Control of Energetic Ring Current Ion Spectra by EMIC Waves

English Abstract:

The dense thermal regions in the magnetosphere comprising the plasmasphere and plasmaspheric plumes play a key role in influencing the energetic ion and electron populations not least because electromagnetic ion cyclotron (EMIC) waves are typically generated inside or close to these thermal regions. Here we address the process of gyroresonant pitch-angle scattering of energetic ions by EMIC waves in the inner magnetosphere. Specifically we test the hypothesis that EMIC waves closely control the shapes and intensities of energetic ion spectra. The limiting energetic ion flux of a trapped population is assumed to be determined by a steady-state marginal-stability criterion in which a convective wave gain condition is applied over all frequencies for which EMIC wave growth occurs. We find that, assuming a multi-ion background plasma,the limiting energetic ion spectrum varies as 1/E for large energy E. We compare the numerical model solutions for the limiting spectra with observed energetic ion spectra at Earth measured by the Van Allen Probes.