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1.
邓晓华  王水 《天文学报》1994,35(2):165-175
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   

2.
Filaments and flares are prominent indicators of the magnetic fields of solar activity. These instability phenomena arise from the influence of weak transport effects (radiation and resistivity, respectively) on coronal magnetodynamics and energy flow. We have previously shown that the filament and flare (tearing or reconnection) mechanisms are resistively coupled in sheared magnetic fields of the kind existing in active regions. The present paper expands this treatment to include the effects of compressibility and viscosity, which are most prominent at short wavelengths. The results show that compressibility affects the radiative mode, including a modest increase of its growth rate, and that viscosity modifies the tearing mode, partially through a decrease of its growth rate. A comprehensive discussion of the mode structures and flows is presented. The strongest effect found is a reversal, at very long wavelengths, of the radiative cooling of the resistive interior layer of the tearing mode, caused by compressional heating.  相似文献   

3.
在电流片两侧的基态速度场与磁场相互平行的位形下,讨论了速度切变对撕裂模不稳定性的影响.结果表明,只有当速度场的相对变化率与磁场的相对变化率符号相同(Sign(v_0/v_0)=sign(B_0~')),|v_0~'/v_0|≥|B_0'/B_0|时,速度切变有利于不稳定增长,否则不利于不稳定增长.  相似文献   

4.
We derived the energy balance equation in steady state and made numerical calculations for 37 sets of parameter values for three layers of the atmosphere. The main results are: 1) The energy density released by tearing mode instability rapidly falls with rising temperature. It is much greater in the colmnar pinch than in the thin plate pinch. 2) The critical temperature increases with the intensity of the sheared magnetic field and decreases with its width. It is higher in the columnar pinch, and the rise time is shorter, than in the thin plate pinch. 3) At the surface of the photosphere, the energy released by tearing mode instability raises the local temperature by less than 1 K. 4) When the sheared width of the magnetic field is below a certain threshold, the local temperature is raised sharply to over (+6) K. This threshold is an increasing function of the strength of the sheared field. Our calculated results fit the observations to within one order of magnitude.  相似文献   

5.
Wang  S.  Liu  Y. F.  Zheng  H. N. 《Solar physics》1997,173(2):409-426
Satellite observations of the heliospheric current sheet indicate that the internal structure of sector boundaries is a very complex structure with many directional discontinuities in the magnetic field. This implies that the heliospheric current sheet is not a single surface but a constantly changing layer with a varying number of current sheets. In this paper, we investigate magnetic reconnection caused by the resistive tearing mode instability in non-periodic multiple current sheets by using two-dimensional magnetohydrodynamic simulation. The results show that it is complex unsteady magnetic reconnection. Accompanying the nonlinear development of the tearing mode, the width of each magnetic island in multiple current sheets increases with time, and this leads to new magnetic reconnection. At the same time, the width of each current sheet increases, and the current intensity decreases gradually. Finally, the reverse current disappears, and a big magnetic island is formed in the central region. This process is faster when the separation between the current sheets is smaller. We suggest that the occurrence of multiple directional discontinuities observed at sector boundary crossings in the heliosphere may be associated with the magnetic islands and plasmoids caused by magnetic reconnection in multiple current sheets.  相似文献   

6.
In this paper, the observational data in H, radio, soft X-ray, hard X-ray, and -ray emissions for the 3B/X3.0 solar flare on 4 February, 1986 are collected. This flare is studied in detail by using the flare-filament current model. The momentum equations and the energy equations of the filament current have been solved. The influence of the highly sheared background magnetic field on the motion of the filaments is studied through numerical calculation. The results show that the resistive tearing instability is a possible pre-heating mechanism in the preflare phase, and both the rotation of the spiral sunspots and the highly sheared background field are necessary for the energy storage of this flare. The high-energy data of the flare imply that the current-loop coalescence instability is a possible eruptive mechanism.  相似文献   

7.
The process of magnetic reconnection in anisotropic plasmas is studied numerically using a 2-dimensional, 3-component hybrid simulation. The results of the calculation show that, when the plasma pressure in the direction perpendicular to magnetic field is larger than that in the parallel direction (e.g. P/P = 1.5), instability may greatly increase, speeding up the rate of reconnection. When P is smaller than P, (e.g., when P/P = 0.6), fire hose instability appears, which will restrain the tearing mode instability and the process of magnetic reconnection.  相似文献   

8.
Dispersion relations for the resistive tearing instability are analytically found in the hydromagnetic approximation for a current sheet with a small normal component of the magnetic field. A strong stabilizing influence of the normal component on the development of the tearing instability is shown to exist. These results are also obtained from physical considerations, and so a simple interpretation of the stabilization effect of the normal component is given. The results of the present paper are compared with those of previous works on the topic, and the previous negative results are explained.  相似文献   

9.
The X-ray activity of anomalous X-ray pulsars and soft γ-ray repeaters may result from the heating of their magnetic corona by direct currents dissipated by magnetic reconnection. We investigate the possibility that X-ray flares and bursts observed from anomalous X-ray pulsars and soft γ-ray repeaters result from magnetospheric reconnection events initiated by development of the tearing mode in magnetically dominated relativistic plasma. We formulate equations of resistive force-free electrodynamics, discuss the relation of the latter to ideal electrodynamics, and give examples of both ideal and resistive equilibria. Resistive force-free current layers are unstable towards the development of small-scale current sheets where resistive effects become important. Thin current sheets are found to be unstable due to the development of the resistive force-free tearing mode. The growth rate of the tearing mode is intermediate between the short Alfvén time-scale  τA  and a long resistive time-scale  τR: Γ∼ 1/(τRτA)1/2  , similar to the case of non-relativistic non-force-free plasma. We propose that growth of the tearing mode is related to the typical rise time of flares, ∼10 ms. Finally, we discuss how reconnection may explain other magnetar phenomena and ways to test the model.  相似文献   

