Vertical Propagation of Acoustic Waves in the Solar Inter-Network as Observed by IRIS

We investigate the Interface Region Imaging Spectrograph (IRIS) observations of the quiet-Sun (QS) to understand the propagation of acoustic waves in transition region (TR) from photosphere. We selected a few IRIS spectral lines, which include the photospheric (Mn~{\sc i} 2801.25~{\AA}), chromospheric (Mg~{\sc ii} k 2796.35~{\AA}) and TR (C~{\sc ii} 1334.53~{\AA}), to investigate the acoustic wave propagation.The wavelet cross-spectrum reveals significant coherence (about 70\% locations) between photosphere and chromosphere. Few minutes oscillations (i.e., period range from 1.6 to 4.0 minutes) successfully propagate into chromosphere from photosphere, which is confirmed by dominance of positive phase lags. However, in higher period regime (i.e., greater than $\approx$ 4.5 minutes), the downward propagation dominates is evident by negative phase lags. The broad spectrum of waves (i.e., 2.5-6.0 minutes) propagates freely upwards from chromosphere to TR. We find that only about 45\% locations (out of 70\%) show correlation between chromosphere and TR. Our results indicate that roots of 3 minutes oscillations observed within chromosphere/TR are located in photosphere. Observations also demonstrate that 5 minute oscillations propagate downward from chromosphere. \textbf{However, some locations within QS also show successful propagation of 5 minute oscillations as revealed by positive phase lags, which might be the result of magnetic field}. In addition, our results clearly show that a significant power, within period ranging from 2.5 to 6.0 minutes, of solar chromosphere is freely transmitted into TR triggering atmospheric oscillations. Theoretical implications of our observational results are discussed.