Frequencies and Amplitudes of HighDegree Solar Oscillations.
Abstract
Measurements of some of the properties of high degree solar p and fmode oscillations are presented. Using highresolution velocity images from Big Bear Solar Observatory, we have measured mode frequencies, which provide information about the composition and internal structure of the Sun, and mode velocity amplitudes (corrected for the effects of atmospheric seeing), which tell us about the oscillation excitation and damping mechanisms. We present a new and more accurate table of the Sun's acoustic vibration frequencies, nu _{rm nl}, as a function of radial order n and spherical harmonic degree l. These frequencies are averages over azimuthal order m and approximate the normal mode frequencies of a nonrotating, spherically symmetric Sun near solar minimum. The frequencies presented here are for solar p and fmodes with 180 <= l <= 1920, 0 <= n <= 8, and 1.7 mHz <= nu_{ rm nl} <= 5.3 mHz. The uncertainties, sigma_ {rm nl}, in the frequencies are as low as 3.1 muHz. The theoretically expected fmode frequencies are given by omega ^2 = gk_ h ~ gl/R_odot , where g is the gravitational acceleration at the surface, k_ h is the horizontal component of the wave vector, and R_odot is the radius of the Sun. We find that the observed frequencies are significantly less than expected for l > 1000, for which we have no explanation. Observations of highdegree oscillations, which have very small spatial features, suffer from the effects of atmospheric image blurring and image motion (or "seeing"), thereby reducing the amplitudes of their spatialfrequency components. In an attempt to correct the velocity amplitudes for these effects, we have simultaneously measured the atmospheric modulation transfer function (MTF) by looking at the effects of seeing on the solar limb. We are able to correct the velocity amplitudes using the MTF out to l ~ 1200. We find that the frequency of the peak velocity power (as a function of l) increases with l. We also find that the mode energy is approximately constant out to l ~ 200, at which point it begins to decrease. Mode energy is expected to be constant as a function of l if the modes are excited by stochastic interactions with convective turbulence in the solar convection zone. Finally, we discuss the accuracy of the seeing correction and a test of the correction using the 1989 March 7 partial solar eclipse.
 Publication:

Ph.D. Thesis
 Pub Date:
 1991
 Bibcode:
 1991PhDT.........3K
 Keywords:

 Physics: Astronomy and Astrophysics;
 Amplitude Modulation;
 Atmospheric Optics;
 Image Motion Compensation;
 Modulation Transfer Function;
 Resonant Frequencies;
 Solar Oscillations;
 Sound Waves;
 Vibration Mode;
 Convective Flow;
 Solar Eclipses;
 Solar Interior;
 Solar Limb;
 Stellar Structure;
 Stochastic Processes;
 Turbulence Effects;
 Velocity Measurement;
 Vibration Damping;
 Solar Physics