Low-energy He+ and H+ distributions and proton cyclotron waves in the afternoon equatorial magnetosphere

Characteristics of low energy (similar to 10 to 50 eV) H+ and He+ distributions in the afternoon magnetosphere from L = 4 to 9 are studied using Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) data. He+ distributions in this study routinely display features indicating resonant heating by proton cyclotron waves in the outer magnetosphere (L > 7) whereas protons exhibit minimal evidence of heating. Low energy H+ distributions have T-perpendicular to >> T-parallel to for L < 5 but T-perpendicular to << T-parallel to for L > 7. The change is due to a decrease in T-perpendicular to with increasing L and is consistent with adiabatic cooling in the perpendicular direction as the ions convect sunward. By contrast, low-energy He+ distributions have T-perpendicular to >> T-parallel to independent of L. The failure of the He+ to cool adiabatically indicates that He+ experiences transverse heating as it convects sunward through the outer magnetosphere. The occurrence of proton cyclotron waves in the afternoon magnetosphere is evaluated using AMPTE/CCE magnetometer data and shows that low energy ions convecting sunward from the afternoon plasmasphere have a 20% chance of being exposed to cyclotron waves by the time they reach L = 5, 50% by L = 6 and > 95% by L = 7. The majority of cyclotron waves in the early afternoon outer magnetosphere occur above the local He+ gyrofrequency and will interact resonantly with He+ but nonresonantly with H+. The combined effects of adiabatic sunward convection and interactions between the low-energy ions and proton cyclotron waves therefore account for the observed characteristics of both the H+ and He+ distributions in the afternoon magnetosphere.