Time-distance helioseismology: Noise estimation

L. Gizon, A. C. Birch

Research output: Contribution to journalArticle

Abstract

As in global helioseismology, the dominant source of noise in time-distance helioseismology measurements is realization noise due to the stochastic nature of the excitation mechanism of solar oscillations. Characterizing noise is important for the interpretation and inversion of time-distance measurements. In this paper we introduce a robust definition of travel time that can be applied to very noisy data. We then derive a simple model for the full covariance matrix of the travel-time measurements. This model depends only on the expectation value of the filtered power spectrum and assumes that solar oscillations are stationary and homogeneous on the solar surface. The validity of the model is confirmed through comparison with SOHO MDI measurements in a quiet-Sun region. We show that the correlation length of the noise in the travel times is about half the dominant wavelength of the filtered power spectrum. We also show that the signal-to-noise ratio in quiet-Sun travel-time maps increases roughly as the square root of the observation time and is at maximum for a distance near half the length scale of supergranulation.

Original languageEnglish (US)
Pages (from-to)472-489
Number of pages18
JournalAstrophysical Journal
Volume614
Issue number1 I
DOIs
StatePublished - Oct 10 2004

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Keywords

  • Methods: Data analysis
  • Sun: Granulation
  • Sun: Helioseismology
  • Sun: Interior
  • Sun: Oscillations
  • Waves

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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