Spectral line similarity in the geomagnetic dipole field variations and length of day fluctuations

Abstract

Power spectral density analysis using Burg's maximum entropy method (MEM) was applied to the geomagnetic dipole field and its rate of change for the years 1901–1969. Both spectra indicate relative maxima at 0.015 cycle/yr and its harmonics. These maxima correspond approximately to 66‐, 33‐, 22‐, 17‐, 13‐, 11‐, and 9‐year spectral lines. The application of the same analysis techniques to the length of day (l.o.d.) fluctuations for the period 1865–1961 reveals similar spectral characteristics. The existence of the common spectral peaks with periods of 66 and 33 years in the l.o.d. fluctuations and the geomagnetic dipole field is clearly established. The existence of the higher harmonics with periods of 22, 17, 13, 11, and 9 years is somewhat uncertain because of the line‐splitting problem in the MEM spectral analysis. Furthermore, the 22‐ and 11‐year spectral lines cannot be attributed to the solar magnetic cycle and the solar cycle unambiguously because they are the higher harmonics of the 66‐year period. We suggest that the spectral line similarity in the l.o.d. fluctuations and the dipole field variations is related to the motion within the earth's fluid core during the past 100 years.

On the nonstationarity of the decadal periodicities of the length of day

“…Currie (1973) analysed LoD data primarily from the 18th and 19th centuries using the maximum entropy method and found a periodicity of 57.5 yr. The same method was used by Jin and Thomas (1977), who found well-defined signals at 66 and 33 yr. Some years later, Zatman and Bloxham (1997) showed that the fluid flow at the surface of the core is consistent with the presence of two torsional oscillations travelling inside the core with periods of 76 and 53 yr. Hide et al (2000) identified a dominant variability period of approximately 65 yr in LoD data.…”

Geomagnetic variation on decadal time scales: What can we learn from Empirical Mode Decomposition?

“…Whilst it has been successfully identified in length of day (LOD) data [e.g., Jin and Thomas, 1977;Braginsky, 1984;Hide et al, 2000;Roberts et al, 2007], there has been an equal divide between those successfully [e.g., Braginsky, 1984;Hide et al, 2000;Roberts et al, 2007] and unsuccessfully [e.g., Barraclough et al, 2000] identifying it in geomagnetic data. This particular period is attributed to the fundamental mode of torsional oscillations by Braginsky [1970].…”

“…Jin and Thomas [1977] applied the Maximum Entropy Method to LOD and annual magnetic data and found well defined signals at 66 and 33 years. Hide et al [2000] used LOD to assess variations in core angular momentum and found a period of 65 years.…”

Decade Variations of the Earth's Rotation and Geomagnetic Core-Mantle Coupling

“…The period spectra of some characteristics of both quantities showed nearly equal periods (e.g. JIN and THOMAS, 1977;GREINER-MAI, 1989). The period spectrum of the decade fluctuations of the polar motion is also similar to that of the sv (JOCHMANN, 1991).…”