The inverse-C shape of solar chromospheric line bisectors

Spatially averaged intensity profiles of the chromospheric Na I D andCa II infrared lines exhibit a pronounced red asymmetry in their cores with bisectors in the shape of an inverse C. This shape stands in stark contrast to the regular C shape of photospheric spectral line bisectors, which on average exhibit a blue shift as a result of the asymmetry in surface area subtended by convective upflows over downflows. The nature of the inverse chromospheric C shape is investigated by comparing spatially averaged profiles of the Na I D and Ca II infrared lines with mean profiles computed through three-dimensional snapshots of a hydrodynamic convection simulation and a one-dimensional simulation of chromospheric radiation hydrodynamics. In part the red asymmetry is the result of the asymmetry in time the atmosphere spends in downward motion compared to upward motion when it is traversed by acoustic shocks. Profiles from convection simulations without shocks suggest that convective motions play a limited role in shaping the chromospheric line asymmetry. Further simulations that include effects of both convection and shock wave formation are needed to reach a definitive conclusion on the origin of the inverse-C shaped bisectors.