共查询到20条相似文献，搜索用时 31 毫秒
1.
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
2.
The problem of electromagnetic field interacting with viscous fluid without and with zero-mass scalar field has been studied. It has been shown that electromagnetic field cannot interact with viscous fluid for spherically-symmetric Robertson-Walker metric. Exact solutions corresponding to the problem of electromagnetic field interactions in presence of viscous fluid and zero-mass scalar field have been obtained subject to various physical conditions. It presents a scope for the study of imperfect fluid FRW models showing the existence of the electromagnetic field due to the presence of zero-mass scalar field. 相似文献
3.
本文按常α无力场模型计算了1980年10月23日Boulder 2744活动区前导黑子的纵向磁场随高度的变化,并与用CIV 1548谱线观测得到的色球一日冕过渡区的磁场资料相结合,求得CIV 1548发射区的有效高度。这些结果与文献[4]中对同一黑子用势场模型推求的结果有很大差别。从而表明,势场和无力场在某些方面导致的结果是极不相同的。鉴于观测已表明活动区上空存在电流的事实,在活动区磁场的模拟中,特别是在强扭曲活动区磁场的计算中,应当避免采用势场,而尽可能采用无力场模型。 相似文献
4.
N. Seehafer 《Solar physics》1986,105(2):223-235
The field lines of closed magnetic structures above the photosphere define a mapping from the photosphere to itself. This mapping is discontinuous, and the field line connectivity to the boundary can change discontinuously in response to continuous changes of field strength and direction, if field lines either end in a singular point of the field or are tangential to the photosphere at one end. Whereas the general existence of singular points is questionable, the field has typically a cell structure due to the presence of segments of the zero line of the photospheric longitudinal field on which the transversal field is directed from negative (pointing into the Sun) to positive fields. The cell boundaries are made up of field lines which all touch the photosphere on one of these line segments. Within each of the cells the field line mapping is continuous. When during a slow evolution a substantial part of a coronal loop or of an arcade has passed from one cell into another a fast dynamic instability may set in which was previously prevented by the anchoring of field lines in the dense photosphere. 相似文献
5.
Hirokazu Yoshimura 《Solar physics》1977,52(1):41-52
The magnetic field lines of the corona associated with the solar-cycle surface general magnetic field are calculated by a potential-field approximation to study the solar-cycle evolution of the geometry of the coronal field. The surface field evolution used here is the radial field evolution, predicted by a model of the solar cycle driven by the dynamo action of the global convection, and justified observationally using Mount Wilson magnetic synoptic chart data. The evolution of the calculated coronal general field is now good for comparison with observations and shows the following. (i) The field of the polar and high-altitude corona has dipolar structure in almost all phases of the solar cycle except in a short time interval around maximum phase despite the quadrupolar structure of the general magnetic field at the surface; quadrupolar field forms loop-like structure in the lower corona. The almost-dipolar structure of the polar and high-latitude corona and the loop formation of the equatorial lower corona explain the appearance of the undisturbed minimum corona observed at eclipses. (ii) The polar field lines are directed almost radially at the minimum phase, which should be responsible for polar plumes. The field lines slowly open up to participate in the loop-like structure of the equatorial lower corona, and rapidly change their structure and polarity at the maximum phase, to resume the almost radial configuration slowly, (iii) During the rapidly changing maximum phase, the field lines do not penetrate deep into the interplanetary space resulting in the absence of polar plumes and the appearance of the circular corona- the maximum corona. In this phase, the coronal field should not be approximated by a dipole field. The surface field evolution which can explain such behaviors of the corona is characteristic of the solar-cycle process dominated by the latitudinal gradient of the differential rotation. If the radial gradient dominated in the subsurface process, the coronal evolution would look quite different and would show latitudinal propagation of enhancement of activity. Although nonaxisymmetric features should be superposed on the axisymmetric general field to express the real corona, the general field can be a basic coronal field in studying long-term interaction between the convection zone and the interstellar space especially in studying the magnetic braking of the solar rotation. 相似文献
6.
The potential and linear force-free field models for the magnetic field in the solar corona are often used in the analysis of flares. The field is calculated using boundary values measured in the low solar atmosphere. The topology of the field calculated using these models is then compared to the position of flare emissions. We demonstrate that the topology of the field according to each of these models, with the same boundary conditions in place, is not in general even qualitatively equivalent. An argument is given for a similar discrepancy between a linear force-free field solution and a nonlinear force-free field solution. 相似文献
7.
