[1] Particle-in-Cell simulations of magnetic reconnection with an H + current sheet and a mixed background plasma of H + and O + ions are completed using physical mass ratios. Four main results are shown. First, the O + presence slightly decreases the reconnection rate and the magnetic reconnection evolution depends mainly on the lighter H + ion species in the presented simulations. Second, the Hall magnetic field is characterized by a two-scale structure in presence of O + ions: it reaches sharp peak values in a small area in proximity of the neutral line, and then decreases slowly over a large region. Third, the two background species initially separate in the outflow region because H + and O + ions are accelerated by different mechanisms occurring on different timescales and with different strengths. Fourth, the effect of a guide field on the O + dynamics is studied: the O + presence does not change the reconnected flux and all the characteristic features of guide field magnetic reconnection are still present. Moreover, the guide field introduces an O + circulation pattern between separatrices that enhances high O + density areas and depletes low O + density regions in proximity of the reconnection fronts. The importance and the validity of these results are finally discussed.

[1] We report a 3D magnetohydrodynamics simulation that studies the formation of dipolarization fronts during magnetotail reconnection. The crucial new feature uncovered in the present 3D simulation is that the process of reconnection produces flux ropes developing within the reconnection region. These flux ropes are unstable to the kink mode and introduce a spontaneous structure in the dawn-dusk direction. The dipolarization fronts forming downstream of reconnection are strongly affected by the kinking ropes. At the fronts, a density gradient is present with opposite direction to that of the acceleration field and leads to an interchange instability. We present evidence for a causal link where the perturbations of the kinking flux ropes with their natural and well defined scales drive and select the scales for the interchange mode in the dipolarization fronts. The results of the simulation are validated against measured structures observed by the THEMIS mission. Citation: Lapenta, G., and L. Bettarini (2011), Self-consistent seeding of the interchange instability in dipolarization fronts, Geophys. Res. Lett., 38, L11102,

To model the interaction between the solar wind and the interstellar wind, magnetic fields must be included. Recently Opher et al. 2003 found that, by including the solar magnetic field in a 3D high resolution simulation using the University of Michigan BATS-R-US code, a jet-sheet structure forms beyond the solar wind Termination Shock. Here we present an even higher resolution threedimensional case where the jet extends for 150AU beyond the Termination Shock. We discuss the formation of the jet due to a de Laval nozzle effect and it's subsequent large period oscillation due to magnetohydrodynamic instabilities. To verify the source of the instability, we also perform a simplified two dimensionalgeometry magnetohydrodynamic calculation of a plane fluid jet embedded in a neutral sheet with the profiles taken from our 3D simulation. We find remarkable agreement with the full three-dimensional evolution. We compare both simulations and the temporal evolution of the jet showing that the sinuous mode is the dominant mode that develops into a velocity-shear-instability with a growth rate of 5 × 10 −9 sec −1 = 0.027 years −1 . As a result, the outer edge of the heliosphere presents remarkable dynamics, such as turbulent flows caused by the motion of the jet. Further study, e.g., including neutrals and the tilt of the solar rotation from the magnetic axis, is required before we can definitively address how this outer boundary behaves. Already, however, we can say that the magnetic field effects are a major player in this region changing our previous notion of how the solar system ends.

Three-dimensional numerical simulations of the tearing instability in the framework of compressible and resistive magnetohydrodynamics are presented. Simulations have been performed with a novel Eulerian conservative high order code, including an explicit resistivity, which uses implicit high order numerical schemes having higher spectral resolution than classical schemes. The linear and non linear evolution of the tearing instability has been followed for force-free and pressure-balanced initial equilibrium configurations. Pressure equilibrium configurations are subject to a secondary instability which drives the system toward a quasi two dimensional structure oriented perpendicularly to the initial configuration. The development of secondary instabilities is suppressed by a guide field allowing the coalescence instability to fully develop in the system. Force-free initial configurations follow an intermediate path with respect the previous cases: Strong coalescence of magnetic islands, due to the non linear evolution of the tearing instability, is observed before the system enters in a phase dominated by 3D modes. The histories of the differing initial current-sheet equilibria have counterparts in the energy spectra that, for all three cases, are observed to be strongly anisotropic.