  This work presents a 2.5-dimensional simulation study of the instability of
current-sheets located in a medium with a strong density variation along the
current layer. 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 leading the system to a final chaotic configuration.
Yet, the onset of the fast phase is not determined by the presence of any
anomalous enhancement in plasma's local resistivity, but rather is the result
of a new mechanism discovered in Lapenta (2008)* and captured only when
sufficient resolution is present. Hence, the effects of the global resistivity,
the global viscosity and the plasma beta on the overall dynamics are
considered. This mechanism allowing the transition from slow to fast
reconnection provides a simple but effective model of several processes taking
place within the solar atmosphere from the high chromosphere up to the low
corona. In fact, the understanding of a spontaneous transition to a
self-feeding fast reconnection regime as well as its macroscopic evolution is
the first and fundamental step to produce realistic models of all those
phenomena requiring fast (and high power) triggering events (* Lapenta G. 2008,
Phys. Rev. Lett., 100, 235001).
