Nonasymptotic helioseismic inversion: iterated seismic solar model

Abstract

A nonasymptotic technique based on the variational formulation of the equations of stellar oscillations is used for helioseismic inversion. The resulting nonlinear integral equations are solved iteratively to obtain a seismic model of the Sun. Tests using pairs of known solar models show that this technique is capable of determining the sound speed to an accuracy of better than 0.2% and the density to an accuracy of better than 1% in most of the solar interior. It is demonstrated that the resulting seismic solar model is not particularly sensitive to the choice of regularization parameter. Influence of systematic differences in the observed frequencies is studied to find that it is probably the dominant source of uncertainties in helioseismic inversions. The seismic solar model is found to be close to a standard solar model including the diffusion of helium and heavy elements. Further, just below the HeII ionization zone OPAL equation of state is found to be in better agreement with that of solar material as compared to the MHD equation of state.