Non-zero phase-shifts of acoustic waves in the lower solar atmosphere observed in realistic radiative hydrodynamic simulations and their role in local helioseismology

Previous studies analyzing the evanescent nature of acoustic waves in the lower solar atmosphere (up to 300km above the photosphere) have shown an unexpected phase-shift of an order of 1s between different heights. These studies investigated the atomic absorption line Fe I 6173.3 Å, which is commonly used for helioseismic observations. Such phase-shifts can contribute to a misinterpretation of the measured travel time in local helioseismology, complicating estimations of, e.g., the deep meridional flow. In this study, we carry out phase-shift computations using a simulated, fully radiative and convective atmosphere (exhibiting naturally excited acoustic waves) from which the Fe I 6173.3 Å line is synthesized. The resulting phase-shifts across multiple heights indicate the presence of evanescent waves, similar to what is observed for real data. Comparing the Doppler-velocities, estimated from the synthesized absorption line with the true plasma velocities, we find substantial differences in phase-shifts as a function of height. This leads us to hypothesize that the non-adiabaticity of the solar atmosphere yields extra phase-shift contributions to Doppler-velocities. Such contributions must be considered for the accurate reconstruction of solar interior properties.