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Is the Earth's magnetic field a constant ? On seasonal variations of the geomagnetic field: a legacy of Poisson

J-L. Le Mou\"elF. LopesV. CourtillotD. GibertJ-B. Boul\'e
Mar 2023
In the report he submitted to the Acad\'emie des Sciences, Poisson imagined aset of concentric spheres at the origin of the Earth's magnetic field. It maycome as a surprise to many that Poisson as well as Gauss both considered themagnetic field to be constant. We propose in this study to test this surprisingassertion for the first time evoked by Poisson (1826). First, we will present adevelopment of Maxwell's equations in the framework of a static electric fieldand a static magnetic field in order to draw the necessary consequences for thePoisson hypothesis. In a second step, we will see if the observations can be inagreement with Poisson (1826). To do so, we have chosen to compare 1) the polarmotion drift and the secular variation of the Earth's magnetic field, 2) theseasonal pseudo-cycles of day length together with those of the sea levelrecorded by different tide gauges around the globe and those of the Earth'smagnetic field recorded in different magnetic observatories. We then propose amechanism, in the spirit of Poisson, to explain the presence of the 11-year inthe magnetic field. We test this mechanism with observations and finally westudy closely the evolution of the g10 coefficient of the IGFR over time.