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  • SOLAR PHYSICS  (8)
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  • 1
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    Non-local transport in turbulent MHD convection (1995)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The nonlocal non-diffusive transport of passive scalars in turbulent magnetohydrodynamic (MHD) convection is investigated using transilient matrices. These matrices describe the probability that a tracer particle beginning at one position in a flow will be advected to another position after some time. A method for the calculation of these matrices from simulation data which involves following the trajectories of passive tracer particles and calculating their transport statistics, is presented. The method is applied to study the transport in several simulations of turbulent, rotating, three dimensional compressible, penetrative MDH convection. Transport coefficients and other diagnostics are used to quantify the transport, which is found to resemble advection more closely than diffusion. Some of the results are found to have direct relevance to other physical problems, such as the light element depletion in sun-type stars. The large kurtosis found for downward moving particles at the base of the convection zone implies several extreme events.
    Keywords: SOLAR PHYSICS
    Type: ESA, Proceedings of 4th SOHO on Helioseismology. Volume 2: Posters; p 253-258
    Format: text
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  • 2
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    Is there a seismic signature of the sun's magnetic field? (1995)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Both weak magnetic fields and latitudinally dependent acoustic perturbations remove the degeneracy of the azimuthal quantum number, m, of acoustic modes of otherwise spherically symmetrical solar model. In the case of acoustic perturbations, the degeneracy is removed because the range of latitude in which a mode propagates depends on m, and therefore modes of like principle order n and degree l sample the aspherical scalar sound speed distribution differently. In the magnetic case, the removal of the degeneracy is caused by the same geometrical effect, and is influenced by the anisotropy of the Lorentz forces. Asymptotic analysis is used to show that the frequency splittings cannot be unambiguously attributed to the direct effect of a magnetic field, and that the effect of such a field on frequency splittings can be reproduced by a perturbation to the sound speed.
    Keywords: SOLAR PHYSICS
    Type: ESA, Proceedings of 4th SOHO on Helioseismology. Volume 2: Posters; p 73-76
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  • 3
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    Changes in the upper turning point due to magnetism (1995)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: It was observed that the p-mode power is substantially suppressed in magnetic regions. One possible explanation is that the upper turning point, the acoustic cut-off point of the solar p-modes is lowered in the presence of a magnetic field. A related possibility is that the attenuation length scale in the evanescent region is reduced in the presence of a magnetic field. It is likely that the observations sample a different position in the evanescent tails of the eigenfunctions in magnetic regions because of different temperature structures in these regions. A model is used to quantify the first of these effects.
    Keywords: SOLAR PHYSICS
    Type: ESA, Proceedings of 4th SOHO on Helioseismology. Volume 2: Posters; p 63-67
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  • 4
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    Driven acoustic oscillations within a vertical magnetic field (1995)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The effects of a vertical magnetic field on p-mode frequencies, line widths, and eigenfunctions, are examined. A solar model, consisting of a neutrally stable polytropic interior matched to an isothermal chromosphere, is applied. The p-modes are produced by a spatially distributed driver. The atmosphere is threaded by a constant vertical magnetic field. The frequency shifts due to the vertical magnetic field are found to be much smaller than the shifts caused by horizontal fields of similar strength. A large vertical field of 2000 G produces shifts of several nHz. It is found that the frequency shifts decrease with increasing frequency and increase with field strength. The coupling of the acoustic fast mode to the escaping slow modes is inefficient. Constant vertical magnetic field models are therefore incapable of explaining the high level of absorption observed in sunspots and plage.
    Keywords: SOLAR PHYSICS
    Type: ESA, Proceedings of 4th SOHO on Helioseismology. Volume 2: Posters; p 77-82
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  • 5
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    Magnetohydrodynamic waves and particle acceleration in a coronal loop (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A model is presented for the acceleration of electrons in a flaring solar coronal loop. It is suggested that the primary energy release event in the flare establishes a spectrum of MHD waves within the loop which accelerate electrons by wave-particle resonant interactions. The nature of the MHD oscillations are described and numerical and analytic solution of the dispersion relation are examined. It is found that the behavior of a particle depends on whether or not it is in resonance with the wave spectrum. Resonant particles follow chaotic trajectories and can diffuse in velocity space between wave effective potentials, thereby gaining energy.
    Keywords: SOLAR PHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 336; 1059-107
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  • 6
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    Evolution to nonequilibrium in simple models of prominence filaments (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The support of cold plasma against gravity by a line-tied magnetic field is considered. The normal component of the field at the boundaries, the mass/unit flux, and the magnetic shear are all specified. Fixing the first two of these quantities, and varying the third leads to evolutionary sequences. When the magnetic shear is less than a critical value, an infinite number of solutions exist. When the magnetic shear is sufficiently large, there are no solutions with the desired topology. These results support the idea that prominences erupt as a result of evolution to magnetic nonequilibrium.
    Keywords: SOLAR PHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 376; 761-770
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  • 7
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    The formation of current sheets in the solar atmosphere (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A simple model of the effect of slow photospheric motions on the coronal magnetic field is examined. In this model, a magnetic field is stretched between two conducting plates and slow tangential displacements of the plates cause the field to evolve quasi-statically. If the field is treated as a continuum, the interior magnetic topology is uniquely specified by the displacement of the boundaries, while the topology of a field made up of discrete flux tubes is not determined by the boundary displacement. Treating the field as a continuum, it is found that small perturbations of a constant vertical or weakly sheared field can come to an equilibrium free of singularities if the separation between the plates is finite.
    Keywords: SOLAR PHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 312; 423-430
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  • 8
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    Electron acceleration by magnetosonic waves in solar flares (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Results of a simulation of electron acceleration by magnetosonic waves in a solar coronal loop are presented. The dependence of the energization rate on the strength and shape of the wave spectrum is discussed. Comparisons are made with the predictions of quasilinear theory and of a model describing stochastic acceleration by waves as isotropic diffusion in momentum space.
    Keywords: SOLAR PHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 17; 2051-205
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