Normalized CO 3-2 R14 line center intensity images, HMI line-of-sight magnetograms, and horizontal magnetic field. The top (a–c) and bottom (d–f) panels are for September 8 and 11, 2018, respectively. The white contours indicate the dark regions (cold bubbles) in the normalized CO 3–2 R14 line center intensity map with a level of I_{3-2 R14}/I_continuum = 0.77. The red arrows indicate the cold bubbles near or surrounded by magnetic fields. The colored dots in panels a and d give the positions where we show the normalized CO spectral lines in Figs. 4a,b. The green line in panel e is the slit on which we show the changes of the CO 3–2 R14 line center intensity and B_l strength in Fig. 8a.