Magnetohydrodynamic Simulations of Reconnection and Particle Acceleration: Three-Dimensional Effects

Abstract

The magnetic fields can change their topology through a process known as magnetic reconnection. This process in not only important for understanding the origin and evolution of the large-scale magnetic field, but is seen as a possibly efficient particle accelerator producing cosmic rays mainly through the first order Fermi process. In this work we study the properties of particle acceleration in reconnection zones and show that the velocity component parallel to the magnetic field of test particles inserted in nearly non-resistive magnetohydrodynamic (MHD) domains of reconnection without including kinetic effects, such as pressure anisotropy, the Hall term, or anomalous effects, increases exponentially. Also, the acceleration of the perpendicular component is always possible in such models. We have found that within contracting magnetic islands or current sheets the particles accelerate predominantly through the first order Fermi process, as previously described, while outside the current sheets and islands the particles experience mostly drift acceleration due to magnetic fields gradients. Considering two dimensional MHD models without a guide field, we find that the parallel acceleration stops at some level. This saturation effect is however removed in the presence of an out-of-plane guide field or in three dimensional models. Therefore, we stress the importance of the guide field and fully three dimensional studies for a complete understanding of the process of particle acceleration in astrophysical reconnection environments.

The energetics of relativistic magnetic reconnection: ion-electron repartition and particle distribution hardness

“…Large scale magnetic reconnection events may be at the origin of large scale transient jets in microquasar systems (de Gouveia dal Pino & Lazarian 2005;de Gouveia Dal Pino et al 2010;Kowal et al 2011;Dexter et al 2014), for example via a magnetic field reversal accreted onto a magnetically arrested disk. Reconnection then occurs in the accretion disk coronae, near the black hole, where particle densities and magnetic fields are high.…”

“…It is a candidate for explaining (i) particle acceleration at pulsar wind termination shocks (Kirk & Skjaeraasen 2003;Pétri & Lyubarsky 2007;Sironi & Spitkovsky 2011a); (ii) the flat radio spectra from galactic nuclei and AGNs (Birk et al 2001) and from extragalactic jets (Romanova & Lovelace 1992); (iii) GeV flares from the Crab nebula (Bednarek & Idec 2011;Uzdensky et al 2011;Cerutti et al 2012aCerutti et al ,b, 2013; (iv) flares in active galactic nuclei (AGN) jets (Giannios et al 2009) or in gamma-ray bursts (Lyutikov 2006a;Lazar et al 2009); (v) the heating of AGN and microquasar coronae and the observed flares (Di Matteo 1998;Merloni & Fabian 2001;Goodman & Uzdensky 2008;Reis & Miller 2013;Romero et al 2014;Zdziarski et al 2014); (vi) the heating of the lobes of giant radio galaxies (Kronberg et al 2004); (vii) transient outflow production in microquasars and quasars (de Gouveia dal Pino & Lazarian 2005;de Gouveia Dal Pino et al 2010;Kowal et al 2011;Dexter et al 2014); (viii) gamma-ray burst outflows and non-thermal emissions (Drenkhahn & Spruit 2002;Giannios & Spruit 2007;McKinney & Uzdensky 2012); (ix) X-ray flashes (Drenkhahn & Spruit 2002); or (x) soft gamma-ray repeaters (Lyutikov 2006b;Uzdensky 2011).…”

“…It can be efficient in collisionless plasmas (Drake et al 2006(Drake et al , 2010Bessho & Bhattacharjee 2012) or in collisional plasmas (Kowal et al 2011) where reconnection is fast because of turbulence. For a closer analysis, the ratio of Larmor radii to typical sizes of the magnetic islands is key.…”

“…This mechanism does not rely on a direct acceleration by the reconnection electric field E rec when particles are demagnetized at the center of the diffusion region, but makes use of the motional electric field in the inflow. It is thus efficient in non-relativistic and collisional plasmas (Kowal et al 2011) where direct acceleration by E rec is known to be negligible. It requires particles crossing the current sheet and bouncing off the other side, i.e., having a Larmor radius in the asymptotic field that is larger than the sheet width, which is generally true only for pre-accelerated particles or hot inflows.…”

“…But to what extent this kinetic energy is distributed between the bulk flow velocity of the outflows, their thermal energy, and a possible high-energy tail, as well as the properties of the high-energy tail, are open questions. This mechanism is inefficient for non-relativistic reconnection because the acceleration zone is too short (along the outflow direction) (Drake et al 2010;Kowal et al 2011;Drury 2012) and affects too few particles, but is efficient under relativistic conditions where the larger reconnection electric field creates a wider acceleration zone (Zenitani & Hoshino 2001. It has indeed been found, with PIC simulations of relativistic reconnection, that power-law tails are produced through particle acceleration by E rec .…”