Computer Science – Sound
Scientific paper
Aug 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt.........7n&link_type=abstract
Thesis (PhD). STANFORD UNIVERSITY, Source DAI-B 61/02, p. 892, Aug 2000, 116 pages.
Computer Science
Sound
Scientific paper
The Sun is permeated by acoustic oscillations. The findings in this dissertation address the characteristics of the source exciting these waves and is consistent with the following proposed excitation mechanism: blobs of hot gas continually rise in the outer layer of the convection zone where they are cooled and collapse. This volume change results in monopolar emission of sound. Cool, dense parcels of gas then accelerate downward into the intergranular lanes and lead to dipolar acoustic emission due to the monopole source. Finally, the void left behind by the downflow is filled by horizontal flow resulting in Reynolds stresses which produce quadrupolar emission. During this process of acoustic excitation by turbulent convection there is photospheric darkening seen in the intensity observations. Power spectra of these oscillations obtained with the Michelson Doppler Imager instrument on-board the Solar and Heliospheric Observatory are asymmetric about their central peaks. At frequencies above the acoustic cutoff frequency, the asymmetry is reduced. Surprisingly, a reversal in asymmetry is seen, along with a high frequency shift between velocity and intensity; where the velocity power drops off rapidly compared to the intensity power. The observed phase difference between velocity and intensity jumps in the vicinity of an eigenfrequency and is not 90° as predicted by adiabatic theory of oscillations below the acoustic cutoff frequency. The granulation signal is partially correlated with the oscillations, observed as photospheric darkening, and is related to the strength of the acoustic source. A model to explain the observed power spectra and the phase difference shows that the correlated signal is higher in intensity than in velocity. A novel asymmetric formula is derived and used to fit the power spectra, thus allowing accurate determination of the eigenfrequencies, resulting in more precise information about the solar interior and rotation. Finally, different types of excitation sources at various depths are studied, and a best match with observations occur when monopole and quadrupole acoustic sources are placed in the superadiabatic layer at a depth of 75 km below the photosphere where the turbulence is most intense and consistent with the proposed excitation mechanism.
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