Local Helioseismology

Abstract

We review the current status of local helioseismology, covering both theoretical and observational results. After a brief introduction to solar oscillations and wave propagation through inhomogeneous media, we describe the main techniques of local helioseismology: Fourier-Hankel decomposition, ring-diagram analysis, time-distance helioseismology, helioseismic holography, and direct modeling. We discuss local helioseismology of large-scale flows, the solar-cycle dependence of these flows, perturbations associated with regions of magnetic activity, and solar supergranulation.

Modeling the Subsurface Structure of Sunspots

“…time -distance helioseismology, helioseismic holography, acoustic imaging, ring-diagram analysis; see Gizon and Birch, 2005 and references therein) is the only means by which we can investigate subphotospheric structure. However, interpretations of data have been somewhat ambiguous and inconsistent for applications of local-helioseismic methods in solar active regions.…”

“…Helioseismology has shown (see Gizon and Birch, 2005 for a review) that wave propagation is different in sunspots in the quiet Sun. For example, single-bounce wave travel times are altered by (order of magnitude) about thirty seconds; the details depend strongly on the data-analysis filters, the first-bounce distance, and the wave frequency (e.g.…”

Time – Distance Analysis of the Emerging Active Region NOAA 10790

“…The obtained set of cross-correlation functions is then averaged spatially over the quiet Sun to obtain an estimate for the reference cross-correlation function, C ref (∆ i , t). This is used to measure one-way travel-time perturbations at each location following Gizon and Birch (2002, also Jensen and Pijpers, 2003 approach:…”

“…The errors due to realisation noise are estimated by processing the 24 quietSun datacubes, calculating noise covariance matrix in travel-time measurements (Jensen, 2001;Gizon and Birch, 2004;Couvidat, Gizon, Birch et al, 2005), and then evaluating noise in inversions according to (Jensen, 2001). …”

“…The travel-time estimates are obtained by cross-correlating and averaging the Dopplervelocity signal at different locations at the Sun's surface and then obtaining the crosscorrelation function. The travel times are then computed from cross-correlations either via Gabor wavelet fitting or by comparing with a reference cross-correlation function (see Gizon and Birch, 2005 and references therein). Because of the stochastic nature of solar oscillations, substantial spatial and temporal averaging of the data is required to measure the frequencies and travel times accurately.…”