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

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-Kúzmin cutoff (∼1019 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 magnetohydrodynamic energies. These particles are correlated temporally and spatially with the formation of threedimensional 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 magnetic fields merge (Brown 1999; Priest & Forbes 2000). In the simplest, two-dimensional picture, the interface where the inflowing magnetofluid stagnates contains a current sheet to support 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 transverse to the inflow, exits the reconnection region with a speed not exceeding that of a magnetohydrodynamic (MHD) or Alfvén wave of the coupled fluid and field. In 1/2 v p B/ (4pr) A this conventional two-dimensional geometry, the inflow and outflow regions are distinguished by field lines that meet at the center of the reconnection layer where the magnetic field is identically zero. This location is known as the X-point, and the X-line is the extension of the X-point normal to the plane and in the direction of both the electric field and the current. The electric field at the X-point (of magnitude ) E p dw/dtFx can accelerate ions along the X-line. However, since the electric field is in the direction, in the twov B E · B { 0 dimensional picture. Ions that remain precisely on the X-line (where ) can be accelerated appreciably. However, relaB { 0 tively few ions will participate in this acceleration unless some additional features such as multiple X-lines, current sheets, or turbulent bubbles (Ambrosiano et al. 1988) trap them near the region of most intense electric field. In three dimensions, the magnetic field can have a guiding component normal to the plane. Whether it is of external origin or self-consistently generated (Cothran et al. 2002), a guide field can enhance particle acceleration by keeping ions near the intense electric fields in the reconnection region. In fact, a viable definition of three-dimensional reconnection at a point at which B does not vanish (Schindler, Hesse, & Birn 1988; Hesse & Schindler 1988) is the appearance of an electric field along the guide field. Several models of reconnection include mechanisms for generation of out of plane fields (Shay et al. 1998). Threedimensional solar structures have been both observed (Fletcher et al. 2001) and simulated (Birn et al. 2000). Particle acceleration in reconnection geometries is a subject of intense research (Somov & Kosugi 1997; Aschwanden, Schwartz, & Dennis 1998; de Gouveia Dal Pino & Lazarian 2000). We report a measurement of energetic ions (protons) accelerated along the X-line coincident with the formation of threedimensional magnetic reconnection structures. Ion detectors are spatially aligned with the anticipated X-line, and detection is temporally correlated with the magnetics. We have clear evidence of a component of the magnetic field along the XB line. We have also mapped out the energy distribution of the ions and find it to be superthermal and super-Alfvénic. Measurements are consistent with an accelerated ion distribution characterized by a drift energy of about 90 eV and thermalized to 30 eV with eV. E 1 200 max There have been recent reports of reconnection-driven bidirectional sub-Alfvénic outflows observed on the Sun (Innes et al. 1997) and in the magnetospheres of the Earth (Phan et al. 2000; Oieroset et al. 2001) and Jupiter (Russell et al. 1998). There has also been significant evidence of energetic particles associated with solar flares from the Yohkoh satellite (Masuda et al. 1994). Yohkoh observations show hard X-ray sources (150 keV) located at the loop-tops and footpoints of flares, suggesting electron acceleration along and above the loops. The maximum observed energies of particles of solar origin are ≥5 GeV (Ryan, Lockwood, & Debrunner 2000). Magnetic reconnection may play a significant role in the acceleration of charged particles on astrophysical scales. The key feature is that the reconnection electric field is of order , where vB is a typical MHD flow velocity and B is a typical magnetic v field strength. If magnetic reconnection plays a role in particle acceleration, then maximum particle energies should scale as the reconnection electromotive force (EMF) , E p E · dl ∼ vBL ∫ where L is a characteristic length of the system along the electric field (Makishima 1999). Indeed, this scaling is observed in vBL many systems where both can be estimated and energetic vBL particle measurements are available. For example, in our exL64 MAGNETIC RECONNECTION Vol. 577 Fig. 1.—Idealized two-dimensional magnetic reconnection. Two regions contain inflowing magnetofluid with oppositely directed magnetic fields. Reconnected fields are convected away in the outflow regions. The separatrices (dashed lines) distinguish the four topologically distinct regions. There is a layer of current density (and Ohmic electric field) centered at the X-point. Fig. 2.—Top and end views of the SSX. Spheromaks of either helicity are formed on either side by magnetized plasma guns. The toroidal and poloidal components of each spheromak’s helical field are shown in red and blue, respectively. Reconnection occurs through large slots in the back walls at the midplane. The three-dimensional magnetic probe array and retarding grid energy analyzers (green and red) are located at the midplane. Magnified views illustrate magnetic probe locations and data shown as arrows. The view in (a) is the plane; the view in (b) is the plane. Of the 200 vectors, only ˆ ˆ ˆ ˆ x-z x-y B those on facing planes are highlighted. The diameter of the chamber is about 0.6 m. periment m s )( )(0.1 m V, in the 5 E p vBL ≤ (10 0.05T ) p 500 solar corona m s )( )(10 m V, 5 10 E p vBL ≤ (10 0.01T ) p 10 and in exotic objects like the Crab pulsar 6 E p v BL ≤ (10 rot m s )( )(10 m V. 7 17 10 T ) p 10 There is clear evidence for the existence of cosmic rays with energies in excess of 10 eV. Yet, energetic particles born beyond about 100 Mpc should have been slowed by the GreisenZatsepin-Kúzmin (GZK) energy cutoff (∼ eV; Cronin 19 5 # 1