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Title:
Local magnetohelioseismology of active regions
Authors:
Cally, P. S.
Affiliation:
AA(Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800, Australia;)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 358, Issue 2, pp. 353-362. (MNRAS Homepage)
Publication Date:
04/2005
Origin:
MNRAS
MNRAS Keywords:
Sun: helioseismology, Sun: magnetic fields, sunspots
DOI:
10.1111/j.1365-2966.2005.08742.x
Bibliographic Code:
2005MNRAS.358..353C

Abstract

Solar active regions are distinguished by their strong magnetic fields. Modern local helioseismology seeks to probe them by observing waves which emerge at the solar surface having passed through their interiors. We address the question of how an acoustic wave from below is partially converted to magnetic waves as it passes through a vertical magnetic field layer where the sound and Alfvén speeds coincide (the equipartition level), and find that (i) there is no associated reflection at this depth, either acoustic or magnetic, only transmission and conversion to an ongoing magnetic wave; and (ii) conversion in active regions is likely to be strong, though not total, at frequencies typically used in local helioseismology, with lower frequencies less strongly converted. A simple analytical formula is presented for the acoustic-to-magnetic conversion coefficient.

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