1D hybrid simulations of planetary ion-pickup: Energy partition

Abstract

Ion pickup in planetary environments results in unstable newborn ion populations with sufficient free energy to generate electromagnetic plasma waves. Estimates of the SO2+ pickup rates near the Jovian moon, Io, based on ion cyclotron wave observations assumed that the newborn pickup ions lose 50% of their energy to wave growth. Using one-dimensional initial-value hybrid simulation, we test this assumption and predict that at most ∼25% of the energy of the newborn ion population is lost to wave growth for conditions at Io. This energy is lost over >1400 SO2+ gyroperiods (∼1 hour), with the majority lost after saturation of the instability. Thus the saturation wave energy is low, at <3% of the newborn ion population's energy. In contrast, newborn ions in cometary environments saturate at substantially higher wave energies (∼30% of the newborn ion population's energy), but do not continue to lose energy afterwards. Copyright 2007 by the American Geophysical Union.