In previous chapters we have illustrated cyclotron wave–particle interactions using, as examples, the electron radiation belts. However, all the physical effects discussed are valid for the proton radiation belts too. In particular, a sufficiently dense cold plasma is needed for the proton cyclotron instability to be switched on. Thus the plasmapause and detached plasma filaments outside the plasmasphere are the most likely regions where the proton cyclotron instability can develop, as is the case for the electron radiation belt (Section 11.3).

As for electrons, the inward transport of protons across L shells serves as the main source, and the losses are determined by wave–particle interactions (which are most important during the main phase of a geomagnetic storm) and by other mechanisms of energetic ion loss; see Lyons and Williams (1984) and Watt and Voss (2004) for more details. However, when considering the temporal and spatial properties of an Alfvén cyclotron maser compared with a whistler-mode cyclotron maser, we find that the quantitative differences are rather large. Both the characteristic frequency and the growth rate of the proton CI are less by the ratio of their masses (M/m ∼ 2000) in comparison with the electron CI. But we have to remember that the density of energetic protons is larger than the density of energetic electrons, so that the actual difference in growth is not so big. The ratio of the Alfvén and whistler wavelengths λA/λw ≳ (M/m)½ 40 » 1.