Formation of the K I 7699 Å Line in Sunspots

Abstract

The resonance line of the neutral potassium K I 7699 Å in the spectrum of large sunspot umbrae shows a variety in shape and width, which cannot be accounted for by the changes in intensity between large sunspots observed at different times in the activity cycle and at different positions on the disk; in addition it has a reversal in the core which is not symmetric.

We use the version MUL20 of the NLTE code by Mats Carlsson with improved opacities, and our K I atomic model to build synthetic profiles of the K I resonance line in spot umbra model atmospheres.

We estimate several effects on line formation due to the spot atmospheric model, using spot models based on different approaches, and conclude that the core reversal should not be due to the residual NLTE sensitivity of the line source function to a chromospheric temperature rise.

We also explore the effects due to the Zeeman splitting and to a possible insufficient correction for stray light, using simple models to be included in the line formation. Finally we consider downflows as a possible cause of the line core asymmetry.