Wave propagation in the sun and the interpretation of helioseismic data

Details Wave propagation in the sun and the interpretation of helioseismic data Wave propagation in the sun and the interpretation of helioseismic data

Oct 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt........17b&link_type=abstract

Thesis (PhD). STANFORD UNIVERSITY, Source DAI-B 63/04, p. 1888, Oct 2002, 170 pages.

Computer Science

Sound

5

Scientific paper

In order to better understand the physics of the Sun it is necessary to obtain observational constraints on the temporal variations of the flows and structures in the interior of the Sun. Helioseismology, the use of waves to probe the solar interior, has been developed over the course of several decades and has led to numerous exciting results. This dissertation is mainly an effort to develop the theory necessary for the interpretation of time-distance helioseismology data. The results contained in this dissertation can be placed in three broad categories: oscillations in magnetized atmospheres, models for the interpretation of time- distance helioseismology data, and observational results derived using global mode and time-distance helioseismology. We show that in magnetized polytrope atmospheres the magnetic modes can become unstable in the vicinity of avoided crossings. Time-distance helioseismology, which measures the travel times for wave packets moving between distinct points on the solar surface, has provided many intriguing results. It is not yet well understood how to interpret the data, i.e. how to relate subsurface perturbations to the observed travel times. We demonstrate that the Born approximation agrees well with direct numerical results for weak sound-speed perturbations. We also develop a general recipe, using a physically motived distributed wave-source model, for the calculation of the sensitivity of travel times to subsurface perturbations. We perform Optimally Localized Averaging (OLA) inversions of MDI and GONG normal-mode frequency splittings to estimate a near-surface average of the north-south symmetric component of rotation. Closer than 20° to the poles we obtain a rotation rate that is 10 nHz slower than would be expected from a smooth extrapolation from lower latitudes. We show synoptic charts, for three Carrington rotations, of sound-speed throughout the convection zone, inferred by the inversion of deep- focusing time-distance data. These charts show what may be structures in the deep convection zone or may be the effect of near-surface magnetic field.

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