Abstract
The onset of collisionless magnetic reconnection in current sheets containing a localized enhancement of the initial normal magnetic field component is examined using 2D and 3D particle-in-cell simulations that treat a closed system. In the 2D case, the current sheet is found to remain stable for at least several hundred inverse ion cyclotron times. In 3D, however, the system is found to be unstable to a ballooning/interchange type of mode with wavenumber kyρin∼1 (where ρin is the ion gyroradius in the normal field Bz). These modes evolve to form intense “heads” of strongly enhanced Bz; in the wake of the heads are regions of strongly reduced or reversed Bz. These local field reversals lead to the onset of reconnection with reconnection electric fields several times more intense than typical values seen for 2D reconnection.
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