Self consistent one dimensional hybrid code (particle ions, massless electron fluid) simulations are used to examine the possible structure and evolution of a dipolarizing field reversal in the near earth geotail. Here, we run the simulations in both the ion kinetic (hybrid) limit, where the ion moments and the electromagnetic fields are all well resolved on ion Larmor scales, and the quasi- MHD limit, where all ion trajectories are still fully resolved but the ion moments are taken, and the electromagnetic fields are advanced on spatial scales larger than the gyroradii of the ions. One simulation selfconsistently includes the ion current and density resolved on ion Larmor scales whereas the other does not; both simulations have sufficient bandwidth to well resolve all the low frequency wave modes expected from kinetic (in the hybrid case) or 2 fluid (in the quasi- MHD case) theory. It is shown that the evolution of the electromagnetic field and ion pressure tensor are markedly different in the two cases. The results suggest that the selfconsistent quasi- MHD solution obtained in this way cannot be obtained from a coarse graining of the full kinetic (hybrid) selfconsistent solution
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