ACOUSTIC INTERFEROMETRY OF THE SOLAR ATMOSPHERE - P-MODES WITH FREQUENCIES NEAR THE ACOUSTIC CUTOFF

High-frequency p-mode intensity data, obtained from the South Pole in 1987, 1988, 1990 and 1994, show a sharp variation in the phase-shift f unction and in the frequency spacings near 5.5 mHz. Using a simple the oretical model, we demonstrate that this behaviour is caused by an aco ustic resonance in the atmosphere between the excitation source and th e upper reflection level. We discuss the diagnostic properties of this resonance, which is sensitive to the acoustic reflectivity of the sol ar atmosphere and to the location and parity of the excitation source. When applied to the solar data, our model indicates that the average acoustic reflectivity increases with increasing solar activity. The mo del also shows that the acoustic source has composite parity and is lo cated within one pressure scaleheight of the base of the photosphere.