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  f-mode strengthening from a localised bipolar subsurface magnetic field

Singh, N. K., Raichur, H., Käpylä, M. J., Rheinhardt, M., Brandenburg, A., & Käpylä, P. J. (2020). f-mode strengthening from a localised bipolar subsurface magnetic field. Geophysical and Astrophysical Fluid Dynamics, 114(1-2), 196-212. doi:10.1080/03091929.2019.1653461.

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Item Permalink: http://hdl.handle.net/21.11116/0000-0005-EE4D-A Version Permalink: http://hdl.handle.net/21.11116/0000-0006-4732-2
Genre: Journal Article

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Singh, Nishant K.1, Author              
Raichur, Harsha1, Author
Käpylä, Maarit J.2, Author              
Rheinhardt, Matthias, Author
Brandenburg, Axel, Author
Käpylä, Petri J., Author
Affiliations:
1Max Planck Research Group and ERC Consolidator Grant: Solar and Stellar Dynamos - SOLSTAR, Max Planck Institute for Solar System Research, Max Planck Society, Justus-von-Liebig-Weg 3, 37077 Göttingen, DE, ou_2265638              
2Department Sun and Heliosphere, Max Planck Institute for Solar System Research, Max Planck Society, ou_1832289              

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 Abstract: 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 kω 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.

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Language(s): eng - English
 Dates: 2020
 Publication Status: Published in print
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 Rev. Method: Peer
 Identifiers: DOI: 10.1080/03091929.2019.1653461
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Title: Geophysical and Astrophysical Fluid Dynamics
Source Genre: Journal
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Publ. Info: New York : Gordon and Breach Science Publishers.
Pages: - Volume / Issue: 114 (1-2) Sequence Number: - Start / End Page: 196 - 212 Identifier: ISSN: 0309-1929
CoNE: https://pure.mpg.de/cone/journals/resource/958480220815