Periodograms of the daily number of dust impacts with morphological types A and B for the three months of each year that correspond either to the orbital segment against the ISD inflow (February to April, blue curve, top panel) or with the ISD inflow direction (August to October, red curve, bottom panel). The two panels are scaled identically; the estimated 95% significance thresholds are indicated by grey shaded areas. Frequencies of interest are marked by vertical lines as in Fig. 9.