Chromospheric oscillations observed with OSO 8. IV. Power and phase spectra for C IV

OSO 8 time series of profiles of the C IV line at lambda1548 and for the continuum near lambda1900 are analyzed to obtain the properties of solar oscillations at the 100,000 K level (lower transition region) and to obtain phase delays between the temperature-minimum region and the lower transition region. A majority of the time delay between successive maxima of approximately (< or =100%) fluctuations with a mean time delay between successive maxima of approximately 5 minutes. In approximately 20% of the cases these low-amplitude fluctuations are periodic with maximum power in the 3--5 mMZ band. Similar periodic oscillations with the same frequency at lambda1900 and lambda1548 are consistent with the time delay expected for waves propagating vertically at the sound speed. Furthermore, the low-amplitude aperiodic fluctuations can be interpreted as sound waves whose periodicity has been destroyed by variable transit time through the chromosphere. Thus we conclude that sound waves are present at the 100,000 K level most of the time. In bright network and plage regions the amplitude of intensity fluctuations in these low-amplitude events is proportional to the intensity, which may be interpreted as evidence for shock waves. However, the fluctuation amplitude indicates that only very weak shocks, if any, are present. The energy flux carried in the combined periodic and aperiodic sound waves is estimated at approximately 1 x 10-4ergs cm-2s-1.Large-amplitude (> or approx. =200%) transients are frequently observed in C IV in plage and network regions, but they are still much less common than the low-amplitude fluctuations. A study of these large-amplitude events will be reported in a separate paper