The transition to Earth-like torsional oscillations in magnetoconvection simulations

Evidence for torsional oscillations (TOs) operating within the Earth's fluid outer core has been found in the secular variation of the geomagnetic field. These waves arise via disturbances to the predominant (magnetostrophic) force balance believed to exist in the core. The coupling of the core and mantle allow TOs to affect the length-of-day of the Earth via angular momentum conservation. Encouraged by previous work, where we were able to observe TOs in geodynamo simulations, we perform 3-D magnetoconvection simulations in a spherical shell in order to reach more Earth-like parameter regimes that proved hitherto elusive. At large Ekman numbers we find that TOs can be present but are typically only a small fraction of the overall dynamics and are often driven by Reynolds forcing at various locations throughout the domain. However, as the Ekman number is reduced to more Earth-like values, TOs become more apparent and can make up the dominant portion of the short timescale flow. This coincides with a transition to regimes where excitation is found only at the tangent cylinder, is delivered by the Lorentz force and gives rise to a periodic Earth-like wave pattern, approximately operating on a 4 to 5 year timescale. The core travel times of our waves also become independent of rotation at low Ekman number with many converging to Earth-like values of around 4 years

Mineurs étrangers non accompagnés sans protection en Europe : recherche conduite en France dans le cadre du projet PUCAFREU

Les enfants étrangers, migrant de façon indépendante pour différentes raisons et/ou en quête de protection internationale, sont de plus en plus nombreux en Europe et la France est un pays de destination pour beaucoup d’entre eux. La situation de ces enfants – dénommés mineurs isolés étrangers en France- préoccupe de nombreux acteurs (institutionnels, associatifs, académiques). De manière générale, ces enfants, de nationalités variées, aspirent fortement à une protection. Ils sont demandeurs, souvent avides d'apprendre le français, de se former et de travailler. Pourtant, leur accueil est considéré souvent comme une charge. Face à cette problématique, et depuis une vingtaine d’années, les pouvoirs publics peinent à mettre en place une politique claire et coordonnée et les dispositifs de protection de l’enfance adressés restent insuffisants

Forward models of torsional waves: dispersion and geometric effects

Alfvén waves are a set of transverse waves that propagate in an electrically conducting fluid in the presence of an ambient magnetic field. Studies of such waves in the Earth's interior are important because they can be used to make inferences about the structure and physical properties of the core that would otherwise remain inaccessible. We produce 1-D forward models of cylindrical torsional Alfvén waves in the Earth's core, also known as torsional oscillations, and study their evolution in a full sphere and an equatorially symmetric spherical shell. Here, we find that travelling torsional waves undergo significant geometric dispersion that increases with successive reflections from the boundaries such that an initial wave pulse becomes unidentifiable within three transits of the core. Low amplitude wakes trail behind sharply defined pulses during propagation, a phenomenon that we interpret using the failure of Huygens’ principle in even dimensions. We investigate the relationship between geometric dispersion and wavelength, concluding that long-wavelength features are more dispersive than short-wavelength features. This result is particularly important because torsional waves that have been inferred in the Earth's core from secular variation are relatively long wavelength, and are therefore likely to undergo significant dispersion within the core. When stress-free boundary conditions on angular velocity are applied, waves are reflected at the equator of the core–mantle boundary with the same sign as the incident wave. Waves that pass through the rotation axis undergo a pseudo-reflection and display a more complicated behaviour due to a phase shift. In an equatorially symmetric shell, we identify a weak reflection at the tangent cylinder due to geometric effects