Mirror-mode structures at the Galileo-Io flyby: Instability criterion and dispersion analysis

The mirror mode is typically excited in high-beta plasmas when there is la significant pressure anisotropy, with the greatest plasma pressure perpendicular to the magnetic field, B. Large-amplitude mirror-mode structures were identified in the Galileo magnetic field data on the edges of the cold Io wake. Here, despite the high ambient B field and low plasma beta, the enhanced perpendicular pressure due to the ring-type velocity distributions of heavy Iogenic pickup ions overcomes the instability threshold for the mirror mode and provides the free energy. In the center of the Io wake, the local pickup velocities are very low and hence the perpendicular pressure contribution is small, while farther from Io in the torus, the corotating isotropic plasma dominates and the mirror mode is not unstable but instead ion cyclotron waves grow. Thus the spatial region in which the mirror mode dominates is narrow. A warm plasma dispersion analysis is performed for the multispecies plasma conditions appropriate to the edges of the Io wake. In a multispecies plasma, multiple ion cyclotron modes are possible, each with a growth rate dependent upon the anisotropy and free energy provided by the specific gyroresonant ion species component. However, the mirror mode can dominate, since its growth rate depends on the combined anisotropic pressure of all ions present.