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  • SOLAR PHYSICS  (7)
  • Optics  (1)
  • 1
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    Line-of-sight magnetic flux imbalances caused by electric currents (1995)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Several physical and observational effects contribute to the significant imbalances of magnetic flux that are often observed in active regions. We consider an effect not previously treated: the influence of electric currents in the photosphere. Electric currents can cause a line-of-sight flux imbalance because of the directionality of the magnetic field they produce. Currents associated with magnetic flux tubes produce larger imbalances than do smoothly-varying distributions of flux and current. We estimate the magnitude of this effect for current densities, total currents, and magnetic geometry consistent with observations. The expected imbalances lie approximately in the range 0-15%, depending on the character of the current-carying fields and the angle from which they are viewed. Observationally, current-induced flux imbalances could be indicated by a statistical dependence of the imbalance on angular distance from disk center. A general study of magnetic flux balance in active regions is needed to determine the relative importance of other- probably larger- effects such as dilute flux (too weak to measure or rendered invisible by radiative transfer effects), merging with weak background fields, and long-range connections between active regions.
    Keywords: SOLAR PHYSICS
    Type: Solar Physics (ISSN 0038-0938); 157; 1-2; p. 185-197
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  • 2
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    Reduction, analysis, and properties of electric current systems in solar active regions (1995)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The specific attraction and, in large part, the significance of solar magnetograms lie in the fact that they give the most important data on the electric currents and the nonpotentiality of active regions. Using the vector magnetograms from the Marshall Space Flight Center (MSFC), we employ a unique technique in the area of data analysis for resolving the 180 deg ambiguity in order to calculate the spatial structure of the vertical electric current density. The 180 deg ambiguity is resolved by applying concepts from the nonlinear multivariable optimization theory. The technique is shown to be of particular importance in very nonpotential active regions. The characterization of the vertical electric current density for a set of vector magnetograms using this method then gives the spatial scale, locations, and magnitude of these current systems. The method, which employs an intermediate parametric function which covers the magnetogram and which defines the local `preferred' direction, minimizes a specific functional of the observed transverse magnetic field. The specific functional that is successful is the integral of the square of the vertical current density. We find that the vertical electric current densities have common characteristics for the extended bipolar (beta) (gamma) (delta)-regions studied. The largest current systems have j(sub z)'s which maximizes around 30 mA/sq m and have a linear decreasing distribution to a diameter of 30 Mn.
    Keywords: SOLAR PHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 445; 2; p. 982-998
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  • 3
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    Transformation of vector magnetograms and the problems associated with the effects of perspective and the azimuthal ambiguity (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Off-center vector magnetograms which use all three components of the measured field provide the maximum information content from the photospheric field and can provide the most consistent potential field independent of the viewing angle by defining the normal component of the field. The required transformations of the magnetic field vector and the geometric mapping of the observed field in the image plane into the heliographic plane have been described. Here we discuss the total transformation of specific vector magnetograms to detail the problems and procedures that one should be aware of in analyzing observational magnetograms. The effect of the 180-deg ambiguity of the observed transverse field is considered as well as the effect of curvature of the photosphere. Specific results for active regions AR 2684 (September 23, 1980) and AR 4474 (April 26, 1984) from the Marshall Space Flight Center Vector magnetograph are described which point to the need for the heliographic projection in determining the field structure of an active region.
    Keywords: SOLAR PHYSICS
    Type: Solar Physics (ISSN 0038-0938); 126; 21-36
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  • 4
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    Concerning the extrapolation of solar nonlinear force-free magnetic fields (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: This paper contains a review and discussion of the mathematical basis of the extrapolation techniques involved in using photospheric vector magnetograms to obtain the coronal field above the surface. The two basic techniques employing the Cauchy initial value problem and the variational techniques are reviewed in terms of the mathematical and practical applications. A short review is presented of the current research on numerical modeling techniques in the area of extrapolating vector magnetograms; specifically, algorithms to extrapolate nonlinear force-free magnetic fields from the photosphere are considered.
    Keywords: SOLAR PHYSICS
    Type: Societa Astronomica Italiana, Memorie (ISSN 0037-8720); 61; 2, 19
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  • 5
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    Off disk-center potential field calculations using vector magnetograms (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A potential field calculation for off disk-center vector magnetograms that uses all the three components of the measured field is investigated. There is neither any need for interpolation of grid points between the image plane and the heliographic plane nor for an extension or a truncation to a heliographic rectangle. Hence, the method provides the maximum information content from the photospheric field as well as the most consistent potential field independent of the viewing angle. The introduction of polarimetric noise produces a less tolerant extrapolation procedure than using the line-of-sight extrapolation, but the resultant standard deviation is still small enough for the practical utility of this method.
    Keywords: SOLAR PHYSICS
    Type: Solar Physics (ISSN 0038-0938); 120; 2, 19; 235-247
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  • 6
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    Linear force-free magnetic fields for solar extrapolation and interpretation (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: This paper discusses the interconnection of the various linear force-free magnetic field formulations, the specific phenomenological and topological parameters of these formulations, and their usefulness. Particularly, the limitations and usefulness of linear force-free fields are discussed. Specific field configurations are related to magnetographic interpretation. The relationship of the integral and Fourier procedures is shown explicitly. The physical interpretation of linear force-free fields is shown by analytic models and from the Marshall Space Flight Center solar vector magnetograms.
    Keywords: SOLAR PHYSICS
    Type: Astrophysical Journal Supplement Series (ISSN 0067-0049); 69; 323-348
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  • 7
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    Solar Confocal Interferometers for Sub-Picometer-Resolution Spectral Filters (2006)
    In:  Other Sources
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    Publication Date: 2019-07-18
    Description: The confocal Fabry-Perot interferometer allows sub-picometer spectral resolution of Fraunhofer line profiles. Such high spectral resolution is needed to keep pace with the higher spatial resolution of the new set of large-aperture solar telescopes. The line-of-sight spatial resolution derived for line profile inversions would then track the improvements of the transverse spatial scale provided by the larger apertures. The confocal interferometer's unique properties allow a simultaneous increase in both etendue and spectral power. Methods: We have constructed and tested two confocal interferometers. Conclusions: In this paper we compare the confocal interferometer with other spectral imaging filters, provide initial design parameters, show construction details for two designs, and report on the laboratory test results for these interferometers, and propose a multiple etalon system for future testing of these units and to obtain sub-picometer spectral resolution information on the photosphere in both the visible and near-infrared.
    Keywords: Optics
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  • 8
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    The effects of viewing angle on the inference of magnetic shear in preflare active regions (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Analytic models for nonpotential force-free fields representative of preflaring active regions have been used to determine the degree of magnetic shear along the magnetic neutral line of these fields as a function of location and orientation of the active region on the solar disk. The results indicate that, with the exception of regions close to the disk center, the position of the inferred neutral line differs significantly from the actual neutral line. It is suggested that the significant variation of the inferred degree of shear with the position and orientation of the region is due to such factors as geometric projection effects, the shift of the inferred neutral line relative to its true position, and the variations in the reference potential field.
    Keywords: SOLAR PHYSICS
    Type: Solar Physics (ISSN 0038-0938); 119; 1 19
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