Panels a)–d): stack plots along slices “A−B” and “C−D” (dashed lines in Fig. 2b) and slices “E−F” and “G−H” (dashed lines in Fig. 4b). Red arrows and dotted lines show the repeated upflows. Panel e): intensity variation of a BP in panel a) at 1330 Å. Each data point of the profile is the vertical sum of the intensity in area “A1” (two dashed lines in panel a)). Red arrows point to the peaks of the quasi-periodic intensity profile. Panel f): horizontal slice (black curve) along the dashed line (“L1”) in panel d) at 1330 Å.