The reconnection of magnetic fields between plasmas with different densities: Scaling relations

Using two-dimensional compressible-magnetohydrodynamic computer simulations, the reconnection of magnetic fields between two plasmas with different mass densities is examined. Antiparallel, undriven merging is studied from a Harris-sheet initial condition with the sheet separating the two different plasmas. Reconnection is initiated with a resistive spot in the center of the Harris sheet. Density ratios from 1 to 320 are simulated. It is found that the rate of magnetic-field reconnection R is give approximately as R=0.07 upsilon(Ah), where upsilon(Ah)=B/[4 pi(0.5 rho(1)+0.5 rho(1))](1/2) is a hybrid Alfven speed constructed by averaging the densities of the two plasmas. In the symmetric-density case, plasma jetting speeds are equal to the local ExB velocity; in the asymmetric-density cases, the plasma jetting speeds exceed the local ExB velocity. In the asymmetric cases, plasma jetting tends to be parallel to the local magnetic field. In the symmetric-density case the plasma jetting arises from the site of the X-line and resides in the region of magnetic-field reversal; in the asymmetric case, jetting tends to reside in the lower-density plasma. (C) 2007 American Institute of Physics.