Accueil > Séminaires d’Astrophysique > MHD Reconnection in Astrophysical Systems


MHD Reconnection in Astrophysical Systems

Nuno Loureiro (Instituto de Plasmas e Fusao Nuclear, IST Lisbon) — au LATT

Par Francois RINCON - 15/02/2010


Séminaires d’Astrophysique

de l’Observatoire Midi-Pyrénées

Séminaire de

Nuno Loureiro

(Instituto de Plasmas e Fusao Nuclear, IST Lisbon)

Jeudi 4 Mars 2010 à 11h

OMP, salle Coriolis

MHD Reconnection in Astrophysical Systems

Magnetic reconnection is a ubiquitous plasma physics phenomenon, characterized by rapid reconfiguration of the magnetic field topology. Within the simplest plasma framework - single fluid resistive MHD - the Sweet-Parker (SP) model provides the currently accepted description of reconnection. Famously, however, the SP model predicts reconnection rates which are orders of magnitude too slow to explain observations. It is known that fast reconnection rates can be obtained in more complex, collisionless descriptions of the plasma. However, in some situations (e.g., the solar chromosphere, the ISM, inside stars and accretion discs, and in the high-energy-density environments in central engines of -ray bursts (GRB) and core-collapse supernovae), the density is so high that the current layer is collisional and resistive MHD should apply. Can reconnection be fast in those environments ?

The SP model disregards two essential facts : first, that most, if not all, plasmas where reconnection takes place are likely to be turbulent ; second, that the current layers predicted by the SP theory are now realized to be violently unstable to plasmoid (secondary island) formation. In this talk, we discuss the effects of turbulence and plasmoids in MHD reconnection. We present the first analytical theory of the instability of SP-like current sheets and formation of plasmoid chains. Results of direct numerical simulations are shown, validating the theoretical predictions and detailing the complex nonlinear evolution of this instability. These results strongly suggest that high-Lundquist-number reconnection is inherently time-dependent and hence call for a substantial revision of the standard Sweet-Parker quasi-stationary picture. When turbulence is added to the background, we obtain reconnection rates whose dependence on the plasma resistivity eta is much shallower than the SP eta^1/2 dependence, indicating that fast reconnection is possible within the resistive MHD framework.



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