Petschek vs. species separation vs. plamoids: an investigation of the long term evolution of collisionless magnetic reconnection

Fifty years ago, scientists were struggling to explain fast magnetic reconnection. Then, several mechanisms for fast reconnection were unveiled: formation of switch-off slow shocks flanking the diffusion region (Petschek’s model, [Vasyliunas 1975]), species separation, with formation of an ion and an electron diffusion region [Birn 2001], plasmoid formation [Loureiro 2007]. The different mechanisms are supposed to be triggered at different levels of collisionality and system size [Ji and Daughton 2011].In the (collisionless) solar wind, the expected fast reconnection mechanism is species separation. However, both Petschek-like exhausts [Mistry 2015] and plasmoid-dominated current sheets [Eriksson 2014] are observed there. Hence the questions: how are the different fast reconnection mechanisms related? How does a system move in the Daughton & Ji phase diagram, when an ongoing reconnection event modifies the system characteristics?We investigate the issue through large domain, long 2D3V Particle In Cell simulations.We observe the development of a mixed scenario, where initial plasmoid emission disrupts the formation of a Petschek-like exhaust. When a monster plasmoid [Loureiro 2012] forms at the X point preventing further plasmoid emission, a Slow Shock/ Rotational Discontinuity (SS/RD) compound structure forms, at large distances from the X point, and performs the switch-off of the in plane tangential component of the magnetic field. The SS and RD are characterised as such via Rankine- Hugoniot jump conditions and Walén test. For the first time, switch-off discontinuities and plasmoid-unstable current sheets are observed together in a collisionless reconnection event initiated with a GEM-like perturbation [Innocenti 2015].