The dark side of solar photospheric G-band bright points

¹ Max-Planck-Institut für Sonnensystemforschung (MPS), Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: riethmueller@mps.mpg.de
² School of Space Research, Kyung Hee University, Yongin, 446-701 Gyeonggi, Republic of Korea

Received: 22 September 2016
Accepted: 22 December 2016

Abstract

Bright, small-scale magnetic elements found mainly in intergranular lanes at the solar surface are named bright points (BPs). They show high contrasts in Fraunhofer G-band observations and are described by nearly vertical slender flux tubes or sheets. A recent comparison between BP observations in the ultraviolet (UV) and visible spectral range recorded with the balloon-borne observatory Sunrise and state-of-the-art magnetohydrodynamical (MHD) simulations revealed a kilogauss magnetic field for 98% of the synthetic BPs. Here we address the opposite question, namely which fraction of pixels hosting kilogauss fields coincides with an enhanced G-band brightness. We carried out 3D radiation MHD simulations for three magnetic activity levels (corresponding to the quiet Sun, weak and strong plage) and performed a full spectral line synthesis in the G-band. Only 7% of the kilogauss pixels in our quiet-Sun simulation coincide with a brightness lower than the mean quiet-Sun intensity, while 23% of the pixels in the weak-plage simulation and even 49% in the strong-plage simulation are associated with a local darkening. Dark strong-field regions are preferentially found in the cores of larger flux patches that are rare in the quiet Sun, but more common in plage regions, often in the vertices of granulation cells. The significant brightness shortfall in the core of larger flux patches coincides with a slight magnetic field weakening. Kilogauss elements in the quiet Sun are, on average, brighter than similar features in plage regions. Almost all strong-field pixels display a more or less vertical magnetic field orientation. Hence, in the quiet Sun, G-band BPs correspond almost one-to-one with kilogauss elements. In weak plage, the correspondence is still very good, but not perfect.