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Register a new userMagnetic reconnection and stochastic plasmoid chains in high-Lundquist-number plasmas
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A numerical study of magnetic reconnection in the large-Lundquist-number ($S$), plasmoid-dominated regime is carried out for $S$ up to $10^7$. The theoretical model of Uzdensky {it et al.} [Phys. Rev. Lett. {bf 105}, 235002 (2010)] is confirmed and partially amended. The normalized reconnection rate is $ ormEeffsim 0.02$ independently of $S$ for $Sgg10^4$. The plasmoid flux ($Psi$) and half-width ($w_x$) distribution functions scale as $f(Psi)sim Psi^{-2}$ and $f(w_x)sim w_x^{-2}$. The joint distribution of $Psi$ and $w_x$ shows that plasmoids populate a triangular region $w_xgtrsimPsi/B_0$, where $B_0$ is the reconnecting field. It is argued that this feature is due to plasmoid coalescence. Macroscopic monster plasmoids with $w_xsim 10$% of the system size are shown to emerge in just a few Alfven times, independently of $S$, suggesting that large disruptive events are an inevitable feature of large-$S$ reconnection.
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We use extensive 3D resistive MHD simulations to study how large-scale current sheets will undergo fast reconnection in the high Lundquist number $S$ limit (above $sim 10^4$), when the system is subject to different externally driven turbulence levels and the self-generated turbulence produced by 3D reconnection dynamics. We find that the normalized global reconnection rate $sim 0.01 - 0.13$, weakly dependent on $S$. Global reconnection with the classic inflow/outflow configurations is observed, and 3D flux ropes are hierarchically formed and ejected from reconnection regions. A statistical separation of the reconnected magnetic field lines follows a super-diffusive behavior, from which the rate is measured to be very similar to that obtained from the mixing of tracer populations. We find that the reconnection rate scales roughly linearly with the turbulence level during the peak of reconnection. This scaling is consistent with the turbulence properties produced by both the externally driven and self-generation processes. These results imply that large-scale thin current sheets tend to undergo rigorous reconnection.
We present the results of two-dimensional and three-dimensional magnetohydrodynamical numerical simulations of relativistic magnetic reconnection, with particular emphasis on the dynamics of the plasma in a Petschek-type configuration with high Lundquist numbers, Ssim 10^5-10^8. The numerical scheme adopted, allowing for unprecedented accuracy for this type of calculations, is based on high order finite volume and discontinuous Galerkin methods as recently proposed by citet{Dumbser2009}. The possibility of producing high Lorentz factors is discussed, showing that Lorentz factors close to sim 4 can be produced for a plasma parameter sigma_m=20. Moreover, we find that the Sweet-Parker layers are unstable, generating secondary magnetic islands, but only for S > S_c = 10^8, much larger than what is reported in the Newtonian regime. Finally, the effects of a mildly anisotropic Ohm law are considered in a configuration with a guide magnetic field. Such effects produce only slightly faster reconnection rates and Lorentz factors of about 1% larger with respect to the perfectly isotropic Ohm law.
The shock structure of a plasmoid in magnetic reconnection in low-beta plasmas is investigated by two-dimensional magnetohydrodynamic simulations. Using a high-accuracy code with unprecedented resolution, shocks, discontinuities, and their intersections are resolved and clarified. Contact discontinuities emanate from triple-shock intersection points, separating fluids of different origins. Shock-diamonds inside the plasmoid appear to decelerate a supersonic flow. New shock-diamonds and a slow expansion fan are found inside the Petschek outflow. A sufficient condition for the new shock-diamonds and the relevance to astrophysical jets are discussed.
(abridged) Magnetic reconnection is the topological reconfiguration of the magnetic field in a plasma, accompanied by the violent release of energy and particle acceleration. Reconnection is as ubiquitous as plasmas themselves, with solar flares perhaps the most popular example. Over the last few years, the theoretical understanding of magnetic reconnection in large-scale fluid systems has undergone a major paradigm shift. The steady-state model of reconnection described by the famous Sweet-Parker (SP) theory, which dominated the field for ~50 years, has been replaced with an essentially time-dependent, bursty picture of the reconnection layer, dominated by the continuous formation and ejection of multiple secondary islands (plasmoids). Whereas in the SP model reconnection was predicted to be slow, a major implication of this new paradigm is that reconnection in fluid systems is fast (i.e., independent of the Lundquist number), provided that the system is large enough. This conceptual shift hinges on the realization that SP-like current layers are violently unstable to the plasmoid instability - implying, therefore, that such current sheets are super-critically unstable and thus can never form in the first place. This suggests that the formation of a current sheet and the subsequent reconnection process cannot be decoupled, as is commonly assumed. This paper provides an introductory-level overview of the recent developments in reconnection theory and simulations that led to this essentially new framework. We briefly discuss the role played by the plasmoid instability in selected applications, and describe some of the outstanding challenges that remain at the frontier of this subject. Amongst these are the analytical and numerical extension of the plasmoid instability to (i) 3D and (ii) non-MHD regimes. New results are reported in both cases.
Using fully kinetic simulations, we study the scaling of the inflow speed of collisionless magnetic reconnection from the non-relativistic to ultra-relativistic limit. In the anti-parallel configuration, the inflow speed increases with the upstream magnetization parameter $sigma$ and approaches the light speed when $sigma > O(100)$, leading to an enhanced reconnection rate. In all regimes, the divergence of pressure tensor is the dominant term responsible for breaking the frozen-in condition at the x-line. The observed scaling agrees well with a simple model that accounts for the Lorentz contraction of the plasma passing through the diffusion region. The results demonstrate that the aspect ratio of the diffusion region remains $sim 0.1$ in both the non-relativistic and relativistic limits.
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