Barriers in the transition to global chaos in collisionless magnetic reconnection. I. Ridges of the finite time Lyapunov exponent field

The transitional phase from local to global chaos in the magnetic field of a reconnecting currentlayer is investigated. Regions where the magnetic field is stochastic exist next to regions where thefield is more regular. In regions between stochastic layers and between a stochastic layer and anisland structure, the field of the finite time Lyapunov exponent (FTLE) shows a structure withridges. These ridges, which are special gradient lines that are transverse to the direction of minimumcurvature of this field, are approximate Lagrangian coherent structures (LCS) that act as barriers forthe transport of field lines.