Signal-to-noise enhancement in ground-based intensity observations of solarp modes

Intensity observations of solarp modes are needed to form a complete picture of wave propagation in the photosphere. Ground-based intensity observations are severely hampered by terrestrial atmospheric noise. Partial cancellation of the noise power can be achieved if two spectra having disparate signal-to-noise ratios, and based on time series acquired simultaneously at the same site, are combined. A method of combining the spectra is suggested in which one amplitude is scaled and subtracted from the other. The result is squared yielding a positive-definite power density. To test the method, the intensity of light scattered by the Earths atmosphere was recorded at fifteen-second intervals in two narrow bands centered on 0.5 and 1.6. When the two resulting spectra were combined, the noise power was attenuated by a factor of 2.7. The scale factor was varied about its optimum value, revealing that noise peaks have a different signature than signal peaks, and opening up the possibility of a new tool in discrimination against noise peaks. Maxima at symmetry-allowed frequencies and minima at symmetry-forbidden frequencies indicate that the probability that these results are obtained by chance is only 6.1 × 10^–4. The positions of these maxima and minima also support the solar-cycle dependent frequency shifts found by Pallé, Régulo, and Roca Cortés (1990a).