Fig. 7
From: Recovery of the 6-year signal in length of day and its long-term decreasing trend
Comparison of the results from two filtering methods, a time domain and b frequency domain. a The green dashed lines show the ±0.1 ms; the wavelet packet analysis of the LOD signals on 5~8-year scales showing an abruptly obvious decreasing phenomenon in amplitude during the 1990s, which also can be seen from previous studies; while the result from NMWT shows gradually weakening during the past over 50 years; b the result from NMWT has a much narrower frequency domain than the result from wavelet packet analysis, which means the result from NMWT should be a very good representative of the pure 6-year signal