The detection of low-degree solar gravity mode oscillations by differential radius observations, 1986-1987.

Abstract

Solar oscillations are observed by examination of time-varying properties of the photosphere. Over 800 hours of solar limb intensity profile observations in the equatorial region were collected during 1986 and 1987 at the SCLERA Mt. Bigelow site and an analysis using the FFTD was performed in an effort to detect low-degree g-mode oscillation signals in the solar photosphere. Fourier analysis of the data yielded power spectra whose peak frequencies and overall power were tested for the presence of g-mode signals at 245 classified low-degree g-mode eigenfrequencies of Hill and Gu (1990). Significant results arose from comparison of the detected eigenmode power in the 1979 data to the power density at those same classified eigenfrequencies in spectra from 1986-87 data. Also, significant correlation was found between the earlier and later mode power distributions using several different methods. Use of the parameter D(i,j) to characterize the horizontal spatial behavior of eigenfunctions yielded significant results consistent with its known solar properties. Also, the observed relative signal power densities at those classified eigenfrequencies were found to be consistent with the theoretical development of Berthomieu and Provost (1990), and with previous SCLERA observations. It was noted that overall detected oscillation amplitudes in the photosphere are on average 4.0 $\times$ 0.6 times those observed in 1979, and the signal-to-noise ratio is substantially worse. The effects of instrumental defects, solar active regions, and terrestrial atmospheric noise were found to be minimal. Substantial agreement with the results of analysis of 1979 SCLERA data provides strong evidence of the correctness of the low-order g-mode classifications. The increase in g-mode signal power implied by these data, together with the other results, suggests that the photospheric behavior of these modes is variable, and perhaps solar-cycle dependent.