LIMITS ON CORONAL REFLECTION USING HIGH-FREQUENCY SOLAR OSCILLATIONS

Acoustic waves in the Sun with frequencies above about 5.3 mHz can pro pagate in the chromosphere. We examine imaged solar intensity data for evidence of reflection of these waves in the upper chromosphere, wher e the temperature increases by a large factor over a short distance. O ur method is to compare the observed and theoretically derived frequen cy spacings between peaks in the power spectrum. We find that our theo retical frequencies provide the best fit to the data when the reflecti on in the upper atmosphere is eliminated. In particular, the model of Kumar (1993b), which includes the source depth, and radiative damping, in the calculation of power spectra but ignores chromospheric reflect ion, gives peak frequencies that are in good agreement with the observ ations. For acoustic waves of frequency greater than 6 mHz we put an u pper limit to the reflectivity of chromosphere and corona, using our m ethod, of about 10%. At a given spherical harmonic degree, the frequen cy spacing between peaks in the data generally decreases with increasi ng frequency, because the lower turning point of the waves is moving i nward. However, between 5 and 5.5 mHz the frequency spacing increases slightly. This feature is probably associated with the acoustic cutoff frequency in the solar atmosphere, i.e., it indicates a transition fr om trapped waves to propagating waves. We are able to reproduce the ob served behavior by a crude modeling of the solar atmosphere. Further s tudy of these peaks should provide an independent way of exploring the mean structure of the solar atmosphere, particularly around the tempe rature minimum region.