Mirror modes in space plasmas

Mirror modes are generated in anisotropic plasmas with high plasma beta by the pressure balance of perpendicular thermal pressure and magnetic pressure. They are solutions of the general plasma dispersion relation in the very low frequency limit. In the plasma frame, mirror modes do not propagate. In space plasmas, they are usually observed by spacecraft passing over these standing magnetic field and particle density structures. Mirror modes were observed in the magnetosheaths of Mercury, Venus, Earth, Mars, Jupiter and Saturn, where high beta plasma conditions meet adiabatic perpendicular heating and thus, an increased temperature anisotropy. The presence of mirror modes near Jupiter’s moon Io was also detected by the Galileo spacecraft, even though the temperature anisotropy arises from freshly ionized pick-up ions instead of adiabatic perpendicular heating. However, the Juno magnetometer data from two recent flybys at Io do not confirm the presence of mirror modes there.