Excitation of chromospheric wave transients by collapsing granules

The excitation of acoustic waves is studied using three-dimensional numerical simulations of the nonmagnetic solar atmosphere and the upper convection zone. Transient acoustic waves in the atmosphere are excited at the top of the convective zone (the cooling layer) and immediately above in the convective overshoot zone, by small granules that undergo a rapid collapse, in the sense that upflow reverses to downflow, on a timescale shorter than the atmospheric acoustic cutoff period (3 minutes). These collapsing granules tend to be located above downflows at the boundaries of mesogranules where the upward enthalpy flux is smaller than average. An extended downdraft between larger cells is formed at the site of the collapse. The waves produced are long wavelength, gravity modified acoustic waves with periods close to the 3 minute cutoff period of the solar atmosphere. The oscillation is initially horizontally localized with a size of about 1 Mm. The wave amplitude decays in time as energy is transported horizontally and vertically away from the site of the event. Observed "acoustic events" and darkening of intergranular lanes could be explained by this purely hydrodynamical process. Furthermore, the observed "internetwork bright grains" in the Ca II H and K line cores and associated shock waves in the chromosphere may also be linked to such wave transients.