Generation of Magnetohydrodynamic Waves in Low Solar Atmospheric Flux Tubes by Photospheric Motions

Mumford, S. J. and Fedun, V. and Erdélyi, Róbert (2015) Generation of Magnetohydrodynamic Waves in Low Solar Atmospheric Flux Tubes by Photospheric Motions. ASTROPHYSICAL JOURNAL, 799 (1). ISSN 1538-4357

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Abstract

Recent ground- and space-based observations reveal the presence of small-scale motions between convection cells in the solar photosphere. In these regions small-scale magnetic flux tubes are generated due to the interaction of granulation motion and background magnetic field. This paper studies the effects of these motions, on magnetohydrodynamic wave excitation from broadband photospheric drivers. Numerical experiments of linear magnetohydrodynamic wave propagation in a magnetic flux tube embedded in a realistic gravitationally stratified solar atmosphere between the photosphere and the low choromosphere (above β = 1) are performed. Horizontal and vertical velocity field drivers mimic granular buffeting and solar global oscillations. A uniform torsional driver as well as Archimedean and logarithmic spiral drivers mimic observed torsional motions in the solar photosphere. The results are analysed using a novel method for extracting the parallel, perpendicular and azimuthal components of the perturbations, which caters for both the linear and non-linear case. Employing this method yields the identification of the wave modes excited in the numerical simulations and enables a comparison of excited modes via velocity perturbations and wave energy flux. The wave energy flux distribution is calculated, to enable the quantification of the relative strengths of excited modes. The torsional drivers excite primarily Alfvén modes (≈ 60% of the total flux) with small contributions from the slow kink mode, and, for the logarithmic spiral driver, small amounts of slow sausage mode. The horizontal and vertical drivers excite primarily slow kink or fast sausage modes respectively, with small variations dependent upon flux surface radius.

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