The magnetic field at the surface of the Sun is confined to widely separated small, intense magnetic flux bundles or fibrils with most of the gas in a relatively field free state in the interstices. There is evidently a systematic field and fluid separation effect in operation, overriding the tendency for turbulent mixing of the two phases. It is well known that a constantly rotating volume of fluid soon excludes any exterior magnetic field. To take the next theoretical step, this paper examines the exclusion of magnetic field from a circular cylinder with oscillating angular velocity. The result is that the field is effectively excluded from any oscillating or tumbling body of fluid that maintains coherence over a sufficiently long time. However, there is no expectation of such long term coherence in turbulent convection in the Sun, suggesting that the observed separation of field and fluid does not come about by the exclusion of field from convective cells. This leads to the conclusion that the separation of fluid and field is a consequence of extraction of fluid from the field through buoyancy and reconnection. 相似文献
8.
The structure of the solar corona is dominated by the magnetic field because the magnetic pressure is about four orders of
magnitude higher than the plasma pressure. Due to the high conductivity the emitting coronal plasma (visible, e.g., in SOHO/EIT)
outlines the magnetic field lines. The gradient of the emitting plasma structures is significantly lower parallel to the magnetic
field lines than in the perpendicular direction. Consequently information regarding the coronal magnetic field can be used
for the interpretation of coronal plasma structures. We extrapolate the coronal magnetic field from photospheric magnetic
field measurements into the corona. The extrapolation method depends on assumptions regarding coronal currents, e.g., potential
fields (current-free) or force-free fields (current parallel to magnetic field). As a next step we project the reconstructed
3D magnetic field lines on an EIT-image and compare with the emitting plasma structures. Coronal loops are identified as closed
magnetic field lines with a high emissivity in EIT and a small gradient of the emissivity along the magnetic field. 相似文献
9.
P. J. Luyten 《Astrophysics and Space Science》1989,153(1):13-29
The effect of a helical magnetic field on the oscillations and the stability of a homogeneous self-gravitating rotating cylinder is investigated. The axial field has a tendency to stabilise long wave numbers and to destabilise small wave numbers so that maximum instability occurs for a finite wave number. If the toroidal and the axial component of the field have the same sign, the instability associated with the toroidal field can be removed by the rotation or by the axial field. Rotational instability is reduced but cannot removed by the field. If the components of the field have the opposite sign, rotational instability is increased. The maximum growth rate of the magnetic instability is reduced by a small axial field and tends to a finite value for large axial fields. 相似文献
10.
《天文和天体物理学研究(英文版)》2017,(10)
We calculate the mass-radius relationship of quark stars with the magnetized densitydependent quark mass model in this work, considering two magnetic field geometries: a statistically isotropic, tangled field and a force-free configuration. In both cases, magnetic field production decreases in the case of maximum quark star mass. Furthermore, a tangled, isotropic magnetic field has a relatively smaller impact on the mass and radius, compared to the force-free configuration, which implies that the geometry of the interior magnetic field is at least as important as the field strength itself when the influence of the strong magnetic field on the mass and radius is assessed. 相似文献
11.
We consider the expulsion of the magnetic field from the super-conducting core of a neutron star and its subsequent decay in the crust. Particular attention is paid to a strong feedback of the distortion of magnetic field lines in the crust on the expulsion of the flux from the core. This causes a considerable delay in the core flux expulsion if the initial field strength is larger than 1011 G. It is shown that the hypothesis on the magnetic field expulsion induced by the neutron-star spin-down is adequate only for a relatively weak initial magnetic field B ≈1011 G. The expulsion time-scale depends not only on the conductivity of the crust, but also on the initial magnetic field strength itself. Our model of the field evolution naturally explains the existence of the residual magnetic field of neutron stars. Its strength is correlated with the impurity concentration in neutron-star crusts and anticorrelated with the initial field strengths. 相似文献
12.
