Electromagnetic ion cyclotron wave amplification and source regions in the magnetosphere

The amplification of electromagnetic ion cyclotron (EMIC) waves in the magnetosphere is studied by integrating local convective growth rates along the wave path. The integrated wave amplification is used to investigate the radial extent of wave source regions. Typical heavy ion concentrations (He + ∼10‐20%, O + ∼1‐10%) are considered in the calculation. In contrast to previous studies, the results presented indicate that the radial structure of the integrated wave amplification is controlled by the ion composition in the magnetosphere, as well as by the cold (or thermal) plasma density profile. It is found that the source region of the lowest‐frequency wave branch (i.e., the O + wave branch here) is confined to the plasmapause and inside the plasmasphere, whereas the other wave branches (i.e., the He + and H + wave branches here) can be amplified over most of the magnetosphere. It is suggested that the structured Pc 1‐2 waves observed mainly on the ground are due to the lowest‐frequency wave branch, and the unstructured Pc 1‐2 waves observed both on the ground and in space result from the other wave branches.