the front of the plasmoid separates from the plasmoid and coalesces to the force-free current loop associated with the magnetic reconnection. The core of the plasmoid stays outside the reconnection region, maintaining high density. The core can be confined by the current system produced around the plasmoid. This collison process between a current loop and a plasmoid may explain the triggering of solar flares observed byYohkoh. 相似文献
10.
Numerical simulation of magnetic field reconnection at IMF sector boundaries shows that the reconnection line may be carried by the solar wind out of the region of the anomalous resistivity. This makes it possible to observe magnetic loops at the Earth's orbit open to the Sun as well as from it. Besides, it is shown that the current sheet in the vicinity of the reconnection line has to split into two currents.Experimental data on the structure of the sector boundaries are analyzed, and it is shown that the currents at sector boundaries are indeed often splitted.The thickness of the splitted boundaries may amount to 18×106 km; taking into account this value, the heliocentric distance of the region of anomalous resistivity in the interplanetary current sheet is estimated as 0.4–0.5 AU.The probability of observing magnetic loops open towards the Sun seems to be greater than that of loops open from the Sun, which suggests an essential asymmetry of the field reversal regions. 相似文献
11.
本文在细长柱位形下数值研究了具有剪切流动的电流片中电阻撕裂模不稳定性的非线性演化。结果表明，电流片附近Ｓｅｃｈ形式的剪切流动，将导致电阻撕裂模不稳定性的发展，且不稳定性增长率随着剪切参数Ｒｒ的增加而增长，导致磁岛的形成和快速的磁能释放。这种剪切流动和撕裂模的耦合过程以及超热不稳定性的相互作用，改变了磁拱中的磁场剪切强度或者说电流密度梯度，从而驱动电阻撕裂模不稳定性的发展，这种过程对于等离子体电流密 相似文献
12.
Lutz Janicke 《Solar physics》1982,76(1):29-43
The apparent stability of coronal neutral sheets with respect to the resistive tearing mode has been attributed by previous authors to the influence of a weak normal component of the confining magnetic field. To check this hypothesis a normal mode analysis is performed applying rigorously singular perturbation technique. Allowance is made for a value of the normal component which is large measured in the appropriate units deduced from the dynamics of the one-dimensional tearing mode. The structure of the eigenmodes is completely changed: the singular layer decays into a broad band of filaments with antiparallel flow directions and spatial oscillations in the perturbed current density appear. Surprisingly, the growth rate is not changed. If parameters for a typical neutral sheet in the middle corona (0.5 solar radii) are inserted, the result is that no stabilization by a normal component occurs, if the value of the growth time predicted by the one-dimensional theory is far shorter than ten minutes - independent of the values assumed for the width of the neutral sheet or the resistivity. 相似文献
13.
14.
In this article, we investigate the possibility of transient growth in the linear perturbation of current sheets. The resistive magnetohydrodynamics operator for a background field consisting of a current sheet is non-normal, meaning that associated eigenvalues and eigenmodes can be very sensitive to perturbation. In a linear stability analysis of a tearing current sheet, we show that modes that are damped as \(t\rightarrow \infty \) can produce transient energy growth, contributing faster growth rates and higher energy attainment (within a fixed finite time) than the unstable tearing mode found from normal-mode analysis. We determine the transient growth for tearing-stable and tearing-unstable regimes and discuss the consequences of our results for processes in the solar atmosphere, such as flares and coronal heating. Our results have significant potential impact on how fast current sheets can be disrupted. In particular, transient energy growth due to (asymptotically) damped modes may lead to accelerated current sheet thinning and, hence, a faster onset of the plasmoid instability, compared to the rate determined by the tearing mode alone. 相似文献
15.
A longitudinal stability is considered for the quasi-steady current sheet which is uniform along the current. In the MHD approximation, the stability problem is solved for the plane neutral sheet and small disturbances propagating along the current. The current sheet is shown to break-up into the system of cooler and more dense filaments due to radiative cooling. The filaments are parallel to magnetic field lines. This process corresponds to the condensation mode of a thermal instability and can play a trigger role for a solar flare. Moreover, at the nonlinear stage of development, it can lead to the formation of very dense cold filaments surrounded by high-temperature low-density plasma inside the current sheet. Flowing into the filaments, hot plasma is cooled by radiation and compressed. Then the cold dense plasma flows out from the current sheet along the filaments. We think that the process under consideration is responsible for the often observed picture of an arcade of cold loops in the solar corona.The text of this paper was written by B. V. Somov after the death of Prof. S. I. Syrovatskii. 相似文献
16.
The study of resistive ballooning instabilities in line-tied coronal magnetic fields is extended by including viscosity in the stability analysis. The equations that govern the resistive ballooning instabilities are derived and the effects of parallel and perpendicular viscosity are included using Braginskii's stress tensor. Numerical solutions to these equations are obtained under the rigid wall boundary conditions for arcades with cylindrically-symmetric magnetic fields. It is found that viscosity has a stabilizing effect on the resistive ballooning instabilities with perpendicular viscosity being more important by far than parallel viscosity. The strong stabilizing effect of perpendicular viscosity can lead to complete stabilization for realistic values of the equilibrium quantities.Research Assistant at the Belgian Fund for Scientific Research. 相似文献
17.
A. Ciardi S. V. Lebedev J. P. Chittenden D. J. Ampleford S. N. Bland B. S. Bott J. Rapley 《Astrophysics and Space Science》2005,298(1-2):277-286
The twisting of magnetic fields threading an accretion system can lead to the generation on axis of toroidal field loops.
As the magnetic pressure increases, the toroidal field inflates, producing a flow. Collimation is due to a background corona,
which radially confines this axially growing “magnetic tower”. We investigate the possibility of studying in the laboratory
the dynamics, confinement and stability of magnetic tower jets. We present two-dimensional resistive magnetohydrodynamic simulations
of radial arrays, which consist of two concentric electrodes connected radially by thin metallic wires. In the laboratory,
a radial wire array is driven by a 1 MA current which produces a hot, low density background plasma. During the current discharge
a low plasma beta (β < 1), magnetic cavity develops in the background plasma (β is the ratio of thermal to magnetic pressure). This laboratory magnetic tower is driven by the magnetic pressure of the toroidal
field and it is surrounded by a shock envelope. On axis, a high density column is produced by the pinch effect. The background
plasma has >rsim1, and in the radial direction the magnetic tower is confined mostly by the thermal pressure. In contrast, in the axial direction
the pressure rapidly decays and an elongated, well collimated magnetic-jet develops. This is later disrupted by the development
of m = 0 instabilities arising in the axial column. 相似文献
18.
19.
A localized force-free current is proposed as a model for the observed coronal loops.An upper limit for the growth rate of kink instabilities in this model is found by solving numerically, in cylinder symmetry, the MHD equation of motion, with the boundary condition = 0 outside the loop.For various current densities a spectrum of kinks is found. These instabilities will disrupt the loops that are long or strongly twisted, on a time scale of a few seconds.The kinks in short or barely twisted loops are too long to fit. 相似文献
20.
A. W. Hood 《Solar physics》1986,103(2):329-345
A method for analysing the stability of cylindrical loops and arcades to ballooning modes is presented. Using a WKB method, simple tests for instability (or stability to localised modes) are obtained for the two commonly used, line tying, conditions. In addition, when instabilities are present, the physical growth rate and the radial structure of the least stable can be computed from the solution of a 4th order system of ordinary differential equations. The detailed analysis for a particular field is presented in an Appendix. 相似文献