Electron Acceleration by Wave-Particle Interactions at the Earth and Magnetised Planets

The radiation belts of the Earth and magnetised planets include high energy electrons reaching energies of up to 50 MeV.  Observations at the Earth show that the electron flux is highly variable, and that acceleration must take place inside the planetary magnetic field.   Soon after the radiation belts were discovered it was thought that inward radial diffusion was the main process responsible for the acceleration, but it was difficult to reproduce the timescale for some of the observed variations in the electron flux.  Local electron acceleration via Doppler shifted cyclotron resonance with chorus waves was proposed as an alternative mechanism and has been shown to play a major role in forming the outer electron belt at the Earth reaching energies of several MeV.  Here we review some of the evidence for local acceleration and describe the process of chorus wave acceleration at the Earth.  We review other types of plasma waves, such as magnetosonic waves, that could contribute to electron acceleration and describe the conditions necessary to reach electron energies of several MeV.  We show examples of chorus and other types of plasma waves at Jupiter and Saturn and show how they play an important role in accelerating electrons to form the radiation belts at those planets.  We suggest that wave acceleration is the missing link in a set of process that starts with volcanic gasses from the moon Io and results in the emission of synchrotron radiation from Jupiter.  We suggest that wave acceleration is a universal process operating at the magnetised planets.  Finally, we show how wave acceleration is included into space weather forecasting models to help ensure the safe and reliable operation of satellites on orbit around the Earth.