Identification of Y-Shaped and O-Shaped Diffusion Regions During Magnetic Reconnection in a Laboratory Plasma

Masaaki Yamada, Hantao Ji, Scott Hsu, Troy Carter, Russell Kulsrud, Yasushi Ono, Francis Perkins
1997 Physical Review Letters  
Two strikingly different shapes of diffusion regions are identified during magnetic reconnection in a magnetohydrodynamic laboratory plasma. The shapes depend on the third vector component of the reconnecting magnetic fields. Without the third component (antiparallel or null-helicity reconnection), a thin double-Y -shaped diffusion region is identified. In this case, the neutral sheet current profile is accurately measured to be as narrow as the order of the ion gyro-radius. In the presence of
more » ... In the presence of an appreciable third component (cohelicity reconnection), an O-shaped diffusion region appears and grows into a spheromak configuration. [S0031-9007 (97) 02984-0] PACS numbers: 52.30.Jb, 94.30.Lr, 96.60.Rd Magnetic reconnection, a topological rearrangement of magnetic field lines, is a focal point of magnetohydrodynamic (MHD) plasma phenomena since its treatment invokes fundamental issues of resistive MHD theory of conductive plasmas with large Lundquist number [1] [2] [3] [4] . It is considered to be a key process in the evolution of solar flares [1] [2] [3] [4] [5] [6] , in the dynamics of the Earth's magnetosphere [3] , and during plasma formation and/or configuration change of laboratory plasmas. In recent studies of solar flares through soft x-ray pictures taken by the Yohkoh satellite [6], many large solar flares were observed to interact with themselves, changing their topology rapidly on a much shorter time scale than the value predicted by classical theory. Although the observed activities are attributed to magnetic reconnection, the fundamental physics of the fast topological change is still unknown. No conclusive evidence of a neutral sheet current has been observed yet in the solar corona. Recently, the third component of reconnecting fields, which determines actual merging angle, has been recognized to play an important role in the dayside magnetopause: namely, southward solar winds reconnect with the Earth's dipole field (northward) much faster than northward solar winds [7] . The Magnetic Reconnection Experiment (MRX) [8] has been initiated to elucidate magnetic reconnection as an "elementary process" in a plasma occurring during the interplay between plasma and magnetic fields. We will study how this local reconnection process can affect the global plasma characteristics. Our laboratory experiment creates an environment which satisfies the critical MHD plasma conditions and in which the global boundary conditions can be controlled externally. All three components of the magnetic field B are measured during the reconnection process, and studies of 3D reconnection are possible. The most significant results of the present research are the (1) identification of Y-shaped and O-shaped diffusion regions which strongly depend on the existence and the direction of the third vector component (along the neutral line) of B, (2) observation of very thin (order of the ion gyro-radius r i ø plasma size L) neutral sheet current layers during antiparallel magnetic reconnection (without the third component), and (3) observation of a considerable reduction of the reconnection rate when an appreciable third component is present. To describe the motion of magnetic field lines in a plasma, we derive an equation of motion for B by combining the Maxwell equations and Ohm's law, The first term on the right hand side represents the effect of plasma motion with "frozen-in" field lines, and the second term describes diffusion of the field lines with the diffusion coefficient proportional to the plasma resistivity h. If we define t D ϵ m 0 L 2 ͞h as a diffusion time and t A ϵ L͞V A as the Alfvén time, the ratio of these two time scales, which is called the Lundquist number (S ϵ t D ͞t A ), must be much larger than unity in order for the plasma to be treated as an MHD fluid. For typical MHD plasmas such as solar flares [6], S . 10 10 ; for tokamaks, S . 10 7 ; and for MRX plasmas, S ϳ 10 2 10 3 . Figure 1 (a) presents the most commonly used 2D description of magnetic reconnection [1, 4, 9, 10] in which two sets of field lines are oppositely directed above and below the separatrix. As magnetized plasmas move in from each side toward the separatrix, a strong sheet current develops perpendicular to the plane of the page. The sheet current diffuses due to plasma resistivity in this "diffusion region" where a magnetic field line can lose its original identity and reconnect to another field line. In actual reconnection phenomena, such as in solar flares, the magnetosphere, and most laboratory experiments, the magnetic field has three components as illustrated in Fig. 1(b) . The same 2D pictures of the magnetic field lines shown in Fig. 1(a) to describe the merging of two plasmas carrying identical toroidal currents appear quite differently in the 3D illustrations of 0031-9007͞97͞78(16)͞3117(4)$10.00
doi:10.1103/physrevlett.78.3117 fatcat:o346shdwqzfyhof7kqm7myddqe