Abstract

Reanalysis of geomagnetic field changes over the satellite era have unveiled Magneto-Coriolis (MC) waves in Earth’s core, with a period close to 7 years [1]. These are responsible for a large part of the rapid field changes recorded over the past two decades. Such motions are Quasi-Geostrophic (QG), i.e. invariant along the rotation axis due a dominating Coriolis force. QG MC modes were previously thought to be detectable only on centennial and longer periods [2]. However, an eigenmode study has recently revealed the existence of QG MC solutions on interannual time- scales, presenting a low azimuthal complexity [3]. The dispersion relation for these very dispersive waves indeed indicates that a short period is associated with a shear much stronger along the cylindrical radial direction, and a low azimuthal wavenumber. Their reconstruction is easier in the vicinity of the core equator because there small radial length-scales project onto large horizontal patterns. Our analysis of geophysical observations suggests that (i) their detection is facilitated by the advent of the continuous satellite era since 2000 and (ii) similar waves also exist at decadal periods. This scenario is supported by the reanalysis of synthetic magnetic data computed from geodynamo simulations run at extreme parameters [4], where modulated QG MC waves are triggered over a large range of periods.   [1] Gillet, Gerick, Jault, Schwaiger, Aubert, & Istas, Satellite magnetic data reveal interannual waves in Earth’s core, Proc. Nat. Acad. Sci. 119(13), e2115258119 (2022) [2] Hide, Free hydromagnetic oscillations of the Earth’s core and the theory of the geomagnetic secular variation, Philos. Trans. R. Soc. Lond. A 259 , 615–647 (1966) [3] Gerick, Jault & Noir, Fast quasi-geostrophic Magneto-Coriolis modes in the Earth’s core, Geophys. Res. Lett. 48, e2020GL090803 (2021). [4] Aubert & Gillet, The interplay of fast waves and slow convection in geodynamo simulations nearing Earth’s core conditions, Geophys. J. Int. 225, 1854–1873 (2021).