Experimental investigation of the neutral sheet profile during magnetic reconnection

During magnetic reconnection, a "neutral sheet" current is induced, heating the plasma. The resultant plasma thermal pressure forms a stationary equil ibrium with the opposing magnetic fields. The reconnection layer profile ho lds significant clues about the physical mechanisms which control reconnect ion. In the Magnetic Reconnection Experiment [M. Yamada et al., Phys. Plasm as 4, 1936 (1997)], a quasi steady-state and axisymmetric neutral sheet pro file has been measured precisely using a magnetic probe array with spatial resolution equal to one quarter of the ion gyro-radius. It was found that t he reconnecting field profile fits well with a Harris-type profile [E. G. H arris, Il Nuovo Cimento 23, 115 (1962)], B(x) similar to tanh(x/delta). Thi s agreement is remarkable since the Harris theory does not take into accoun t reconnection and associated electric fields and dissipation. An explanati on for this agreement is presented. The sheet thickness delta is found to b e similar to 0.4 times the ion skin depth, which agrees with a generalized Harris theory incorporating nonisothermal electron and ion temperatures and finite electric field. The detailed study of additional local features of the reconnection region is also presented. (C) 2000 American Institute of P hysics. [S1070-664X(00)95505-8].