Testing Solar Envelope Models Using Intermediate-Degree P-Mode Frequencies

The accurately measured frequencies of intermediate-degree acoustic oscillations permit to infer the frequency dependence of the phase shift, corresponding to the reflection of the trapped acoustic waves from the solar surface. This function is sensitive to the structure of the outermost solar layers down to to the depth of second helium ionization zone. It deviates significantly from the predictions of standard solar models, representing the main source of discrepancies of oscillation frequencies in all the degree range. We report the results of studying this problem in the framework of adiabatic approximation, using both linearized inversions for the reflecting acoustic potential and direct tests of a variety of solar envelope models.