Periodogram of the daily number of dust impacts with morphological types A and B before April 2011, original data (blue curve, top panel) and data with artificially induced 6 d-periodic gaps (orange curve, bottom panel), evaluated at the DFT’s natural frequencies and oversampled by a factor of five. The two panels are scaled identically. Frequencies of interest are marked by vertical lines, as in Fig. 9. The estimated 95% significance thresholds are indicated by grey shaded areas.