One of the most fundamental questions in reconnection physics is how the dynamical evolution will scale to macroscopic systems of physical relevance. This issue is examined for electron-positron plasmas using two-dimensional fully kinetic simulations with both open and periodic boundary conditions. The resulting evolution is complex and highly dynamic throughout the entire duration. The initial phase is distinguished by the coalescence of tearing islands to larger scale while the later phase is marked by the expansion of diffusion regions into elongated current layers that are intrinsically unstable to plasmoid generation. It appears that the repeated formation and ejection of plasmoids plays a key role in controlling the average structure of a diffusion region and preventing the further elongation of the layer. The reconnection rate is modulated in time as the current layers expand and new plasmoids are formed. Although the specific details of this evolution are affected by the boundary and initial conditions, the time averaged reconnection rate remains fast and is remarkably insensitive to the system size for sufficiently large systems. This dynamic scenario offers an alternative explanation for fast reconnection in large-scale systems.

Collisionless magnetic reconnection in large-scale electron-positron plasmas

One major characteristic associated with collisionless space plasmas is the development of non-Maxwellian velocity distribution that in many circumstances can be represented by the κ function characterized by the κ parameter. This paper discusses the mathematical character and physical origin of the κ function by first showing that the κ velocity function may be expressed in terms of exponential functions multiplied by the kinetic energy and its higher orders. The possible development of κ velocity distribution is illustrated by the problem of low-frequency waves and instabilities in uniform magnetized plasmas with bi-Maxwellian distribution. It is observed that the background and perturbed distribution functions bear the same forms as the zeroth- and higher-order terms of the κ function expanded in the limit of κ→∞. The consequence of assuming κ velocity distribution in inhomogeneous plasmas is illustrated by the Vlasov-Maxwell equilibrium problems that show the nonthermal equilibrium characteristic of n...

Mathematical and physical aspects of Kappa velocity distribution

Using resistive magnetohydrodynamic simulations, we investigate the influence of various parameters on the reconnection rate in two scenarios of magnetic reconnection. The first scenario consists of the “Newton Challenge” problem [Birn et al., Geophys. Res. Lett. 32, L06105 (2005)]. In this scenario, reconnection is initiated in a plane Harris-type current sheet by temporally limited, spatially varying, inflow of magnetic flux. The second scenario consists of the well-studied island coalescence problem. This scenario starts from an equilibrium containing periodic magnetic islands with parallel current filaments. Due to the attraction between parallel currents, pairs of islands may move toward each other, forming a current sheet in between. This leads to reconnection and ultimately the merging of islands. In either scenario, magnetic reconnection may be considered as being driven by external or internal forcing. Consistent with that interpretation we find that in either case the maximum reconnection rate (...

Reconnection rates in driven magnetic reconnection

The evolution of many plasma systems is adiabatic, i.e., plasma entropy is conserved in each magnetic flux tube. An apparently surprising result found recently from simulations of a forced magnetic reconnection problem (the “Newton Challenge” [J. Birn et al., Phys. Plasmas 13, 092117 (2006)]) is that even in the presence of a dissipative process such as reconnection, the entropy within a flux tube can still be approximately conserved, due to the strong localization of the dissipation. To address plasma equilibrium with such adiabatic constraints, a novel code has been developed that computes equilibria with entropy profile as input, using the alternating dimension method [H. Grad et al., Proc. Natl. Acad. Sci. USA 72, 3789 (1975)]. The code alternates between solving the two–dimensional (2D) Grad-Shafranov equation to obtain the field configuration (flux function A) from the pressure profile P(A) and a 1D ordinary differential equation that uses the entropy conservation to derive the pressure function P(A...

Adiabatic plasma equilibrium and application to a reconnection problem

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Collisionless Magnetic Reconnection in Large-Scale Electron-Positron Plasmas

A strong violation of the Quark Line Rule is observed if we allow the pion nucleon σ term to attain the present experimental value. This in turn indicates the possibility of a large strangeness content in the proton. In this work we have shown that the value of the σ term is seen to be raised from what is expected from the GMOR scheme of chiral symmetry breaking through a sum rule which shows that the ratio of the meson wave function renormalization constants ZK12/Zπ12 may deviate considerably from unity by using the recent QCD estimate on the quark vacuum condensate ratio O/ O

QCD determination of the pion nucleon σ term and the strangeness content of the proton

Using a QCD estimate on the quark-vacuum condensate ratio (ss)0/(uu)0 the authors show, by deriving a sum rule, that the ratio of the meson wavefunction renormalization constants ZK12//Zpi 1/2 may deviate considerably from the SU(3) symmetric value of unity. This in turn raises the value of the pion-nucleon sigma -term from what is normally expected from the conventional GMOR scheme of chiral symmetry breaking.

Shukla Basu

Papers

QCD determination of the pion nucleon σ term and the strangeness content of the proton

Pion-nucleon σ-term : a QCD assessment