Energy and Power Spectra of the Potential Geomagnetic Field since 1840
It is possible that the radially independent, spatial-spectral components of the energy and power of the potential part of the main geomagnetic field were determined and studied for the first time. Energy is obtained by integrating its known radial density from the core of the Earth to infinity, and power is a time derivative of energy. The total and spectral variations of energy and power from 1840 to 2020 are analyzed based on three generally accepted observational models of the geomagnetic field. The total energy (~6 × 1018 J) and power (~108 W) are determined by the sum of odd harmonics: dipole n = 1, octupole n = 3, etc. The dipole, the energy of which is close to the total energy symmetric with respect to the axis of rotation of the field, is predominant. The energy variations are ~10% and are similar for all models with the exception of the “burst” of the international geomagnetic reference field (IGRF) model in 1945–1950. Comparative spectral analysis showed that the “burst” is concentrated at n = 9 and 10, and the variations of the other harmonics are similar in all models. In this case, n = 3 dominates over n = 2. From n = 3 to 8, it decreases, and further n = 9 dominates over 8 and 10. The mean powers close to zero for n> 1 indicate an almost periodic behavior of the nondipole field, and significant power variations indicate a strong nonlinearity of the geodynamo. The results of the work are consistent with modern geodynamo-like models. The fact that such a significant IGRF “burst” that can have a non-linear geodynamic nature is a challenge. Alternatively, this may be some consequence of the imperfections of the IGRF model. Two other too-"quiet" models were subjected to excessive smoothing.
The authors are deeply grateful to the anonymous reviewer. His comments helped to improve significantly the presentation of this work, which was done mainly at the expense of the IZMIRAN budget. For geodynamo applications, partial support was provided by the Russian Foundation for Basic Research (project no. 16-05-00507-a).
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