Interannual Fluctuations of the Core Angular Momentum Inferred from Geomagnetic Field Models
Recent models of Earth’s core magnetic field based on geomagnetic satellite data suggest an existence of interannual core dynamics. The related magnetic signal of the interannual core dynamics is found to be small, which makes an observational detection of interannual variations of the core flow and the associated oscillations of the core angular momentum (CAM) difficult. However, it has been shown that the phase of the interannual oscillation is robustly determined, according to numerous CAM computations from diverse core flow models that are all estimated as a result of inverting a single geomagnetic model C3FM2. Here, we show that the phase identification depends on the secular acceleration (SA) of a geomagnetic model. Estimates of the phase still vary with geomagnetic models, C3FM2, gufm1, and COV-OBS, for the pre-satellite era, whose differences are readily recognisable in their SA representations. None of them may be an optimal model for describing the SA. Compared with the SA of a satellite model GRIMM3, C3FM2 is overdamped in time, but it may be improved for resolving the interannual CAM oscillations by properly modifying its temporal smoothness in reference to the SA of satellite models.
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