Ion cyclotron waves at Io

Ion cyclotron waves are left-hand polarized electromagnetic waves that propagate through magnetized plasmas with frequency well below the ion cyclotron frequency and most effectively along a background magnetic field. They are generated through an initial electric or magnetic field fluctuation and grow with temperature anisotropies. A temperature anisotropy appears, if the distribution function in velocity space that is associated with random movement parallel to a magnetic background field differs from that associated with movement perpendicular to it. In the vicinity of Jupiter’s volcanic moon Io, temperature anisotropies arise through freshly generated pickup-ions that escaped from Io’s atmosphere and get ionized through impacts, photo-ionization or charge exchange. These ions usually have a ring- or ring-beam velocity distribution, e. g. the perpendicular temperature is much higher than the parallel. Ions in resonance with the ion cyclotron wave interact with the wave and which leads to growth or decay of the wave amplitude, in highly anisotropic environments to an exponential growth. At Io, the Galileo spacecraft detected periodic magnetic field perturbations close to the ion gyrofrequency of SO₂⁺, SO⁺, SO⁺⁺, S⁺, S⁺⁺, O⁺ and H⁺ in Io’s wake region during six flybys. However, further analysis revealed that the presence of ion cyclotron waves is temporally and spatially variable and can maybe linked to atmospheric escape and ionization processes.