Upper left panel: A brightness temperature (T_B) map from ALMA’s Band 6 observation of the plage region at the start of the time series. The ellipse in the bottom left corner of the panel represents the beam size of the observations and the dashed white rectangle outlines the location of the long-lived fibril of interest. Upper right panel: Zoomed-in view of the selected fibril (using two different color tables for better clarity) with the seven artificial slits placed perpendicular to the fibril axis for wave analysis as discussed in Sect. 3. Middle and lower panels: Co-aligned SDO/HMI continuum and magnetogram and SDO/AIA images at 170, 30.4, 17.1, and 19.3 nm. In the SDO/HMI magnetogram, the line-of-sight photospheric magnetic fields (B_los) are in range of −1116 < B_los (G) < 81.