Figure 6

Time changes of the amplitude and phase of periodic signals at various sites. Amplitude (top) and phase (bottom) obtained for the annual (right) and semiannual (left) periodic signals for the X component at several observatories, for a window length of δ t=5 years, after filtering with an 11-year running mean. The phase of the periodic signal (originally in radians) is translated into the date at which occurs the first maximum of the annual (or semiannual) fit to the data (for each epoch, a phase of zero would correspond to January 1).