共查询到20条相似文献，搜索用时 31 毫秒
1.
Studies of solar flares indicate that the mechanism of flares is magnetic in character and that the coronal magnetic field is a key to understanding solar high-energy phenomena. In our ongoing research we are conducting a systematic study of a large database of observations which includes both coronal structure (from the Soft X-ray Telescope on the Yohkoh spacecraft) and photospheric vector magnetic fields (from the Haleakala Stokes Polarimeter at Mees Solar Observatory). We compare the three-dimensional nonlinear force-free coronal magnetic field, computed from photospheric boundary data, to images of coronal structure. In this paper we outline our techniques and present results for active region AR 7220/7222. We show that the computed force-free coronal magnetic field agrees well with Yohkoh X-ray coronal loops, and we discuss the properties of the coronal magnetic field and the soft X-ray loops. 相似文献
2.
PROBLEMS AND PROGRESS IN COMPUTING THREE-DIMENSIONAL CORONAL ACTIVE REGION MAGNETIC FIELDS FROM BOUNDARY DATA 总被引：2，自引：0，他引：2
An overview of the whole process of reconstructing the coronal magnetic field from boundary data measured at the photosphere is presented. We discuss the errors and uncertainties in the data and in the data reduction process. The problems include noise in the magnetograph measurements, uncertainties in the interpretation of polarization signals, the 180° ambiguity in the transverse field, and the fact that the photosphere is not force-free. Methods for computing the three-dimensional structure of coronal active region magnetic fields, under the force-free assumption, from these boundary data, are then discussed. The methods fall into three classes: the extrapolation technique, which seeks to integrate upwards from the photosphere using only local values at the boundary; the current-field iteration technique, which propagates currents measured at the boundary along field lines, then iteratively recomputes the magnetic field due to this current distribution; and the evolutionary technique, which simulates the evolution of the coronal field, under quasi-physical resistive magnetohydrodynamic equations, as currents injected at the boundary are driven towards the observed values. The extrapolation method is mathematically ill-posed, and must be heavily smoothed to avoid exponential divergence. It may be useful for tracing low-lying field lines, but appears incapable of reconstructing the magnetic field higher in the corona. The original formulation of the current-field iteration method had problems achieving convergence, but a recent reformulation appears promising. Evolutionary methods have been applied to several real datasets, with apparent success. 相似文献
3.
Markus J. Aschwanden 《Solar physics》2013,287(1-2):323-344
We derive an analytical approximation of nonlinear force-free magnetic field solutions (NLFFF) that can efficiently be used for fast forward-fitting to solar magnetic data, constrained either by observed line-of-sight magnetograms and stereoscopically triangulated coronal loops, or by 3D vector-magnetograph data. The derived NLFFF solutions provide the magnetic field components B x (x), B y (x), B z (x), the force-free parameter α(x), the electric current density j(x), and are accurate to second-order (of the nonlinear force-free α-parameter). The explicit expressions of a force-free field can easily be applied to modeling or forward-fitting of many coronal phenomena. 相似文献
4.
The coronal magnetic field cannot be directly observed, but, in principle, it can be reconstructed from the comparatively well observed photospheric magnetic field. A?popular approach uses a nonlinear force-free model. Non-magnetic forces at the photosphere are significant, meaning the photospheric data are inconsistent with the force-free model, and this causes problems with the modeling (De Rosa et?al., Astrophys. J. 696, 1780, 2009). In this paper we present a numerical implementation of the Grad?CRubin method for reconstructing the coronal magnetic field using a magnetostatic model. This model includes a pressure force and a non-zero magnetic Lorentz force. We demonstrate our implementation on a simple analytic test case and obtain the speed and numerical error scaling as a function of the grid size. 相似文献
5.
A brief historical review of solar and interplanetary magnetic field modeling and analysis is presented. The modeling was initiated with the routine observation of longitudinal magnetograms and the application of current free potential magnetic fields for its interpretation. Almost simultaneously, the development of detailed H observation motivated the constant-, force-free magnetic field analysis. Subsequent development of transverse magnetograms leads not only to further refinements of the analysis and modeling but also to many physically meaningful interpretations of the observations. In this review, historical development is traced with the emphasis placed on the yet unanswered questions. 相似文献
6.
Force-free magnetic fields (f.f.f) are considered as the first approximation of magnetic hydrodynamic equations in the case when the energy of the field exceeds the thermal energy of the medium. Such a relation of energies takes place in the upper atmosphere of the Sun in active regions.The consequence of the virial theorem obtained shows that for any solution of the corresponding non-linear system of equations only two cases are possible: either the total energy of the field is given by a divergent integral, or in some regions the force-free character of field is destroyed. This permits the conclusion that it is impossible to build f.f. current systems everywhere, and therefore boundary problems for this type of fields are of the same importance as for harmonic fields.Integral relations are obtained which are the necessary conditions for the solution of boundary problems. According to the classical principle of Thompson the harmonic fields are always stable, while f.f.f. may be stable or unstable.It is shown that: (1) arbitrary f.f.f. are stable to small changes of boundary conditions; (2) among f.f.f. the hydrodynamically stable configurations exist.The hydrodynamic stability condition restricts the size of force-free currents in such configurations. 相似文献
7.
