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  • 1
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    Radiative and reconnection instabilities - Filaments and flares (1984)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: The way in which the initial development of solar filament radiative cooling and the magnetic reconnection of a solar flare can occur in the center of a field-shear layer is demonstrated. Since the present treatment unites these two mechanisms, it indicates the common as well as the disparate features they possess. Unstable radiation serves to increase the Coulomb resistivity at the X-point, so that the reconnection is not self-quenching. The surprising dominance of the magnetic component of the perturbation in the midwavelength range indicates the need to examine the nonlinear saturation of the energy transport of the radiative mode, taking the accompanying magnetic reconnection and potential-energy release into account, for comparison with observations of filaments as well as for clues to the character of the preflare state.
    Keywords: SOLAR PHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 280; 391-398
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  • 2
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    The stability of coronal loops - Finite-length and pressure-profile limits (1983)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: Results are described from a quickly converging, necessary-and-sufficient, MHD-stability test for coronal-loop models. The primary stabilizing influence arises from magnetic line tying at the photosphere, and this end conditions requires a series expansion of possible loop excitations. The stability boundary is shown to quickly approach a limit as the number of terms increases, providing a critical length for the loop in proportion to its transverse magnetic scale. Several models of force-free-field profiles are tested and the stability behavior of a localized current channel, embedded in an external current-free region, is shown to be superior to that of other, broader, current profiles. Pressure-gradient effects, leading to increased or decreased stability, are shown to be amplified by line tying. Long loops must either conduct low net current, or exhibit an axial-field reversal coexisting with a low-pressure core. The limits on stability depend on the magnetic aspect ratio, the plasma-to-magnetic pressure ratio, and the field orientation at the loop edge. Applications of these results to the structure of coronal loops are described.
    Keywords: SOLAR PHYSICS
    Type: Solar Physics (ISSN 0038-0938); 88; 163-177
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  • 3
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    Nonlinear computations of a solar flare model (1984)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: A cylindrical axisymmetric tearing mode model for solar flares is investigated numerically. Large magnetic energy release only occurs when there are at least two mode rational surfaces in the current-carrying plasma.
    Keywords: SOLAR PHYSICS
    Type: Physics of Fluids (ISSN 0031-9171); 27; 2063-206
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  • 4
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    Simple-loop flares - Magnetic instabilities (1981)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: The era of rocket and spacecraft observations of the sun has provided an entirely new view of the structure and evolution of the solar atmosphere. It is now clear, particularly since the extended series of Skylab flights, that the lower corona is quite nonuniform. In active regions, the strong ambient magnetic fields collimate and confine the emissive plasma into a myriad of loops and arcades. These features are observed to evolve slowly, with a time-scale much longer than the relevant hydromagnetic of Alfven period and, therefore, must be considered generally to be stable. A conspicuous exception is the sporadic flare activity of these loops, which is believed to be due to localized departures from infinite-conductivity behavior.
    Keywords: SOLAR PHYSICS
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  • 5
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    The structure, stability and flaring of solar coronal loops (1982)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: A review is given of recent progress in the theory of the magnetohydrodynamic behavior of coronal loops, beginning with a brief characterization of thy observations. The equilibrium magnetic field is described, along with the consequences of the empirical requirement for short-term, or infinite-conductivity, stability which is shown to be dominated by the end-effect influence of thy quasi-rigid photosphere. A new loop-flare model is then developed, which takes account of the finite loop length. The primary resistive-sausage-mode instability exhibits the necessary threshold behavior, and produces a number of spatially and energetically distinct flare-release manifestations.
