Spontaneous non-steady magnetic reconnection within the solar environment

Context. A fundamental step to produce realistic models of the solar phenomena requiring fast (and high power) triggering events is to understand the feasibility of a spontaneous transition from a slow to a fast reconnection regime in the solar environment and its macroscopic evolution within the theoretical framework of pure resistive magnetohydrodynamics. Aims. We show the dynamical evolution of a reconnecting force-free magnetic field in a simplified solar atmosphere (high chromosphere-low corona) described by a pressure-balanced configuration with a variable density modeling the transition region. Magnetic reconnection plays a fundamental role in this region and we show the efficient working of a non-steady and self-feeding reconnection process whose development as determined by characteristic solar parameters (global resistivity, global viscosity, plasma beta) is followed. Methods. This work presents a 2.5-dimensional simulation study of the instability of force-free current-sheets located in a medium with a strong density variation along the current layer. In order to reach the needed high resolution and to reduce the influence of spurious numerical effects, we use a code with a fully-implicit-particle (or Flip) algorithm to solve an Eulerian-Lagrangian formulation of resistive and viscous magnetohydrodynamics equations. Results. The initial force-free configuration is observed to undergo a two-stage evolution consisting of an abrupt regime transition from a slow to a fast reconnection process, which leads the system to a final chaotic configuration. Yet the onset of the fast phase is not determined by any anomalous enhancement in the plasma's local resistivity. An asymmetric development of the whole structure is observed and the related magnetic field topology and energetic features are described. Conclusions. This mechanism can be used as a simple but effective model of several (explosive) processes taking place from the high chromosphere up to the low corona. Our simplified model of the solar atmosphere allows us to obtain a realistic oriented path for the evolution of the overall flow and reconnecting current-sheet. In the present work, the numerical experiment provides key information and observables (like the energetic fluxes) to be compared with observations.