I. Contopoulos 《Solar physics》2013,282(2):419-426
We present a new improved version of our force-free electrodynamics (FFE) numerical code in spherical coordinates that extrapolates the magnetic field in the inner solar corona from a photospheric vector magnetogram. The code satisfies the photospheric boundary condition and the condition ??B=0 to machine accuracy. The performance of our method is evaluated with standard convergence parameters, and is found to be comparable to that of other nonlinear force-free extrapolations. 相似文献
8.
J. P. McGuire E. Tandberg-Hanssen K. R. Krall S. T. Wu J. B. Smith D. M. Speich 《Solar physics》1977,52(1):91-100
A large long-lived soft X-ray emitting arch system was observed during the last Skylab mission. This arcade stayed in the same approximate position for several solar rotations. We suggest that these long-lived arches owe their stability to the stable coronal magnetic-field configuration. A global constant force-free magnetic field analysis, as developed by Nakagawaet al. (1977), is used to describe the arches, and results in a marked resemblance between the theoretical magnetic-field configuration and the observed X-ray emitting feature. 相似文献
9.
The similarity between the spiral topology of chromospheric fibrils and filaments observed in H near sunspots and the configuration of an axisymmetric force-free magnetic field is examined. It is suggested that some of the observed features could be interpreted in terms of the configuration of lines of force of an axisymmetric force-free chromospheric magnetic field. Implications of the results of analysis to the possible interpretations of other observed topological features near a sunspot are discussed.Visiting scientist at the High Altitude Observatory.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Sponsored by the Office of Naval Research, Contract No. N 00014-67-C-0290. 相似文献
10.
Rolf Boström 《Astrophysics and Space Science》1973,22(2):353-380
A review is given of some of the basic properties of force-free fields under circumstances when the conductivity of the medium is finite. Then the electric current density is related not only to the magnetic field, but also by Ohm's law to the electric field and plasma velocity, which must be considered in the solutions. It is pointed out that the natural constraint that the electric field and plasma velocity should be finite everywhere is not fulfilled in some previously used models. Models with a constant ratio of the electric current density and magnetic field intensity have been used extensively in the past. They are of some importance since solutions with the plasma at rest are possible only if is constant. However, it is shown that solutions of constant cannot be matched to an external current-free region of finite conductivity since can have no discontinuity in a medium where the conductivity varies continuously. Hence, for most applications models with a varying and a moving plasma must be used. Some new, simple and consistent models, especially of cylindrical symmetry, are derived by prescribing the form of the magnetic field lines, or one electic field component, or one time-dependent electric current component. 相似文献
11.
Su Qing-Rui 《Solar physics》1982,75(1-2):229-236
In this paper, we extend B. C. Low's study on nonlinear force-free magnetic fields. Based on Low's mathematical method, a revised boundary-value problem of the two-dimensional nonlinear force-free magnetic field is solved analytically. The solution shows that higher magnetic loops evolve towards preflare loops when the gradient of longitudinal magnetic field at the photospheric level and the angle (shear) included between the magnetic field line and magnetic neutral line increase with time. The density, temperature and the current density are higher in the preflare loops than in the high-lying magnetic loops. We believe it is the loops that provide conditions for the eruption of the flare.The original has been published in the Acta Astronomica Sinica
21 (1980), 152, in Chinese. The present paper completes the discussion and revises some of the preliminary results. 相似文献
12.
Using the boundary element method (BEM) for constant-, force-free fields, the vector magnetic field distributions in the chromosphere of a flare-productive active region. AR 6659 in June 1991, are obtained by extrapolating from the observed vector magnetograms at the photosphere. The calculated transverse magnetic fields skew highly from the photosphere to the chromosphere in the following positive polarity sunspot whereas they skew only slightly in the main preceding sunspot. This suggests that more abundant energy was stored in the former area causing flares. Those results demostrate the validity of the BEM solution and the associations between the force-free magnetic field and the structure of the AR 6659 region. It shows that the features of the active region can be revealed by the constant- force-free magnetic field approximation. 相似文献
13.
On practical representation of magnetic field 总被引：2，自引：0，他引：2
Various manners of determination of a magnetic field are reviewed briefly from the standpoint of practicality and uniqueness. Then a practical representation of magnetic fields in terms of a class of force-free magnetic field is described. The proposed scheme is based on the physical consideration that in the chromosphere and lower corona a quasistatic magnetic field must be nearly force-free and that for the class of force-free magnetic field, i.e., ×B=B with = constant, the magnetic field can be determined uniquely from the observed distribution of the vertical component of a magnetic field. The applicability of the representation is demonstrated by examples and the limitations are discussed.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
14.
