Chromospheric dynamics — What can be learnt from numerical simulations

Abstract

Observations of the solar chromosphere are often interpreted using methods derived from static modeling (e.g., the Vernazza et al. 1981 model atmospheres and work based on such models) or linear theory (e.g., phase relations). Recent numerical simulations have shown that such an analysis can be very misleading. It is found that enhanced chromospheric emission, which corresponds to 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 field free internetwork regions. This dynamic picture is consistent with observations in CO lines and the calcium H and K bright grains. More opaque lines, on the other hand, seem to show emission all of the time. This indicates the existence of a hotter, magnetic, component that increases in importance with height.

The simulations closely match the observed behaviour of Ca II H_2v bright grains down to the level of individual grains. The bright grains are produced by shocks near 1 Mm above where the optical depth is unity at 500 nm (τ ₅₀₀=1). These shocks are primarily due to waves from the photosphere with a frequency slightly above the acoustic cutoff frequency. The concept of a fixed formation height is of little use in the chromosphere. The temperature spikes at shock fronts may produce doublypeaked intensity contribution functions with one peak at τ _v=1 and another at the shock. The mean height of formation for lines and continua formed around 1 Mn can vary greatly with time and does not necessarily correspond to the actual layers emitting the photons. When waves in the chromosphere have large amplitude, linear perturbation theory is not valid since the passage of waves changes the atmosphere fundamentally.