A new concept for magnetic reconnexion
1er juillet 2006
Magnetic reconnection is the process that allows rapid conversion of energy stored in a magnetic configuration in others kinds of energies (heating, kinetic energy, ...). This mechanism appears in almost all magnetized plasmas and thus almost every where in the Universe. In particular, magnetic reconnection is involved of some of the most violent phenomena of our solar system, such as solar flares and solar eruptions (see movie, size : 15 Mo, credits : NASA/SVS). Recently a team from Paris Observatory has provided more understanding of the mechanism, through 3D numerical simulations. Magnetic reconnection also happens in the Earth magnetosphere, as in laboratory experiments, inside tokamaks, like in the forthcoming ITER. In these devices, magnetic reconnection has to be constrained because it limits the efficiency of the fusion plasma confinement.
The storing and release of magnetic energy is analogous with what happens when one twist and then cut an elastic. Similarly, magnetic energy is released when magnetic field lines are cut, i.e., when one change their connectivity. This change of connectivity is at the heart of magnetic reconnection. In order that magnetic field lines can be reconnected, narrow layers of intense electric currents must be present. Although reconnection start to be well understood in two dimensions (2D), numerous questions, still unanswered, have been raised these last ten years about the conditions of formation of these current layers and the nature of magnetic reconnection in three dimensions (3D).
Recently, a research group of the Paris Observatory [1] has numerically simulated in 3D, the build-up of these current layers and the following reconnection, using the multi-processors facilities of the Service Informatique de L’Observatoire de Paris. This group has generalized in 3D the necessary conditions for current layers to develop, linking them which general geometries of the magnetic field that do not include any topological singularities, called "hyperbolic flux tubes". This association allows to predict the location of the magnetic reconnection triggering, and eventually to better control it in magnetic confinement laboratory experiments.
Figure 1 : Formation of a horizontal current layer (in green, right panel), at the level of a "hyperbolic flux tube" (in red and yellow, left panel). These numerical simulations have revealed a completely new behaviour of magnetic field during reconnection. In opposition with 2D, where magnetic field lines are cut and glued, in 3D, magnetic field lines continuously modify their connectivity and slide along each-other. (see Fig. 2).
Figure 2 : Sliding of several groups of magnetic field lines during reconnection. Left : perspective view. Right : side view. With these state-of-art numerical simulations, French scientists have brought totally new insight about magnetic reconnection in 3D. Some phenomena associated with solar flares can now be understood and interpreted with a new viewpoint. In particular, the motions of hard X ray emitting regions on the solar surface would be due to the rapid sliding of impact regions by the particles accelerated during reconnection. 97819main_rhessi_AR10039_320x240.mpegMovie of the 22 july 2002 flare. (size : 4Mo, credits : NASA/BBSO/SVS) Observations by TRACE in white light, then by Big Bear solar Observatory in the Ha line (in grey levels) and then again by TRACE in Ultraviolet (in green and white levels). X and g ray emissions observed by RHESSI are represented in red, purple and blue. The sliding of the blue emissions can be easily understood in the frame of sliding reconnection.
References [1] Pariat E., Aulanier G. & Démoulin P., New concept for magnetic reconnexion, SF2A, 2006. Aulanier , Pariat, Démoulin & DeVore, Slip-running reconnexion in quasi-separatrix layers, 2006, Solar Physics, à paraître. Aulanier, Pariat & Démoulin, Current sheet formation in quasi-separatrix layers and hyperbolic flux tubes, 2005, Astronomy & Astrophysics, 444, 961-976.
Contact Etienne Pariat (Observatoire de Paris, LESIA, et CNRS)
Dernière modification le 4 mars 2013