Magnetic holes have been frequently observed in the magnetosheath of Earth and it is believed that these structures are the result of nonlinear evolution of mirror instability. Mirror mode fluctuations mostly appear as magnetic holes in regions where plasma is marginally mirror stable with respect to the linear instability. We present an expanding box particle in cell simulation to mimic the magnetosheath plasma and produce the mirror mode magnetic holes. We show that magnetic peaks are dominant when plasma is mirror unstable and mirror fluctuations evolve to deep magnetic holes when plasma is marginally mirror stable. Although, the averaged plasma parameters in the simulation are marginally close to mirror instability threshold, the plasma in the magnetic holes is highly unstable to mirror instability and mirror stable in the magnetic peaks.
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