  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.
