Mirror mode disturbances have been reported in many different space plasma environments. We suggest that these structures are fully evolved mirror mode waves that have achieved the condition for marginal stability against further growth. The limiting form is nonlinear, with field variations of the order of 50% of the average field. We argue that as an initially unstable plasma in a uniform field approaches stability, the particle distributions must separate into trapped and untrapped components that respond differently to the changing field. Most of the trapped particles are excluded from the mirror region. Exclusion sufficient to create marginal stability in the vicinity of the magnetic mirrors can be achieved by relatively small field intensifications. The trapped part of the distribution cannot achieve marginal stability without cooling. We envisage the cooling process as a Fermi deceleration achieved as the magnetic wells become deep and the mirror points move apart. Our analysis is both nonlinear and nonquantitative, but it provides an explanation for various aspects of the observations including the commonly reported feature that the mirror waves look like magnetic holes in the ambient field. We describe the pitch angle dependence of the plasma distribution that results from the processes discussed and note that the predicted distributions compare well with the forms observed in plasma data.
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