Cosmic rays reveal event in Earth's magnetic field history
41 000 years ago, the Earth's magnetic field faded and practically disappeared, leaving our planet unprotected from the bombardment of cosmic rays. Evidence for this event has been found in ocean sediment cores by a team from the Centre de Recherche et d'Enseignement de Géosciences de l'Environnement (CEREGE, CNRS/Aix-Marseille Université/IRD/Collège de France). In the cores, the researchers measured variations in concentrations of beryllium-10, a radioactive isotope produced by the action of cosmic rays on oxygen and nitrogen atoms in the atmosphere. The work, published in the Journal of Geophysical Research, is an important step towards developing a new method for studying the history of Earth's magnetic field, which should shed light on why its strength has been declining over the past three thousand years.