Phase Diagram for Magnetic Reconnection in Heliophysical, Astrophysical and Laboratory Plasmas

Physics of Plasmas (Impact Factor: 2.25). 09/2011; 18(11). DOI: 10.1063/1.3647505
Source: arXiv

ABSTRACT Recent progress in understanding the physics of magnetic reconnection is
conveniently summarized in terms of a phase diagram which organizes the
essential dynamics for a wide variety of applications in heliophysics,
laboratory and astrophysics. The two key dimensionless parameters are the
Lundquist number and the macrosopic system size in units of the ion sound
gyroradius. In addition to the conventional single X-line collisional and
collisionless phases, multiple X-line reconnection phases arise due to the
presence of the plasmoid instability either in collisional and collisionless
current sheets. In particular, there exists a unique phase termed "multiple
X-line hybrid phase" where a hierarchy of collisional islands or plasmoids is
terminated by a collisionless current sheet, resulting in a rapid coupling
between the macroscopic and kinetic scales and a mixture of collisional and
collisionless dynamics. The new phases involving multiple X-lines and
collisionless physics may be important for the emerging applications of
magnetic reconnection to accelerate charged particles beyond their thermal
speeds. A large number of heliophysical and astrophysical plasmas are surveyed
and grouped in the phase diagram: Earth's magnetosphere, solar plasmas
(chromosphere, corona, wind and tachocline), galactic plasmas (molecular
clouds, interstellar media, accretion disks and their coronae, Crab nebula, Sgr
A*, gamma ray bursts, magnetars), extragalactic plasmas (Active Galactic Nuclei
disks and their coronae, galaxy clusters, radio lobes, and extragalactic jets).
Significance of laboratory experiments, including a next generation
reconnection experiment, is also discussed.

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    ABSTRACT: Using fully kinetic simulations, we study the scaling of the inflow speed of collisionless magnetic reconnection in electron-positron plasmas from the nonrelativistic to ultrarelativistic limit. In the antiparallel configuration, the inflow speed increases with the upstream magnetization parameter σ and approaches the speed of light when σ > O(100), leading to an enhanced reconnection rate. In all regimes, the divergence of the 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, modified by the compression factor of proper density, remains ∼0.1 in both the nonrelativistic and relativistic limits.
    Physical Review Letters 03/2015; 114(9):095002. DOI:10.1103/PhysRevLett.114.095002 · 7.73 Impact Factor
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    ABSTRACT: Recent progress in the understanding of how externally driven magnetic reconnection evolves is organized in terms of parameter space diagrams. These diagrams are constructed using four pivotal dimensionless parameters: the Lundquist number S, the magnetic Prandtl number P_m, the amplitude of the boundary perturbation \hat \Psi_0, and the perturbation wave number \hat k. This new representation highlights the parameters regions of a given system in which the magnetic reconnection process is expected to be distinguished by a specific evolution. Contrary to previously proposed phase diagrams, the diagrams introduced here take into account the dynamical evolution of the reconnection process and are able to predict slow or fast reconnection regimes for the same values of S and P_m, depending on the parameters that characterize the external drive, never considered so far. These features are important to understand the onset and evolution of magnetic reconnection in diverse physical systems.
  • Physics of Plasmas 01/2015; 22(1):012106. DOI:10.1063/1.4906052 · 2.25 Impact Factor


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