"Has the Earth's magnetic field periodically collapsed since its last reversal, 780.000 ago?" | "Has the Earth's magnetic field periodically collapsed since its last reversal, 780.000 ago?": Invited Speaker: G. Saracco

International audience

anglais. " Has the Earth's magnetic field periodically collapsed since its last reversal, 780.000 ago? " Invited speaker: Saracco Ginette ; Lieu: Institut de Mathematiques d'Orsay, France, 12-14/06/2019 ; Authors: Nicolas, Thouveny, Didier Bourles, Quentin Simon, Ginette Saracco ;Address: CNRS-UMR7330, CEREGE, AMU, CdF, IRD, INRA,Europole de l'Arbois, Aix-en-Provence, France ;Abstract :The hypothetical contribution of the Earth’s axis precession to the geodynamo energy was recentlyreinforced by experimental and numerical geodynamo models, triggering new research on the possibleinfluence of orbital periodicities, e.g. axial precession (≈26.000 years) and obliquity (≈42.000 years) inthe paleomagnetic field spectrum (e.g. Thouveny et al. 2008; Saracco et al. 2009). Oceanic sedimentarysequences contain records of geomagnetic paleointensity, as well as cosmogenic nuclide productionrecords, that provide some appropriate time-series of the geomagnetic dipole moment variations,allowing us to extract all frequency modulation laws of it, from algorithms based on the phase of a lineartime-frequency method as the complex wavelet transform.Beyond the fundamental interest for mechanical constraints on the geodynamo, the interest of suchstudies lies in the fact that, at present and for the last 3 millennia, the dipole moment has been decaying byabout 30% (from 11.5 10^22 A.m^2 to less than 8 10^22 A.m^2 ), either a rate equivalent to that leading to thefull cancellation of the dipole at the time of excursions and reversals of the geomagnetic field.Since the last excursion (LASCHAMP) occurred 41.500 years ago, i.e. about for one obliquity period,solving the question raised, here, takes all its importance by the changes that may occur during themagnetic field inversion at the scale of the Earth.