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  • 1
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    Numerical simulations of microflare evolution in the solar transition region and corona (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Several observers report transient ultraviolet brightenings, often referred to as microflares, in the solar atmosphere. In this paper, the results are presented of a series of one-dimensional numerical simulations examining possible relationships between microflares and the generation of dynamical chromospheric and transition region features. Low-energy and medium-energy microflares eject long-lived cool, dense gas plugs into the corona, with the gas plug traversing the loop apex in the medium energy case. In the case of high-energy microflares, the gas plug is rapidly heated to the temperature of the surrounding corona, and the results resemble the dynamics occurring in standard solar flare thick-target electron beam models.
    Keywords: SOLAR PHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 381; 313-322
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  • 2
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    Emerging flux and flares in NOAA 7260 (1996)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: We have studied the relation between flux emergence and flare activity in the active region NOAA 7260, using images from the Soft X-ray Telescope (SXT) aboard the Yohkoh spacecraft and other supporting ground-based data. It is found that microflares start around the time of flux emergence as recorded in white-light data, which generally precedes a major flare by several hours. We interpret the microflares as due to fast reconnection that takes place intermittently in the slow reconnection stage while more energy is accumulated in preparation for a larger flare.
    Keywords: SOLAR PHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 17; 5-Apr; p. (4/5)201-(4/5)204
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  • 3
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    Nonlinear Parker instability of isolated magnetic flux in a plasma (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The nonlinear evolution of the Parker instability in an isolated horizontal magnetic-flux sheet embedded in a two-temperature layer atmosphere is studied by using a two-dimensional MHD code. In the solar case, this two-layer model is regarded as a simplified abstraction of the sun's photosphere/chromosphere and its overlying much hotter (coronal) envelope. The horizontal flux sheet is initially located in the lower temperature atmosphere so as to satisfy magnetostatic equilibrium under a constant gravitational acceleration. Ideal MHD is assumed, and only perturbations with k parallel to the magnetic-field lines are investigated. As the instability develops, the gas slides down the expanding loop, and the evacuated loop rises as a result of enhanced magnetic buoyancy. In the nonlinear regime of the instability, both the rise velocity of a magnetic loop and the local Alfven velocity at the top of the loop increase linearly with height and show self-similar behavior with height as long as the wavelength of the initial perturbation is much smaller than the horizontal size of the computing domain.
    Keywords: SOLAR PHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 338; 471-492
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  • 4
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    The structure of the coronal soft X-ray source associated with the dark filament disappearance of 1991 September 28 using the Yohkoh Soft X-ray Telescope (1992)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The structure of the coronal soft X-ray source associated with the dark filament disappearance on September 28, 1991, observed with the Soft X-ray Telescope, is examined as a possible example of the 'eruption-reconnection' model of filament disappearance. The results suggest, however, that this model may not fit. There is a strong possibility that much of the dark filament mass remains in the heated unwinding axial field.
    Keywords: SOLAR PHYSICS
    Type: PASJ: Publications of the Astronomical Society of Japan (ISSN 0004-6264); 44; 5; p. L205-L210.
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  • 5
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    Observations of X-ray jets with the Yohkoh Soft X-ray Telescope (1992)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The features of the multiple X-ray jets in the solar corona, revealed by the time series of the Yohkoh Soft X-ray Telescope images are described. The typical size of a jet was from 5 x 10 exp 3 to 4 x 10 exp 5 km, the translational velocity was 30-300 km/s, and the corresponding kinetic energy was estimated to be from 10 exp 25 to 10 exp 28 erg. Many of the jets were found to be associated with flares in X-ray bright points, emerging flux regions, or active regions, and they sometimes occurred several times from the same X-ray feature. One of the jets associated with a flaring bright point was identified as being an H-alpha surge.
    Keywords: SOLAR PHYSICS
    Type: PASJ: Publications of the Astronomical Society of Japan (ISSN 0004-6264); 44; 5; p. L173-L179.
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  • 6
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    Two-dimensional magnetohydrodynamic model of emerging magnetic flux in the solar atmosphere (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The nonlinear undular mode of the magnetic buoyancy instability in an isolated horizontal magnetic flux embedded in a two-temperature layered atmosphere (solar corona-chromosphere/photosphere) is investigated using a two-dimensional magnetohydrodynamic code. The results show that the flux sheet with beta of about 1 is initially located at the bottom of the photosphere, and that the gas slides down the expanding loop as the instability develops, with the evacuated loop rising as a result of enhanced magnetic buoyancy. The expansion of the magnetic loop in the nonlinear regime displays self-similar behavior. The rise velocity of the magnetic loop in the high chromosphere (10-15 km/s) and the velocity of downflow noted along the loop (30-50 km/s) are consistent with observed values for arch filament systems.
    Keywords: SOLAR PHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 345; 584-596
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  • 7
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    A gigantic coronal jet ejected from a compact active region in a coronal hole (1994)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: A gigantic coronal jet greater than 3 x 10(exp 5) km long (nearly half the solar radius) has been found with the soft X-ray telescope (SXT) on board the solar X-ray satellite, Yohkoh. The jet was ejected on 1992 January 11 from an 'anemone-type' active region (AR) appearing in a coronal hole and is one of the largest coronal X-ray jets observed so far by SXT. This gigantic jet is the best observed example of many other smaller X-ray jets, because the spatial structures of both the jet and the AR located at its base are more easily resolved. The range of apparent translational velocities of the bulk of the jet was between 90 and 240 km s(exp -1), with the corresponding kinetic energy estimated to be of order of 10(exp 28) ergs. A detailed analysis reveals that the jet was associated with a loop brightening (a small flare) that occurred in the active region. Several features of this observation suggest and are consistent with a magnetic reconnection mechanism for the production of such a 'jet-loop-brightening' event.
    Keywords: SOLAR PHYSICS
    Type: The Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 431; 1; p. L51-L53
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  • 8
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    Three-dimensional magnetohydrodynamics of the emerging magnetic flux in the solar atmosphere (1993)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The nonlinear evolution of an emerging magnetic flux tube or sheet in the solar atmosphere is studied through 3D MHD simulations. In the initial state, a horizontal magnetic flux sheet or tube is assumed to be embedded at the bottom of MHD two isothermal gas layers, which approximate the solar photosphere/chromosphere and the corona. The magnetic flux sheet or tube is unstable against the undular mode of the magnetic buoyancy instability. The magnetic loop rises due to the linear and then later nonlinear instabilities caused by the buoyancy enhanced by precipitating the gas along magnetic field lines. We find by 3D simulation that during the ascendance of loops the bundle of flux tubes or even the flux sheet develops into dense gas filaments pinched between magnetic loops. The interchange modes help produce a fine fiber flux structure perpendicular to the magnetic field direction in the linear stage, while the undular modes determine the overall buoyant loop structure. The expansion of such a bundle of magnetic loops follows the self-similar behavior observed in 2D cases studied earlier. Our study finds the threshold flux for arch filament system (AFS) formation to be about 0.3 x 10 exp 20 Mx.
    Keywords: SOLAR PHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 414; 1; p. 357-371.
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