Response of thermal ions to electromagnetic ion cyclotron waves

Electromagnetic ion cyclotron waves generated by 10‐50 keV protons in the Earth's equatorial magnetosphere will interact with the ambient low‐energy ions also found in this region. We examine H + and He + distribution functions from ≈ 1 to 160 eV using the Hot Plasma Composition Experiment instrument on AMPTE/CCE to investigate the thermal ion response to the waves. A total of 48 intervals were chosen on the basis of electromagnetic ion cyclotron (EMIC) wave activity: 24 with prevalent EMIC waves and 24 with no EMIC waves observed on the orbit. There is a close correlation between EMIC waves and perpendicularly heated ion distributions. For protons the perpendicular temperature increase is modest, about 5 eV, and is always observed at 90° pitch angles. This is consistent with a nonresonant interaction near the equator. By contrast, He + temperatures during EMIC wave events averaged 35 eV and sometimes exceeded 100 eV, indicating stronger interaction with the waves. Furthermore, heated He + ions have X‐type distributions with maximum fluxes occurring at pitch angles intermediate between field‐aligned and perpendicular directions. The X‐type He + distributions are consistent with a gyroresonant interaction off the equator. The concentration of He + relative to H + is found to correlate with EMIC wave activity, but it is suggested that the preferential heating of He + accounts for the apparent increase in relative He + concentration by increasing the proportion of He + detected by the ion instrument.