Seismic Analysis of the Sun from Intermediate and High-Degree P-Modes

Solar oscillations provide a unique tool to investigate the internal structure and dynamics of the Sun. While accurate measurements of the solar acoustic spectrum characteristics have been performed for low- and intermediate-degree modes, only a limited set of short time baseline measurements for high-degree modes has been available (l > 120). The purpose of this work is to provide an accurate and more reliable set of measurements for these high-degree modes. High spatial resolution (2.2 arc sec per pixel) full-disk dopplergrams of the solar surface have been acquired at the Mt. Wilson 60-feet solar tower during the summer of 1988, using a doppler analyzer based on the sodium magneto -optical filter. Twenty consecutive days of observations have been reduced to spherical harmonic coefficient time series, for degrees l<=q 600. The time series were then Fourier transformed in order to estimate the corresponding power spectra. From these power spectra, frequency splittings have been estimated for degree 20 <=q l <=q 600. At low and intermediate degree (l <=q 120), individual modes could be isolated, and hence reliable tesseral measurements have been obtained. At high-degree (l > 120), the presence of mode blending introduced systematic errors in the tesseral frequency splitting determination. The nature of these systematics has been carefully studied, but our simulations were unable to reproduce in detail the observed systematics, and therefore, an ad hoc procedure was developed to correct for them. Alternatively, sectoral frequency splittings, shown to be less sensitive to mode blending systematics, have provided a more reliable estimate of the frequency splitting for these high-degree modes. From collapsed < m = 0>^ectra modal frequencies, amplitudes and lifetimes have been estimated for 20 <=q l <=q 600. The required corrections for mode blending, point spread function attenuation, and finite observing run have been applied. These new measurements are compared with previous determinations and theoretical predictions. Finally, based on the frequency splittings obtained from the present study, as well as contemporaneous independent measurements, we have inferred the internal rotation rate as a function of depth in the equatorial regions, and discuss some of the implications of such a profile in relation to solar dynamo, global circulation and evolutionary models of rotating stars.