Klaus Jockers 《Solar physics》1978,56(1):37-53
Numerical calculations of two-dimensional force-free fields as models of solar active regions are presented. For a given toroidal component of the photospheric magnetic field two branches of solutions are numerically obtained which merge at the critical point of maximum allowed toroidal magnetic field. Depending on boundary conditions magnetic islands may or may not form. The results are discussed with respect to their relevance to the flare process. 相似文献
15.
J. N. Tokis 《Astrophysics and Space Science》1992,191(1):131-135
The aim of this paper is to discuss systematically two numerical methods that can play the significant role of numerical simulators of stability in the corresponding astrophysical problems to which they are applied. 相似文献
16.
D. D. Barbosa 《Solar physics》1978,56(1):55-66
We have computed the surface Green's function for linear force-free magnetic fields, where × B = B and is a constant, for application to low coronal levels of the solar atmosphere. Boundary conditions are imposed on the normal component of B on two parallel planes which delineate the force-free volume. This procedure ensures that the magnetic field energy remains bounded, and that the field lines have a smooth behavior. A simple bipolar source distribution is treated and representative field line tracings are shown. 相似文献
17.
Helical structures are generally associated with many eruptive solar prominences. Thus, study of their evolution in the solar atmosphere assumes importance. We present a study of a flare-associated erupting prominence of March 11, 1979, with conspicuous helically twisted structure, observed in H line center. We have attempted to understand the role played by twisted force-free magnetic fields in this event. In the analysis, we have assumed that the helical structures visible in H outline the field lines in which prominence tubes are embedded. Untwisting of observed prominence tubes and later, formation of open prominence structures provide evidence of restructuring of the magnetic field configuration over the active region during the course of prominence eruption. Temporal evolution of the force-free parameter is obtained for two main prominence tubes observed to be intertwined in a rope-like structure. Axial electric currents associated with the prominence tubes are estimated to be of the order of 1011 A which decreased with time. Correspondingly, it is estimated that the rate of energy release was 1028 erg s–1 during the prominence eruption. 相似文献
18.
We present a method to include stereoscopic information about the three-dimensional structure of flux tubes into the reconstruction of the coronal magnetic field. Due to the low plasma beta in the corona we can assume a force-free magnetic field, with the current density parallel to the magnetic field lines. Here we use linear force-free fields for simplicity. The method uses the line-of-sight magnetic field on the photosphere as observational input. The value of is determined iteratively by comparing the reconstructed magnetic field with the observed structures. The final configuration is the optimal linear force-free solution constrained by both the photospheric magnetogram and the observed plasma structures. As an example we apply our method to SOHO MDI/EIT data of an active region. In the future it is planned to apply the method to analyse data from the SECCHI instrument aboard the STEREO mission. 相似文献
19.
We present a set of cylindrically-symmetric force-free magnetic fields with non-constant scalar function scalar. We found that the kink instability of the fields can be suppressed by reducing the length of the flux tube. By using the pressure profile in coronal magnetic loops obtained on the basis of the observational data, and by neglecting the effect of gravity, these force-free fields ars modified to non-force-free ones. For the plasma of finite conductivity the time and space dependent magnetic fields are obtained, and the ohmic dissipation per unit volume per second is calculated. For the magnetic fields, presented in the investigation, it is also found that, due to the large electrical conductivity of the plasma, the ohmic dissipation is negligable in comparison to the conduction and the radiation loss. Hence, for the energy equilibrium in a coronal loop, the contribution of ohmic dissipation is insignificant. 相似文献
20.
In order to understand various solar phenomena controlled by the magnetic field, such as X-ray bright points, flares and prominence eruptions, the structure of the coronal magnetic field must be known. This requires a precise extrapolation of the photospheric magnetic field. Presently, only potential or linear force-free field approximations can be used easily. A more realistic modelling of the field is still an active research area because of well-known difficulties related to the nonlinear mixed elliptic-hyperbolic nature of the equations. An additional difficulty arises due to the complexity of the magnetic field structure which is caused by a discrete partition of the photospheric magnetic field. This complexity is not limited to magnetic regions having magnetic nulls (and so separatrices) but also occurs in those containing thin elongated volumes (called Quasi-Separatrix Layers) where the photospheric field-line linkage changes rapidly. There is a wide range for the thickness of such layers, which is determined by the character (bipolar or quadrupolar) of the magnetic region, by the sizes of the photospheric field concentrations and by the intensity of the electric currents. The aim of this paper is to analyse the recent nonlinear force-free field extrapolation techniques for complex coronal magnetic fields. 相似文献