E. M. Drobyshevski 《Astrophysics and Space Science》1977,46(1):41-49
A comparison of the equations for the magnetic field transfer and for the heat transfer by two-dimensional turbulent convection of a conducting compressible medium shows the magnetic field to be transported as a scalar admixture provided it is parallel to the convective rolls. At high magnetic Reynolds numbers the field strength in a convective zone varies proportionally to the density of the medium.A study of the distribution and amplification of the poloidal field in the two-dimensional convection zone of the Sun lying under the supergranulation, together with the processes of field pumping and amplification in other zones, reveals the importance of considering generation mechanisms of thesemi-dynamo type where the amplifying field is excited independently by weak e.m.f.'s of non-electric origin with no feedback which would otherwise produce MHD self-excitation of the field.An illustrative calculation of the solar poloidal field maintained by a weak Coriolis e.m.f. acting in a thin external layer of the convective envelope yields for the general near-polar field, if one somehow takes into account (1) field pumping by three-dimensional supergranulation, (2) field transfer and amplification by two-dimensional convection, and (3) ohmic diffusion of the field into a stable core, a value of the order of 10–1 gauss. 相似文献
13.
We apply the model of flux expulsion from the superfluid and superconductive core of a neutron star, developed by Konenkov & Geppert, both to neutron star models based on different equations of state and to different initial magnetic field structures. Initially, when the core and the surface magnetic field are of the same order of magnitude, the rate of flux expulsion from the core is almost independent of the equation of state, and the evolution of the surface field decouples from the core field evolution with increasing stiffness. When the surface field is initially much stronger than the core field, the magnetic and rotational evolution resembles that of a neutron star with a purely crustal field configuration; the only difference is the occurrence of a residual field. In the case of an initially submerged field, significant differences from the standard evolution only occur during the early period of the life of a neutron star, until the field has been re-diffused to the surface. The reminder of the episode of submergence is a correlation of the residual field strength with the submergence depth of the initial field. We discuss the effect of the re-diffusion of the magnetic field on the difference between the real and the active age of young pulsars and on their braking indices. Finally, we estimate the shear stresses built up by the moving fluxoids at the crust–core interface and show that these stresses may cause crust cracking, preferentially in neutron stars with a soft equation of state. 相似文献
14.
G. RÜDIGER 《Astronomische Nachrichten》1975,296(3):133-141
Continuing an investigation concerning the influence of a uniform mean magnetic field on turbulence (RÜDIGER, 1974) we now consider a weak magnetic field changing spatially weakly and containing a neutral sheet. An originally homogeneous and isotropic turbulent field becomes inhomogeneous and anisotropic if such a magnetic field is present. Because of the finite correlation length the turbulent field is also affected in a neutral sheet. For a special class of spectral functions of two- und three-dimensional turbulence the anisotropic damping of the motions is given in the vicinity of the neutral sheet. Furthermore, we point out the consequence for the mean magnetic field which is affected by such an inhomogeneous turbulent field. Using BOCHNER'S theorem concerning the spectral tensor of the originally homogeneous turbulence we obtain an additional decay of thr mean magnetic field. 相似文献
15.
K.-H. Rdler 《Astronomische Nachrichten》1974,295(6):265-273
This paper deals with turbulent motions in a homogeneous incompressible electrically conducting medium in the presence of a magnetic field which is on average homogeneous and stationary. Using a model in which the turbulence is produced by a stochastic body force, and supposing a weak interaction between motion and magnetic field, a method is developed for calculating the pair correlation tensor of the velocity field from that occuring in a zero magnetic field. As an example, the pair-correlation tensor for a homogeneous stationary turbulence, which is isotropic and mirror-symmetric for zero magnetic field, is determined. With obvious assumptions on the correlation for zero field, two results are obtained. Firstly, the turbulent velocity is reduced by the magnetic field, the component parallel to the field, however, less than those perpendicular to it. Secondly, the correlation length parallel to the field turns out to be greater than the one perpendicular to it, indicating a tendency towards two-dimensional motion. Finally, the possibility of special situations is briefly discussed in which the turbulent velocity is enhanced by the magnetic field, and the anisotropies of the velocity components and the correlation lengths are opposite to those above. 相似文献
16.
