Issue A&A Volume 460, Number 3, December IV 2006 Page(s) 893 - 899 Section The Sun DOI 10.1051/0004-6361:20065313

A&A 460, 893-899 (2006)
DOI: 10.1051/0004-6361:20065313

On the period ratio P /2P in the oscillations of coronal loops

M. P. McEwan, G. R. Donnelly, A. J. Díaz, and B. Roberts

School of Mathematics and Statistics, University of St Andrews, St Andrews, Fife KY169SS, Scotland
    e-mail: mike@mcs.st-and.ac.uk

(Received 29 March 2006 / Accepted 11 September 2006 )

Abstract
Aims.With strong evidence of fast and slow magnetoacoustic modes arising in the solar atmosphere there is scope for improved determinations of coronal parameters through coronal seismology. Of particular interest is the ratio  P₁/2P₂ between the period P₁ of the fundamental mode and the period P₂ of its first harmonic; in an homogeneous medium this ratio is one, but in a more complex configuration it is shifted to lower values.
Methods.We consider analytically the effects on the different magnetohydrodynamic modes of structuring and stratification, pointing out that transverse or longitudinal structuring or gravitational stratification modifies the ratio  P₁/2P₂.
Results.The deviations caused by gravity and structure are studied for the fast and slow modes. Structure along the loop is found to be the dominant effect.
Conclusions.The departure of P₁/2P₂ from unity can be used as a seismological tool in the corona. We apply our technique to the observations by Verwichte et al. (2004), deducing the density scale height in a coronal loop.

Key words: Sun: oscillations -- Sun: magnetic fields -- Sun: corona

© ESO 2006

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