Solar Physics

, Volume 287, Issue 1–2, pp 149–159 | Cite as

Analysis of the Helioseismic Power-Spectrum Diagram of a Sunspot

SOLAR DYNAMICS AND MAGNETISM

Abstract

The continuous high spatial resolution Doppler observation of the Sun by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager allows us to compute a helioseismic kω power-spectrum diagram using only oscillations inside a sunspot. Individual modal ridges can be clearly seen with reduced power in the kω diagram that is constructed from a 40-hour observation of a stable and round sunspot. Comparing this with the kω diagram obtained from a quiet-Sun region, one sees that inside the sunspot the f-mode ridge is more reduced in power than the p-mode ridges, especially at high wavenumbers. The p-mode ridges all shift toward lower wavenumber (or higher frequency) for a given frequency (or wavenumber), implying an increase of phase velocity beneath the sunspot. This is probably because the acoustic waves travel across the inclined magnetic field of the sunspot penumbra. Line-profile asymmetries exhibited in the p-mode ridges are more significant in the sunspot than in the quiet Sun. Convection inside the sunspot is also highly suppressed, and its characteristic spatial scale is substantially larger than the typical convection scale of the quiet Sun. These observational facts demand a better understanding of magnetoconvection and interactions of helioseismic waves with magnetic field.

Keywords

Sun: helioseismology Sun: oscillations Sun: sunspots 

Notes

Acknowledgements

SDO is a NASA mission, and HMI project is supported by NASA contract NAS5-02139. We thank Mark Cheung for useful discussions on convection scales inside sunspots. We also thank the anonymous referee for useful comments that improved the quality of this article.

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Copyright information

© Springer Science+Business Media Dordrecht 2012

Authors and Affiliations

  1. 1.W. W. Hansen Experimental Physics LaboratoryStanford UniversityStanfordUSA
  2. 2.Physics DepartmentNational Tsing Hua UniversityHsinchuTaiwan

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