Chromospheric oscillations observed with OSO 8. II. Average power spectra for Si II

Several hundred time series of Si II emission-line profiles at lambda816.93 and lambda817.45 have been analyzed to determine the mean power spectrum of chromospheric oscillations at a height of about 1200 km in the middle solar chromosphere. Time series of line intensity and Doppler shift measured on the solar disk have power spectra with very similar shapes, i.e., a broad power maximum extending from 2.5 to about 9 MHz superposed on a flat noise spectrum extending to beyond 30 MHz. Comparison of power spectra for quiet and active regions shows the broad peak near 3 MHz to narrow and shift to lower frequencies as the line strengths increase in plages and bright network elements. Statistical tests suggest that all of the power in the 3 MHz peak is of solar origin and that somewhat less than one-half of the flat noise spectrum arises from solar fluctuations. It is shown that gravity waves are unimportant in the middle chromosphere and that sound waves at frequencies above about 3 MHz should propagate. The total energy flux in solar oscillations at all frequencies up to 30 mHz is estimated to be 1 x 104 ergs cm-2 s-2 for the average Sun, which is too small to heat the upper chromosphere and corona