Physics
Scientific paper
Nov 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003phdt.........9g&link_type=abstract
Thesis (PhD). STANFORD UNIVERSITY, Source DAI-B 64/05, p. 2226, Nov 2003, 183 pages.
Physics
10
Scientific paper
In order to better understand the origin and variability of stellar magnetic fields it is necessary to understand mass flows inside stars. With time-distance helioseismology local flows can be inferred in the Sun by measuring the time it takes for seismic waves to propagate between any two points on the solar surface. This dissertation contains new observations of solar plasma flows and a model for the interpretation of time- distance data. It also discusses the prospects for stellar seismology. First, we present new observations of the solar velocity field in the upper convection zone. Using surface-gravity waves, we discover that supergranulation exhibits wave- like properties, undergoing oscillations with periods of 6 9 days. This points to a mechanism involving traveling-wave convection and explains the observations of anomalously fast rotation of the supergranulation pattern. Near the solar surface we detect a large-scale 50 m/s flow converging toward active regions. Deeper inside the convection zone, we detect, using acoustic waves, bands of slower and faster meridional motion with a period of eleven years. The data for this study were obtained with SOHO/MDI. Second, we present a new and physically motivated general framework for calculations of the sensitivity of travel times to small local perturbations to a solar model, taking into account the fact that the sources of solar oscillations are spatially distributed. We employ the first Born approximation to model scattering from local inhomogeneities and we use a clear and practical definition of travel-time perturbation which allows a connection between observations and theory. After developing the general theory we compute the sensitivity of surface-gravity-wave travel times to local perturbations in the wave excitation and damping rates. We show that the simple single-source picture, employed in most time-distance analyses, is not correct as it does not reproduce all of the features seen in the distributed-source sensitivity kernels. Last, we show that future observations of stellar pulsations will provide us with the possibility of determining the angular velocity of a Sun-like star and the inclination angle between the direction of the rotation axis and the line of sight. Measuring the inclination angle is useful to determine the true masses of extra-solar planets detected from the radial velocity shifts of their central star.
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