The probability density function (pdf) of the instantaneous frequencies of each mode of SOLSTICE/Lyman-α (121 nm) line. The histogram is smoothed with a KS (kernel smoothing) density function. The vertical red lines in modes 3–5 mark the harmonics of the solar rotation period, which correspond to ~13.5, ~27, and ~54 days. Maximum frequency distribution occurs at the following frequencies; 0.054, 0.034, and 0.015 in mode 3, 4, and 5, representing 18-day, 29.2-day, and 66.6-day period, respectively.