Spontaneous non-steady magnetic reconnection within the solar environment

摘要

Context. A fundamental step to produce realistic models ofthe solar phenomena requiring fast (and high power) triggering eventsis to understand the feasibility of a spontaneous transition from aslow to a fast reconnection regime in the solar environment and itsmacroscopic evolution within the theoretical framework of pureresistive magnetohydrodynamics. Aims. We show the dynamical evolution of a reconnectingforce-free magnetic field in a ``simplified'' solar atmosphere (highchromosphere-low corona) described by a pressure-balanced configurationwith a variable density modeling the transition region. Magneticreconnection plays a fundamental role in this region and we show theefficient working of a non-steady and self-feeding reconnection processwhose development as determined by characteristic solar parameters(global resistivity, global viscosity, plasma beta) is followed. Methods. This work presents a 2.5-dimensional simulation studyof the instability of force-free current-sheets located in a mediumwith a strong density variation along the current layer. In order toreach the needed high resolution and to reduce the influence ofspurious numerical effects, we use a code with afully-implicit-particle (or Flip) algorithm to solve anEulerian-Lagrangian formulation of resistive and viscousmagnetohydrodynamics equations. Results. The initial force-free configuration is observed toundergo a two-stage evolution consisting of an abrupt regime transitionfrom a slow to a fast reconnection process, which leads the system to afinal chaotic configuration. Yet the onset of the fast phase is notdetermined by any anomalous enhancement in the plasma's localresistivity. An asymmetric development of the whole structure isobserved and the related magnetic field topology and energetic featuresare described. Conclusions. This mechanism can be used as a simple buteffective model of several (explosive) processes taking place from thehigh chromosphere up to the low corona. Our simplified model of thesolar atmosphere allows us to obtain a realistic oriented path for theevolution of the overall flow and reconnecting current-sheet. In thepresent work, the numerical experiment provides key information andobservables (like the energetic fluxes) to be compared withobservations. Key words: magnetohydrodynamics (MHD) - methods: numerical -Sun: atmosphere - Sun: coronal mass ejections (CMEs) - Sun: flares -magnetic fields