Flux-freezing breakdown in high-conductivity magnetohydrodynamic turbulence
| Autor: | Kai Bürger, Charles Meneveau, Hussein Aluie, Gregory L. Eyink, Randal Burns, Ethan T. Vishniac, Cristian Lalescu, Alexander S. Szalay, Kalin Kanov |
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| Rok vydání: | 2013 |
| Předmět: | |
| Zdroj: | Nature. 497:466-469 |
| ISSN: | 1476-4687 0028-0836 |
| DOI: | 10.1038/nature12128 |
| Popis: | A magnetohydrodynamic simulation of a magnetized plasma at high conductivity shows that, whereas the magnetic flux can be considered ‘frozen’ into the medium for laminar flow, in a turbulent medium the motion of the field lines can become indeterministic, leading to a breakdown in flux freezing. A magnetohydrodynamic simulation of a magnetized plasma at high conductivity shows that, whereas the magnetic flux can be considered 'frozen' into the medium for laminar flow, in a turbulent medium the motion of the field lines can become indeterministic, leading to a breakdown in flux freezing. This finding is of importance for the study of astrophysical plasmas such as those found in the Sun's corona, explaining how microscale mechanisms of line slippage can be accelerated to reconnect rapidly in large-scale astronomical structures. The idea of ‘frozen-in’ magnetic field lines for ideal plasmas1 is useful to explain diverse astrophysical phenomena2, for example the shedding of excess angular momentum from protostars by twisting of field lines frozen into the interstellar medium. Frozen-in field lines, however, preclude the rapid changes in magnetic topology observed at high conductivities, as in solar flares2,3. Microphysical plasma processes are a proposed explanation of the observed high rates4,5,6, but it is an open question whether such processes can rapidly reconnect astrophysical flux structures much greater in extent than several thousand ion gyroradii. An alternative explanation7,8 is that turbulent Richardson advection9 brings field lines implosively together from distances far apart to separations of the order of gyroradii. Here we report an analysis of a simulation of magnetohydrodynamic turbulence at high conductivity that exhibits Richardson dispersion. This effect of advection in rough velocity fields, which appear non-differentiable in space, leads to line motions that are completely indeterministic or ‘spontaneously stochastic’, as predicted in analytical studies10,11,12,13. The turbulent breakdown of standard flux freezing at scales greater than the ion gyroradius can explain fast reconnection of very large-scale flux structures, both observed (solar flares and coronal mass ejections) and predicted (the inner heliosheath, accretion disks, γ-ray bursts and so on). For laminar plasma flows with smooth velocity fields or for low turbulence intensity, stochastic flux freezing reduces to the usual frozen-in condition. |
| Databáze: | OpenAIRE |
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