USE OF ACOUSTIC-WAVE TRAVEL-TIME MEASUREMENTS TO PROBE THE NEAR-SURFACE LAYERS OF THE SUN

The variation of solar p-mode travel times with cyclic frequency v is shown to provide information on both the radial variation of the acoustic potential and the depth of the effective source of the oscillations. Observed travel-time data for waves with frequency lower than the acoustic cutoff frequency for the solar atmosphere (almost-equal-to 5.5 mHz) are inverted to yield the local acoustic cutoff frequency v(c) as a function of depth in the outer convection zone and lower atmosphere of the Sun. The data for waves with v > 5.5 mHz are used to show that the source of the p-mode oscillations lies approximately 100 km beneath the base of the photosphere. This depth is deeper than that determined using a standard mixing-length calculation.