Mirror Instability in the Turbulent Expanding Solar Wind

The relationship between a decaying strong turbulence and the mirror instability in a slowlyexpanding plasma is investigated using two-dimensional hybrid expanding boxsimulations. We impose the initial ambient magnetic field perpendicular to thesimulation box (and perpendicular to the radial direction). We start with aspectrum of large-scale, linearly-polarized, random-phase Alfvenic fluctuationswhich have energy equipartition between kinetic and magnetic fluctuations andvanishing correlation between the two fields. A turbulent cascade rapidlydevelops, magnetic field fluctuations exhibit a power-law spectrum at largescales and a steeper spectrum at ion scales. The imposed expansion leads togeneration of an important perpendicular proton temperature anisotropy.Turbulence is able to partly reduce the anisotropy but the system eventuallydrives the mirror instability that generates localized structures, in the formof pressure-balanced magnetic humps. These magnetic humps are locatedtypically at the flanks of magnetic islands and reduce the perpendiculartemperature anisotropy. Mirror instability may coexist with strong plasmaturbulence even in a constrained 2-D regime.