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A two‐dimensional ( x, z ) particle simulation model based on the Darwin approximation to Maxwell's equations is developed for studying collisionless reconnection in the magnetotail. The particles and fields are initialized in accord with a general equilibrium configuration which includes a pressure gradient along the tail axis and tail flaring. The model is used to investigate a number of theoretical issues regarding the spontaneous ion tearing instability under the assumption that the electron dynamics are unimportant. It is demonstrated both numerically and analytically that in a thin current sheet with ρ i 0 ∼ λ (ρ i 0 is the ion Larmor radius based on the lobe field and λ is the characteristic thickness of the current sheet) the growth rates in the absence of a normal field component are much smaller than expected based upon the analytic theory for a thick sheet (ρ i 0 ≪ λ). For such a thin current sheet the presence of a normal field B z on axis of even a few percent strongly inhibits the growth of the instability. This result is not altered by the addition of a constant B y component smaller than the lobe field. It is demonstrated further that the transition to stability occurs when the cyclotron frequency based on B z equals the growth rate of the B z = 0 tearing mode. This requires typically a normal field of the order of 6% of the lobe field. If a sufficiently large external perturbation of the lobe magnetic field reduces the normal field on axis below the stabilization threshold over a significant fraction of a tearing mode wavelength, then one can recover the rapid instability of the one‐dimensional neutral sheet.
Pritchett et al. (Mon,) studied this question.