C. H. Jaroschek

Onset of collisionless magnetic reconnection in thin current sheets: Three-dimensional particle simulations

Three-dimensional (3D) particle-in-cell simulations of collisionless magnetic reconnection are presented. The initial equilibrium is a double Harris-sheet equilibrium and periodic boundary conditions are assumed in all three directions. No magnetic seed island is imposed initially, and no flow conditions are imposed. The current sheet width is assumed to be one ion inertial length calculated with the density in the center of the current sheet. The ion to electron mass ratio is mi/me=150, which suppresses the growth of the drift kink instability. Two different runs have been performed: a simulation with exactly antiparallel magnetic field and a simulation with a constant guide field of the same magnitude as the antiparallel field superimposed. In the antiparallel case the inductive field of the waves excited by the lower hybrid drift instability (LHDI) leads to rapid acceleration of the electrons in the center of the current sheet and subsequently to a current sheet thinning. The current increase in the ce...

Fast reconnection in relativistic pair plasmas: Analysis of particle acceleration in self-consistent full particle simulations

Particle acceleration in collisionless magnetic reconnection is studied in the relativistic regime of an electron-positron plasma. For the first time, the highly dynamic late-time evolution of reconnection is simulated in two dimensions (2D) and the finite size of the acceleration region is resolved in 3D applying a fully electromagnetic relativistic particle-in-cell (PIC) code. The late-time evolution is extremely important with respect to particle acceleration, because thin current sheets show a highly dynamic late-time phase with instabilities evolving in the Alfven velocity vA0 regime. Consequently, since c∼vA0 is valid as a peculiarity of pair plasmas, v×B-contributions become dominant in the accelerating electric field. Most remarkable: Though acceleration regions are highly variable at late times, the power-law shape of the particle energy distribution is smoothed compared to quasi-static reconnection configurations at early times [S. Zenitani and M. Hoshino, Astrophys. J. 562, L63 (2001)]. Spectra...

Stationary plasma states far from equilibrium

The appearance of generalized-Lorentzian distribution functions, a physical generalization of the so-called “κ-distributions,” in collisionless space plasmas is a frequent phenomenon. They represent “stationary states far from thermal equilibrium.” It is argued in which way they can be understood from kinetic theory of quasistationary highly correlated (presumably turbulent) states. Fits of model generalized-Lorentzian distributions to measured distributions in the magnetospheric tail and the auroral plasmas are presented. These fits provide reasonable values of density and temperature in the regions under consideration. Moreover, an expression for the Debye screening length is derived for such generalized-Lorentzian plasmas showing that the screening in correlated plasmas is reduced in comparison with Maxwellian plasmas.

Ultrarelativistic plasma shell collisions in γ- ray burst sources: dimensional effects on the final steady state magnetic field

Ultrarelativistic electron-positron plasma shell collisions as an integral part of generic γ-ray burst (GRB) fireball models are studied in the framework of self-consistent three-dimensional particle-in-cell simulations. We compare scenarios at moderately relativistic (γ0 10) and ultrarelativistic (γ0 100) energies that directly correspond to the regimes of internal and external shell collisions, respectively, in GRB synchrotron emission models. Simulated systems comprise 5 × 108 particles, applying a relativistic, fully electromagnetic, massively parallelized code. It is found that Weibel-generated, steady state magnetic equipartition ratios in external collisions reach up to B ~ 12%, exceeding the respective internal ratios by nearly a power of 10. Enhanced B yields can be explained theoretically by the effective reduction of dimensionality in the ultrarelativistic limit, i.e., the energy-dependent confinement of the three-dimensional Weibel instability within quasi-two-dimensional plasma shell slices.