Magneto-immutability: the resistance of weakly collisional plasmas to changes in magnetic-field strength*

We propose that pressure-anisotropy causes weakly collisional plasmas to self organize so as to resist changes in magnetic field strength. We term this effect “magneto-immutability” by analogy with incompressibility (resistance to changes in density). We study magneto-immutability using simulations of magnetized (Alfvénic) turbulence in the weakly collisional Braginskii model, which show that magneto-immutable turbulence is similar, in most statistical measures, to critically balanced MHD turbulence. However, in order to minimize magnetic-field variation, the flow becomes nearly scalar, and the turbulence is modestly dominated by magnetic energy (a nonzero “residual energy”). This effect represents a key difference between magnetized kinetic and fluid turbulence at the outer scale, and should be observable in the turbulent solar wind.