10.
Magnetic reconnection induced by Kelvin Helmholtz instability   总被引：1，自引：0，他引：1  
MHD simulation study is performed to investigate magnetic reconnection induced by the Kelvin Helmholtz instability in the initially sheared magnetic field geometry as well as in the uniform magnetic field geometry. Slow mode rarefaction structures seen in the uniform field case are not observed in the sheared field case. Dynamo action is less prominent and the conversion of plasma flow energy into the other forms of energy is also smaller in the sheared field case than in the uniform field case. Momentum transport is mostly due to the hydrodynamic stress in the sheared field case, while the electromagnetic stress is dominant in the uniform field case. The long term evolutions are also markedly different in the two cases. In the uniform field geometry, the magnetic field lines twisted due to the Kelvin Helmholtz instability become reconnected and flattened so that they resume the straight field line structure which resembles the initial field geometry. The magnetic field, however, is not uniform with smaller intensity in the central region where the pressure balance is partially maintained by the enhanced thermal pressure. In the initially sheared magnetic field geometry, magnetic reconnection continues to operate until the end of the simulation and the conversion of the flow energy into the thermal energy is still seen.  相似文献   

11.
Equations governing the coupling of the scalar and vector potentials for a resistive electron-positron plasma in a strong magnetic field are derived. It is shown that in the presence of magnetic shear, a tearing instability may occur. The latter can lead to magnetic field line reconnection and the formation of magnetic islands which could affect the dynamics of the pulsar magnetosphere.  相似文献   

12.
Unstable pertubation modes exist in the magnetic field of penumbral electric current and I think the penumbral filaments are formed from the development of such modes. Under the short wave approximation the non-adiabatic dispersion equation is solved in the radial and transverse directions of the sunspot. From the condition of instability the length and width of the penumbral filament can be evaluated and it is found that the filament mode is static in the direction of the length and is non-moving in the direction of the width, that the penumbral filaments are a feature of the sunspot magnetic flow under gravity and that the presence of the filaments implies the existence of a twisted magnetic field.  相似文献   

13.
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   

14.
An unstable arch model of a solar flare   总被引：1，自引：0，他引：1  
The theoretical consequences of assuming that a current flows along flaring arches consistent with a twist in the field lines of these arches are examined. It is found that a sequence of magneto-hydrodynamic (MHD) and resistive MHD instabilities driven by the assumed current (which we refer to as the toroidal current) can naturally explain most manifestations of a solar flare.The principal flare instability in the proposed model is the resistive kink (or tearing mode in arch geometry) which plays the role of thermalizing some of the field energy in the arch and generating X-configured neutral points needed for particle acceleration. The difference between thermal and nonthermal flares is elucidated and explained, in part, by amplitude-dependent instabilities, generally referred to as overlapping resonances. We show that the criteria for the generation of flare shocks strongly depend on the magnitude and gradient steepness of the toroidal current, which also are found to determine the volume and rate of energy release. The resulting model is in excellent agreement with present observations and has successfully predicted several flare phenomena.  相似文献   

15.
Instabilities produced by finite-resistivity effects in a plasma are of great interest in connection with research in fusion devices, solar flares, and geomagnetic substorms. We elucidate here the physical mechanism of this instability, and in particular, identify the tendencies in the system towards the instability and the tendencies opposing it, if any. As an illustration, we consider the example of the so-called gravitational interchange mode wherein a plasma with a statically stable vertical density gradient is situated in a vertical gravitational field and a sheared horizontal magnetic field. The physical picture developed here may be useful in sorting out phenomena that appear when more subtle properties of the resistive modes in a plasma are considered.  相似文献   

16.
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   

17.
Magnetic dips in the solar wind   总被引：1，自引：0，他引：1  
Using magnetic data from the HELIOS-1 fluxgate magnetometer, with a 0.2 s resolution, we have investigated the structure of several interplanetary discontinuities involving magnetic dips and rotations of the magnetic field vector. A minimum variance analysis illustrates the behaviour of the magnetic field through the transition. Using this analysis, quite different structures have been isolated and, in particular, narrow transitions resembling almost one dimensional reconnected neutral sheets. For the thinner cases (scale lengths of the magnetic rotation of the order or smaller than 103 km), we find that the observed structures can be the nonlinear effect of a resistive tearing mode instability having developed on an originally one dimensional neutral sheet at the solar corona.  相似文献   

18.
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   

19.
The resistive tearing instability of a sheet pinch, first investigated by Kuang & Roberts (1990) for the case of a rapidly rotating inviscid fluid, is studied for arbitrary rotation rate in a visco‐resistive fluid. Altogether there are three regimes of the resistive tearing instability which correspond to the particular parameter domain in the (Ω, Pm) plane. Here Ω is the angular velocity of the medium which is normalized to the Alfvén time and Pm is the magnetic Prandtl number. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  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   

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