Lomb-Scargle periodograms, each of them normalized to its maximum power, for subsectors of sectors A (top panel), B (middle panel), and C (bottom panel), computed at a radial distance of 1.8 R_⊙. The latitudinal range of each subsector is indicated in each panel. The peaks at 42 min and 13 min are indicated by arrows and highlighted by a light yellow and a light magenta shaded area, respectively. The 95% and 99% confidence levels are reported as solid yellow and red lines, respectively.