Experimental Observation of Energetic Ions Accelerated by Three-dimensional Magnetic Reconnection in a Laboratory Plasma

Abstract

Magnetic reconnection is widely believed responsible for heating the solar corona as well as for generating X-rays and energetic particles in solar flares. On astrophysical scales, reconnection in the intergalactic plasma is a prime candidate for a local source (!100 Mpc) of cosmic rays exceeding the Greisen-Zatsepin-Kuzmin cutoff (∼10 19 eV). In a laboratory astrophysics experiment, we have made the first observation of particles accelerated by magnetic reconnection events to energies significantly above both the thermal and the characteristic magneto- hydrodynamic energies. These particles are correlated temporally and spatially with the formation of three- dimensional magnetic structures in the reconnection region. Subject headings: acceleration of particles — cosmic rays — magnetic fields — methods: laboratory — MHD — plasmas Magnetic reconnection occurs when two bodies of highly conductive plasma bearing oppositely directed embedded mag- netic fields merge (Brown 1999; Priest & Forbes 2000). In the simplest, two-dimensional picture, the interface where the in- flowing magnetofluid stagnates contains a current sheet to sup- port the curl of the magnetic field and an electric field to support the consumption of magnetic flux (see Fig. 1). Within the bulk of each inflow, the motion of the field and fluid are coupled owing to the high conductivity. At the interface, this condition no longer holds, and field lines convected into this region break and reconnect across the layer, producing a global change in field topology. The reconnection outflow, coplanar and trans- verse to the inflow, exits the reconnection region with a speed not exceeding that of a magnetohydrodynamic (MHD) or Alf- ven wave of the coupled fluid and field. In 1/2