"5-minute" solar oscillations observed in the continuum: Simultaneous three-wavelength photometric measurements with a ground-based instrument

Abstract

This dissertation reports on the first "5-minute" solar oscillation observations obtained with the Photometric Telescope, an instrument constructed in the Department of Physics at the University of Arizona. The instrument is designed to simultaneously acquire spatially resolved intensity images of the solar surface at three wavelengths in the solar continuum: 507 nm, 747 nm, and 1600 nm. Images were recorded at an approximate 30 second cadence, weather permitting, for an approximate 3 month duration during the Spring of 1999. A central rectangular region of the solar disk spanning ±0.53 R(⊙) in a direction parallel to the solar equator and ±0.636 R(⊙) in a direction parallel to the solar axis is used in obtaining the "5-minute" results. Differential spatial filters utilizing the natural logarithm are developed. Proper design of these filters allows great reductions in the sensitivity to terrestrial atmospheric fluctuations as well as instrumental noise sources such as image fitter. The processing techniques utilized allow the simultaneous observation of solar oscillations in the 2500 to 3600 μHz region at each of the three instrumental wavelengths. The spatial filters used have high sensitivity to modes in the ℓ = 4 to ℓ = 8 range. One-day power spectra from 40 long observing days are averaged. Concurrent data from the SOHO satellite's Luminosity Oscillation Imager is analyzed in a similar manner showing results in outstanding agreement with the Photometric Telescope spectra. Further comparison of measured power spectra peak locations with theoretically predicted peak locations verifies the Photometric Telescope as a capable helioseismology instrument. New "5-minute" oscillation results are also presented. The amplitudes of the individual "5-minute" oscillations are on the order of 10-100 ppm, in agreement with previous amplitude measurements. While these amplitudes vary greatly depending on the details of the stochastic excitation, the oscillation amplitude ratios and phase differences of solar oscillations for the 507 nm, 747 nm, and 1600 nm wavelengths can be measured with a high degree of accuracy. This dissertation reports the first such measurements. The amplitude ratios (I'/I)₇₄₇/(I'/I)₅₀₇ ∼ 0.6 and (I'/I)₁₆₀₀/(I'/I)₅₀₇) ∼ 0.25 are found to be independent of frequency over the frequency region studied and nearly independent of the angular degree of the mode. By contrast, the relative phase differences (φ₇₄₇ - φ₅₀₇) and (φ₁₆₀₀ - φ₅₀₇) are found to have a significant frequency dependence and to depend somewhat sensitively on the angular degree of the mode. The measured wavelength dependent amplitude and phase relationships provide an invaluable diagnostic tool which can be used in future work to help identify longer period and lower amplitude oscillatory modes.