The environment, such as an accretion disk, could modify the signal of the gravitational wave from astrophysical black hole binaries. In this article, we model the matter field around intermediatemass binary black holes by means of an axion-like scalar field and investigate their joint evolution. In detail, we consider equal mass binary black holes surrounded by a shell of axion-like scalar field both in spherically symmetric and non-spherically symmetric cases, and with different strengths of the scalar field. Our result shows that the environmental scalar field could essentially modify the dynamics. Firstly,in the spherically symmetric case, with increase of the scalar field strength, the number of circular orbits for the binary black hole is reduced. This means that the scalar field could significantly accelerate the merger process. Secondly, once the scalar field strength exceeds a certain critical value, the scalar field could collapse into a third black hole with its mass being larger than that of the binary. Consequently,the new black hole that collapses from the environmental scalar field could accrete the binary promptly and the binary collides head-on with each other. In this process, there is almost no quadrupole signal produced, and, consequently, the gravitational wave is greatly suppressed. Thirdly, when the scalar field strength is relatively smaller than the critical value, the black hole orbit could develop eccentricity through accretion of the scalar field. Fourthly, during the initial stage of the inspiral, the gravitational attractive force from the axion-like scalar field could induce a sudden turn in the binary orbits, hence resulting in a transient wiggle in the gravitational waveform. Finally, in the non-spherical case, the scalar field could gravitationally attract the binary moving toward the center of mass for the scalar field and slow down the merger process. 相似文献
17.
This paper analyzes the magnetic field structure of active regions at coronal heights determined by means of multi-wavelength observations of polarized radio emission in the microwave range, and compares it with the force-free magnetic field extrapolation into the corona from the photospheric magnetograms. Our method of one-dimensional radio stereoscopy indicates higher magnetic field strength compared with the field reconstructed from photospheric magnetograms. It is shown that the sense of inclinations of the field lines we obtained from the radio data matches the shape of the reconstructed magnetic field lines, although the degree of the inclinations is very different. 相似文献
18.
Observations are reported of field aligned etectron fluxes in the energy range 50–500 eV at altitudes below 270 km from two rocket flights in the auroral zone. The regions of field aligned suprathermal electrons occurred in bursts of a few seconds duration, and in some instances the energy of the peak field aligned flux was in the range 100–500 eV. Theoretical calculations of the pitch angle distribution were made using the Monte Carlo technique for two model atmospheres having exospheric temperatures of 750 and 1500 K bracketing the expected auroral zone exospheric temperature. The calculations were made for the case of incident field aligned suprathermal fluxes with no local parallel electric field and also for the case of a local constant parallel electric field. Comparison of theoretical and experimental pitch angle distributions showed that in one case at 270 km a parallel electric field of 1–2 mV/m fitted the data whereas another burst at 210 km required a parallel electric field of about 10 mV/m to produce a field aligned distribution of 230 eV electrons as pronounced as was observed. Furthermore in this latter case the lack of strong field alignment at 500 eV pointed to localisation of the parallel electric field to an altitude range of 20–30 km about the rocket altitude. 相似文献
19.
In this paper, a potential field extrapolation and three nonlinear force-free (NLFF) field extrapolations (optimization, direct
boundary integral (DBIE), and approximate vertical integration (AVI) methods) are used to study the spatial configuration
of magnetic field in the quiet Sun. It is found that differences in the computed field strengths among the three NLFF and
potential fields exist in the low layers. However, they tend to disappear as the height increases, and the differences are
of the order of 0.1 gauss when the height exceeds ≈ 2000 km above the photosphere. The difference in azimuth angles between
each NLFF field model and the potential field is as follows: for the optimization field, it decreases evidently as the height
increases; for the DBIE field, it almost stays constant and shows no significant change as the height increases; for the AVI
field, it increases slowly as the height increases. Our analysis shows that the reconstructed NLFF fields deviate significantly
from the potential field in the quiet Sun. 相似文献
20.
The equilibrium structure of normal-polarity, quiescent prominences is investigated and the influence of magnetic shear in response to a slow, shearing, photospheric velocity discussed. The results show that the overall field structure predicted by Fiedler and Hood (1992) is largely unaffected but that magnetic shear reduces the plasma beta and lengthens and flattens the magnetic field when viewed from the side. The flatness of the field suggests that the initial condensation can form and, when the mass is sufficient, deform the field slightly into the equilibrium structure calculated here. Thus, it is postulated that the field must be highly sheared for the radiation (or condensation) time to be less than the free-fall time along the field. A simple estimate predicts that the field must lie close to the polarity inversion line with an angle in agreement with observations. Hence, it is apparent that normal polarity prominences will always be observed with a highly sheared field.It is shown that the line-of-sight field component depends on the imposed shear profile and the viewing angle and in certain cases it is possible for this field component to appear to increase with height. Any observed increase of the line-of-sight magnetic field with height may then be due to the angle of the prominence to the line of sight. 相似文献