title: Particle acceleration in turbulence and weakly stochastic reconnection.
abstract: In this Letter we analyze the energy distribution evolution of test particles injected in three dimensional (3D) magnetohydrodynamic (MHD) simulations of different magnetic reconnection configurations. When considering a single Sweet-Parker topology, the particles accelerate predominantly through a first-order Fermi process, as predicted in  and demonstrated numerically in . When turbulence is included within the current sheet, the acceleration rate is highly enhanced, because reconnection becomes fast and independent of resistivity  and allows the formation of a thick volume filled with multiple simultaneously reconnecting magnetic fluxes. Charged particles trapped within this volume suffer several head-on scatterings with the contracting magnetic fluctuations, which significantly increase the acceleration rate and results in a first-order Fermi process. For comparison, we also tested acceleration in MHD turbulence, where particles suffer collisions with approaching and receding magnetic irregularities, resulting in a reduced acceleration rate. We argue that the dominant acceleration mechanism approaches a second order Fermi process in this case.
authors.affiliation: Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão, 1226-Cidade Universitária, CEP 05508-090, São Paulo/SP, Brazil. kowal@astro.iag.usp.br
authors.forename: Grzegorz
authors.initials: G
authors.lastname: Kowal
authors.forename: Elisabete M
authors.initials: EM
authors.lastname: de Gouveia Dal Pino
authors.forename: A
authors.initials: A
authors.lastname: Lazarian