The double-ridge structure of the high-frequency time-distance crosscorrelation function in local helioseismology
Abstract
We model helioseismic high-frequency cross-correlation function and carry out comparison with observational data.We also discuss the source depth of the acoustic waves, one of the model parameters.It has been reported that when time-distance analysis is applied to the high-frequency acoustic waves, with frequencies above the critical cutoff frequency, time-distance cross-correlation function exhibits double-ridge structure. It has been pointed out, however, that in such analyses subcritical components (frequency < 5.3 MHz) may not be completely filtered out, and a hypothesis is that the double ridges are generated as artificial interference patterns of the subcritical waves and the supercritical waves. We test this hypothesis using SDO/HMI data.The data are put through a frequency filter before the cross-correlation function is computed. We vary the width and central frequency of the filter and examine when double ridges appear. When both the supercritical and the subcritical components are present in the filtered power spectrum, double ridges appear. When there is only one of the components, however, double ridges do not appear, confirming that interference between the two components is necessary for the double ridges.Next, we construct a simple model of cross-correlation function by ray-tracing the waves generated at a certain depth. The model reproduces the double-ridge structure well, indeed by interference between the supercritical part and the subcritical part, each of which by itself exhibits only a single ridge. We find that the successful reproduction of the observations depends sharply on the source depth of the acoustic wave, one of the input parameters to the model.This indicates a possibility that we can measure the source depth of the acoustic waves precisely, using the double ridges.
- Publication:
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AAS/Solar Physics Division Abstracts #48
- Pub Date:
- August 2017
- Bibcode:
- 2017SPD....4810902K