title: Magnetic reconnection from a multiscale instability cascade.
abstract: Magnetic reconnection, the process whereby magnetic field lines break and then                 reconnect to form a different topology, underlies critical dynamics of magnetically                 confined plasmas in both nature and the laboratory. Magnetic reconnection involves localized diffusion of                 the magnetic field across plasma, yet observed reconnection rates are typically much                 higher than can be accounted for using classical electrical resistivity. It is generally proposed that the field diffusion                 underlying fast reconnection results instead from some combination of                 non-magnetohydrodynamic processes that become important on the                 'microscopic' scale of the ion Larmor radius or the ion skin depth. A                 recent laboratory experiment demonstrated a transition from slow to                 fast magnetic reconnection when a current channel narrowed to a microscopic scale,                 but did not address how a macroscopic magnetohydrodynamic system accesses the                 microscale. Recent theoretical models and numerical                     simulations suggest that a macroscopic, two-dimensional                 magnetohydrodynamic current sheet might do this through a sequence of repetitive                 tearing and thinning into two-dimensional magnetized plasma structures having                 successively finer scales. Here we report observations demonstrating a cascade of                 instabilities from a distinct, macroscopic-scale magnetohydrodynamic instability to                 a distinct, microscopic-scale (ion skin depth) instability associated with fast                 magnetic reconnection. These observations resolve the full three-dimensional                 dynamics and give insight into the frequently impulsive nature of reconnection in                 space and laboratory plasmas.
authors.affiliation: Applied Physics, California Institute of Technology, Pasadena, California 91125, USA. auna@caltech.edu
authors.forename: Auna L
authors.initials: AL
authors.lastname: Moser
authors.forename: Paul M
authors.initials: PM
authors.lastname: Bellan