Observation of magnetic diffusion in the Earth's outer core from Magsat, Ørsted, and CHAMP data

• Published in: Journal of Geophysical Research
• Main Authors: Chulliat, A., Olsen, N.
• Other Authors: Institut de Physique du Globe de Paris (IPGP), Institut national des sciences de l'Univers (INSU - CNRS)-IPG PARIS-Université Paris Diderot - Paris 7 (UPD7)-Université de La Réunion (UR)-Centre National de la Recherche Scientifique (CNRS), National Space Institute Lyngby (DTU Space), Technical University of Denmark Lyngby (DTU), Niels Bohr Institute Copenhagen (NBI), Faculty of Science Copenhagen, University of Copenhagen = Københavns Universitet (KU)-University of Copenhagen = Københavns Universitet (KU), CNES and by the French embassy in Denmark
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2010
• Subjects: frozen-flux; magnetic diffusion; outer core; [SDU.STU.GP]Sciences of the Universe [physics]/Earth Sciences/Geophysics [physics.geo-ph]; Helena Island; St. Helena; St. Helena Island; South Atlantic Ocean
• Online Access: https://hal.archives-ouvertes.fr/hal-00540455; https://hal.archives-ouvertes.fr/hal-00540455/document; https://hal.archives-ouvertes.fr/hal-00540455/file/2009JB006994.pdf; https://doi.org/10.1029/2009JB006994

International audience The frozen flux assumption consists in neglecting magnetic diffusion in the core. It has been widely used to compute core flows from geomagnetic observations. Here we investigate the validity of this assumption over the time interval 1980–2005, using high‐precision magnetic data from the Magsat, Ørsted, and CHAMP satellites. A detectable change of magnetic fluxes through patches delimited by curves of zero radial magnetic field at the core‐mantle boundary is associated with a failure of the frozen flux assumption. For each epoch (1980 and 2005), we calculate spatially regularized models of the core field which we use to investigate the change of reversed magnetic flux at the core surface. The largest and most robust change of reversed flux is observed for two patches: one located under St. Helena Island (near 20°S, 15°E); the other, much larger, is located under the South Atlantic Ocean. We next calculate frozen‐flux‐constrained field models (i.e., pairs of models for epoch 1980 and 2005 having the same flux through patches delimited by curves of zero radial magnetic field), using a penalty method. We find that the frozen flux constraint does not lead to any significant increase of the global misfit. However, applying the constraint leads to a detectable increase of the scalar residuals at satellite altitude in the region of St. Helena, strongly suggesting a local failure of the frozen flux assumption. The observed flux expulsion within the St. Helena patch could result from the formation of a pair of “core spots,” as predicted by numerical simulations of the geodynamo.