On the origin of facular brightness

TL;DR

This study investigates how CaIIH line core brightness in solar faculae depends on magnetic field properties, wave motions, and convection, revealing that brightness results from both magnetic structure effects and actual heating processes.

Contribution

It provides new insights into the physical mechanisms behind facular brightness, emphasizing the roles of wave propagation, phase shifts, and convective motions in addition to magnetic effects.

Findings

CaIIH brightness correlates with upward-propagating wave power.

Maximum brightness occurs with specific phase shifts between velocity and temperature oscillations.

Facular brightness is influenced by both magnetic structures and real heating processes.

Abstract

This paper studies the dependence of the CaIIH line core brightness on the strength and inclination of photospheric magnetic field, and on the parameters of convective and wave motions in a facular region at the solar disc center. We use three simultaneous datasets obtained at the German Vacuum Tower Telescope (Observatorio del Teide, Tenerife): (1) spectra of BaII 4554 A line registered with the instrument TESOS to measure the variations of intensity and velocity through the photosphere up to the temperature minimum; (2) spectropolarimetric data in FeI 1.56 $\mu$m lines (registered with the instrument TIP II) to measure photospheric magnetic fields; (3) filtergrams in CaIIH that give information about brightness fluctuations in the chromosphere. The results show that the CaIIH brightness in the facula strongly depends on the power of waves with periods in the 5-min range, that propagate upwards, and also on the phase shift between velocity oscillations at the bottom photosphere and around the temperature minimum height, measured from BaII line. The CaIIH brightness is maximum at locations where the phase shift between temperature and velocity oscillations lies within 0-100 degrees. There is an indirect influence of convective motions on the CaIIH brightness. Namely, the higher is the amplitude of convective velocities and the larger is the height where they change their direction of motion, the brighter is the facula. Altogether, our results lead to conclusions that facular regions appear bright not only because of the Wilson depression in magnetic structures, but also due to real heating.