  Based on Grad-Shafranov-like equations, a gyrotropic plasma where the
pressures in the static regime are only functions of the amplitude of the local
magnetic field is shown to be amenable to a variational principle with a free
energy density given by the parallel tension. This approach is used to
demonstrate that small- amplitude static holes constructed slightly below the
mirror instability threshold identify with lump solitons of KPII equation and
turn out to be unstable. It is also shown that regularizing effects such as
finite Larmor radius corrections cannot be ignored in the description of
large-amplitude mirror structures.
