Nonzero phase shift of evanescent acoustic waves in the lower solar atmosphere

Abstract

Context. The phase behavior of acoustic waves in the lower solar atmosphere provides important information about wave propagation. While previous studies have reported nonzero phase shifts in the evanescent frequency range and possible sign reversals near the frequency 3.0 mHz, the observational robustness and physical interpretation of these signatures remain uncertain. Aims. We investigated the phase shifts and travel times of acoustic waves within the photosphere and across the photosphere-chromosphere interface in the quiet-Sun atmosphere and also assessed their height and frequency-dependent behavior. Methods. Multi-height line-of-sight velocities were inferred using the bisector method applied to the photospheric Fe I 6173 Å and chromospheric Hα 6563 Å spectral lines observed with the Swedish Solar Telescope (SST). Fast Fourier transform analysis was employed to determine the phase shifts and travel times between the velocity signals derived at different bisector levels. In addition, k--ω phase-difference diagnostics were used to examine the wave number and frequency-dependent characteristics of wave propagation. Results. Nonzero phase shifts are detected for acoustic waves in the evanescent frequency range within the photosphere, corresponding to phase travel times on the order of 1 s. In contrast to previous studies, no reversal in the sign of the phase shift or travel time is observed near 3 mHz in the Fe I 6173 Å observations. The measured phase shifts maintain the same sign throughout the investigated frequency range, suggesting the influence of complex wave dynamics. Measurements between velocities inferred from the Fe I and Hα lines exhibit a decrease in the 6--8 mHz frequency range. Our k--ω phase-difference analysis shows that this behavior is mainly caused by phase wrapping near 6.5 mHz rather than by an intrinsic change in wave propagation. Conclusions. Our findings support the presence of nonzero phase differences within the evanescent frequency range while revealing discrepancies with earlier reports of phase-shift sign reversals near 3 mHz. We also highlight the importance of proper phase unwrapping for an accurate interpretation of acoustic-wave propagation in the lower solar atmosphere.