Radiation Shock Dynamics in the Solar Chromosphere { Results of Numerical Simulations

We report results from self-consistent non-LTE radiation hydrodynamics simulations of the propagation of acoustic waves through the solar chromosphere. It is found that enhancedchromospheric emission, which correspondsto an outwardly increasing semi-empirical temperature structure, can be produced by wave motions without any increase in the mean gas temperature. Thus, despite long held beliefs, the sun may not have a classical chromosphere in magnetic eld free internetwork regions. The dynamic formation of continuum radiation is described in some detail. Concepts that work well in static models do not necessarily work in the dynamic chromosphere. The contribution function for the intensity may be bimodal with one peak around =1 and another at a shock at smaller optical depth. The mean height of formation will then be between those regions and have no relation to either formation region. Above the photosphere, the source function is so decoupled from the Planck function that variations in intensity can not be taken as a proxy for gas temperature variations. The emergent intensity does not show the discontinuous rise signature of shock waves even for the continua formed where the shocks are strong. Even in the photosphere a one-to-one correspondence between intensity variation and gas temperature variation is not possible because of the dependence of the formation height on the opacity and therefore on the temperature. The modiication of velocity amplitude and phase as a function of frequency and depth in the atmosphere is described with a transfer function. This function is found to be rather insensitive to the input velocity eld in the photosphere. By using the derived transfer function it is possible to construct piston velocities that give photospheric velocities that match observed Doppler shifts. The simulations closely match the observed behaviour of Ca II H 2 V bright grains. The formation of bright grains is described in detail and both the brightness and the wavelength position of the grains are explained. It is found that the grain pattern is completely set by the velocity pattern of the piston. The frequency components around 3 minute periods are found to be most important but with signiicant modulation of the grain behaviour from the low frequency component of the velocity eld. The strength of grains is not directly proportional to the photospheric 3 minute power but a result of interference between many modes.