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f-mode strengthening from a localised bipolar subsurface magnetic field
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Number of Authors: 62020 (English)In: Geophysical and Astrophysical Fluid Dynamics, ISSN 0309-1929, E-ISSN 1029-0419, Vol. 114, no 1-2, p. 196-212Article in journal (Refereed) Published
Abstract [en]

Recent numerical work in helioseismology has shown that a periodically varying subsurface magnetic field leads to a fanning of the f-mode, which emerges from a density jump at the surface. In an attempt to model a more realistic situation, we now modulate this periodic variation with an envelope, giving thus more emphasis on localised bipolar magnetic structures in the middle of the domain. Some notable findings are: (i) compared to the purely hydrodynamic case, the strength of the f-mode is significantly larger at high horizontal wavenumbers k, but the fanning is weaker for the localised subsurface magnetic field concentrations investigated here than the periodic ones studied earlier; (ii) when the strength of the magnetic field is enhanced at a fixed depth below the surface, the fanning of the f-mode in the diagram increases proportionally in such a way that the normalised f-mode strengths remain nearly the same in different such cases; (iii) the unstable Bloch modes reported previously in case of harmonically varying magnetic fields are now completely absent when more realistic localised magnetic field concentrations are imposed beneath the surface, thus suggesting that the Bloch modes are unlikely to be supported during most phases of the solar cycle; (iv) the f-mode strength appears to depend also on the depth of magnetic field concentrations such that it shows a relative decrement when the maximum of the magnetic field is moved to a deeper layer. We argue that detections of f-mode perturbations such as those being explored here could be effective tracers of solar magnetic fields below the photosphere before these are directly detectable as visible manifestations in terms of active regions or sunspots.

Place, publisher, year, edition, pages
2020. Vol. 114, no 1-2, p. 196-212
Keywords [en]
Magnetohydrodynamics (MHD), sun, helioseismology, turbulence, waves
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:su:diva-173118DOI: 10.1080/03091929.2019.1653461ISI: 000483377300001OAI: oai:DiVA.org:su-173118DiVA, id: diva2:1359416
Available from: 2019-10-09 Created: 2019-10-09 Last updated: 2020-01-03Bibliographically approved

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Käpylä, Maarit J.Brandenburg, Axel
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Nordic Institute for Theoretical Physics (Nordita)Department of Astronomy
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