Secondary Energization in Compressing Plasmoids during Magnetic Reconnection

Abstract

Plasmoids-magnetized quasi-circular structures formed self-consistently in reconnecting current sheets-were previously considered to be the graveyards of energetic particles. In this paper, we demonstrate the important role of plasmoids in shaping the particle energy spectrum in relativistic reconnection (i.e., with upstream magnetization sigma(up) >> 1). Using 2D particle-in-cell simulations in pair plasmas with sigma(up) = 10 and 100, we study a secondary particle energization process that takes place inside compressing plasmoids. We demonstrate that plasmoids grow in time, while their interiors compress, amplifying the internal magnetic field. The magnetic field felt by particles injected in an isolated plasmoid increases linearly with time, which leads to particle energization as a result of magnetic moment conservation. For particles injected with a power-law distribution function, this energization process acts in such a way that the shape of the injected power law is conserved, while producing an additional nonthermal tail f(E) mu E-3 at higher energies, followed by an exponential cutoff. The cutoff energy, which increases with time as E-cut mu root t, can greatly exceed sigma(up)m(e)c(2). We analytically predict the secondary acceleration timescale and the shape of the emerging particle energy spectrum, which can be of major importance in certain astrophysical systems, such as blazar jets.