Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003apj...593.1242t&link_type=abstract
The Astrophysical Journal, Volume 593, Issue 2, pp. 1242-1248.
Astronomy and Astrophysics
Astronomy
8
Stars: Atmospheres, Sun: Atmosphere, Sun: Helioseismology
Scientific paper
We investigate the effect of sound-speed perturbations on the characteristics of helioseismic signals in order to demonstrate the feasibility and desirability of wave field helioseismology. The oscillatory nature of the signals with varying frequency content allows us to characterize the waveform of the signals by the amplitudes of the peaks and troughs as well as by the time lags between successive peaks and troughs. Sinusoidal sound-speed perturbations with an amplitude of 1% of the local sound speed produce more than +/-10% changes in the amplitudes of the peaks and troughs. The same sound-speed perturbations produce changes in the time lags between successive peaks and troughs of the signals that are comparable with the variations of first-break travel times of the oscillatory wave packet. The vertical and horizontal sound-speed perturbations result in distinct patterns of changes in the shape of the helioseismic signals. These changes in the amplitudes and travel-time lags are differential for the successive peaks and troughs: the magnitude is higher for the later peaks and troughs. We also observe that these variations in the amplitude and travel-time lags are more sensitive to shorter wavelength sound-speed perturbations at relatively small source-receiver offsets. Therefore, our results indicate that the waveform of the signals is variable and sensitive to sound-speed perturbations. These observations can be explained by the more dispersive nature of the low-frequency components in the seismic signals at shallow depths. The relatively high sensitivity of these perturbations in the waveform of the signals to sound speed, density, and vertical density gradient at shallow depths due to dispersion is an important modeling consideration.
Pain Christopher C.
Thompson Michael J.
Tong C. H.
Warner Michael R.
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