Overview of the observations taken on 2018 July 22 at 08:31:49 UT. The top panels show intensity images of the continuum at 4000 Å (a) and the wavelength-integrated Ca II K line intensity (b). The dashed white box marks the region that we analyse in detail. The red box marks the region of interest (RoI) analysed in Fig. 3. A zoom-in of the area enclosed by the dashed white box is shown in the bottom panels. (c) Fe I 6173 Å wavelength-averaged linear polarisation; (d) Fe I 6173 Å wavelength-averaged circular polarisation; (e) Ca II 8542 Å line core intensity; (f) Ca II K line core intensity. The solid red and dashed white lines in panel (f) show the cuts across the Ca II K bright fibrils that we used (see Sect. 3.1). An analysis of the cut along the dashed white line is presented in Figs. 7 and 8. The white dot marks the zero point on the y-axis in Fig. 7. An animated version of this figure showing the entire time sequence is available online.