First Observation of Switch-Off Slow Shocks in Fully Kinetic Particle in Cell Simulation of Magnetic Reconnection

Abstract:

A perduring challenge in the study of reconnection it has long been the failing attempts to reconcile the large scale MHD view based on the Petschek model with the small scale view based on kinetic theory. The first is based on the existence of standing switch off slow shocks (SSS) that eliminate the horizontal (the x component in the usual GSM coordinates) reconnecting magnetic field component forming vertical magnetic field lines. The second is based on nested diffusion regions where the magnetic field lines become decoupled first from ions and then from electrons. The kinetic picture when observed superficially does seem to have seem resemblance to the Petschek topology, despite the nested boxes being more of a Sweet-Parker concept.
Nevertheless, the question has always been: if expanded to sufficiently large scales, does the kinetic description eventually lead tot the formation os SSS? The question remains answered. 
Recently a first negative answer has been proposed in Ref. [1]. The proposed answer is in essence that SSS are made impossible by the presence of a firehose instability in the reconnection exhaust and by the formation of a plateau in the firehose parameter at a value of 0.25 corresponding to the condition where nonlinear slow and intermediate wave become degenerate. 
We report a new series of simulations where we demonstrate that this is not the case in general. While for the specific case used in Ref [1], we indeed re-obtain the same conclusions reached by the authors. But our study demonstrates that case to be very peculiar and not representative of the more general kinetic answer. We will report direct evidence of the presence of extended SSS (over regions of hundreds of ion inertial lengths) in fully kinetic simulations for parameters typical of the magntotail and of the solar wind. Our results indicate that SSS are the natural extension of kinetic reconnection to large scales.
The simulations required for the study are heroic and were conducted with state of the art massively parallel computers provided by the PRACE system in Europe (specifically the Curie and SuperMUC computers) and by NASA Pleiades. The present work was funded by the NASA MMS mission and by the EC-funded project eHeroes (www.eheroes.eu)

[1] Liu, Yi-Hsin, et al., PoP 18.6 (2011): 062110.