The possible origin of facular brightness in the solar atmosphere

¹ Main Astronomical Observatory, NAS, 03680 Kyiv, Ukraine
e-mail: kostik@mao.kiev.ua
² Instituto de Astrofísica de Canarias, 38205, La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, 38205, La Laguna, Tenerife, Spain

Received: 22 September 2015
Accepted: 21 February 2016

Abstract

This paper studies the dependence of the Ca ii  H line core brightness on the strength and inclination of the photospheric magnetic field, and on the parameters of convective and wave motions in a facular region at the center of the solar disc. We use three simultaneous data sets that were obtained at the German Vacuum Tower Telescope (Observatorio del Teide, Tenerife): (1) spectra of Ba ii 4554 Å 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 Fe i 1.56 μm lines (registered with the instrument TIP II) to measure photospheric magnetic fields; (3) filtergrams in Ca ii  H that give information about brightness fluctuations in the chromosphere. The results show that the Ca ii  H brightness in the facula strongly depends on the power of waves with periods in the 5-min range, which propagate upwards, and also on the phase shift between velocity oscillations at the bottom photosphere and around the temperature minimum height that is measured from Ba ii line. The Ca ii  H brightness is maximum at locations where the phase shift between temperature and velocity oscillations lies within 0°–100°. There is an indirect influence of convective motions on the Ca ii  H brightness. The higher the amplitude of convective velocities is and the greater the height is where they change their direction of motion, the brighter the facula. In summary, our results lead to conclusions that facular regions appear bright not only because of the Wilson depression in magnetic structures, but also owing to real heating.