    Keywords: SOLAR PHYSICS
    Type: Societa Astronomica Italiana; vol. 53
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  • 6
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    Hydromagnetic stability of coronal arcade structures The effects of photospheric line tying (1982)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: A model is given of the magnetic-field equilibrium and possible dynamic excitations of a solar coronal arcade. Such structures are well observed in the spectral range from H-alpha to X-rays and often give rise to two-ribbon flares. However, the preflare state must be stable to ideal magnetohydrodynamic disturbances, and this problem is treated with particular attention to the necessary foot-point boundary conditions. With reasonably general perturbation set, an energy-principle analysis is used to show the strong stabilizing influence of inertial field-line tying at the photosphere.
    Keywords: SOLAR PHYSICS
    Type: Solar Physics; 79; Aug. 198
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  • 7
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    The thermal instability in a sheared magnetic field - Filament condensation with anisotropic heat conduction (1984)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: The condensation-mode growth rate of the thermal instability in an empirically motivated sheared field is shown to depend upon the existence of perpendicular thermal conduction. This typically very small effect (perpendicular conductivity/parallel conductivity less than about 10 to the -10th for the solar corona) increases the spatial-derivative order of the compressible temperature-perturbation equation, and thereby eliminates the singularities which appear when perpendicular conductivity = 0. The resulting growth rate is less than 1.5 times the controlling constant-density radiation rate, and has a clear maximum at a cross-field length of order 100 times and a width of about 0.1 the magnetic shear scale for solar conditions. The profiles of the observable temperature and density perturbations are independent of the thermal conductivity, and thus agree with those found previously. An analytic solution to the short-wavelength incompressible case is also given.
    Keywords: SOLAR PHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 282; 267-273
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  • 8
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    The Queen's flare - Its structure and development; precursors, pre-flare brightenings, and aftermaths (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A limb flare, which started at about 20:20 UT on April 30, 1980, was observed by several of the instruments on the Solar Maximum Mission (SMM) spacecraft. This flare has been the subject of a joint analysis of the SMM instruments. The present investigation represents a continuation of research reported in part by Woodgate et al. (1981) and Gabriel et al. (1981). Several questions are explored regarding the preflare activity, the evolution of the flare, and its decay. It is concluded that the X-ray brightenings observed before the flare were indicative only of the generally high level of activity from this region. They were not connected with the build-up of energy before the flare since similar brightenings were observed in the region after the flare. At least one brightening occurred at the site of the kernel before the flare. There is also some evidence of a tongue.
    Keywords: SOLAR PHYSICS
    Type: Solar Physics (ISSN 0038-0938); 84; April 19
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  • 9
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    Latitudinal anisotropy of the solar far ultraviolet flux - Effect on the Lyman alpha sky background (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: An estimate is given for the anisotropy of the solar full disk flux in the far ultraviolet, as it would be observed for lines of sight within and above the ecliptic plane through the predominant influence of plages at low heliocentric latitudes. The flux anisotropy for Lyman alpha, at a level of solar activity with a sunspot number of roughly 160, is approximately 0.88 for the integrated flux over the Lyman alpha profile and 0.83 for the flux at line center. The effect of this Lyman alpha flux anisotropy on the Lyman alpha sky background intensity, resonantly backscattered from the local interstellar medium that is streaming through the solar system, is examined. It is concluded that the solar Lyman alpha anisotropy should be included in models of the interplanetary background during periods of moderate to high solar activity.
    Keywords: SOLAR PHYSICS
    Type: Astronomy and Astrophysics; 97; 2, Ap; Apr. 198
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  • 10
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    The preflare state (1980)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: The accumulation, storage and irreversible release of the free energy necessary for a solar flare are discussed on the basis of data obtained from the Apollo Telescope Mount on Skylab and other pertinent sources. Skylab and OSO 7 observations of possible flare precursors and flare evolution are presented, and the evolution of the flare of Sept. 5, 1973, the most completely observed flare of the Skylab program, is described in detail, with account given to magnetic structures and H alpha radiation. Theories of the preflare state are then reviewed, with attention given to the force-free fields and coronal arcades, thermal and magnetic structures and the MHD stability of coronal loops.
    Keywords: SOLAR PHYSICS
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