Acoustic waves in two-fluid solar atmosphere model: cut-off periods, chromospheric cavity, and wave tunnelling

We perform numerical simulations of acoustic waves in a two-fluid model of quiet region of the solar atmosphere. The two-fluid model describes partially ionized (non-magnetized) solar plasma, whose main components are neutral atoms, protons, and electrons. The waves are excited by a monochromatic driver, which operates at the bottom of the solar photosphere. Our numerical results show that the driver excites ion and neutral acoustic waves whose propagation is affected by the gravity. As a result, the acoustic waves with periods higher than a local acoustic cut-off period are evanescent, while lower waveperiods are free to reach the solar corona. Acoustic waves, which are evanescent in the photosphere and low chromosphere, tunnel their energy into the upper chromosphere and the transition region. The wave propagation to the solar corona is affected by partial wave reflection that occurs in the transition region, and is responsible for formation of a cavity, where the waves are trapped. Fourier power analysis of temporal characteristic of plasma quantities reveals that a spectrum of various periods is generated. While oscillations traced in ion and neutral velocities look very similar, dynamics of mass densities of ions and neutrals differs a lot. The obtained results clearly show that the two-fluidmodel provides new insights into the acoustic wave propagation in a more realistic (partially ionized) quiet region of the solar atmosphere.