Investigation of Phase Shift and Travel Time of Acoustic Waves in the Lower Solar Atmosphere Using Multi-Height Velocities

Abstract: We report and discuss phase-shift and phase travel time of low-frequency ({\nu} < 5.0 mHz) acoustic waves estimated within the photosphere and photosphere-chromosphere interface regions, utilizing multi-height velocities in the quiet Sun. The bisector method has been employed to estimate seven height velocities in the photosphere within the Fe I 6173 {\AA} line scan, while nine height velocities are estimated from the chromospheric Ca II 8542 {\AA} line scan observations obtained from the Narrow Band Imager instrument installed with the Multi-Application Solar Telescope operational at the Udaipur Solar Observatory, India. Utilizing fast Fourier transform at each pixel over the full field-of-view, phase shift and coherence have been estimated. The frequency and height-dependent phase shift integrated over the regions having an absolute line-of-sight magnetic field of less than 10 G indicates the non-evanescent nature of low-frequency acoustic waves within the photosphere and photosphere-chromosphere interface regions. Phase travel time estimated within the photosphere shows non-zero values, aligning with previous simulations and observations. Further, we report that the non-evanescent nature persists beyond the photosphere, encompassing the photospheric-chromospheric height range. We discuss possible factors contributing to the non-evanescent nature of low-frequency acoustic waves. Additionally, our observations reveal a downward propagation of high-frequency acoustic waves, indicating the refraction from higher layers in the solar atmosphere. This study contributes valuable insights into the understanding of the complex dynamics of acoustic waves within different lower solar atmospheric layers, shedding light on the non-evanescent nature and downward propagation of the acoustic waves.