Convective growth of electromagnetic ion cyclotron waves from realistic ring current ion distributions

Abstract

It is well established that the development of pitch angle anisotropy in ring current ions provides a favorable condition for electromagnetic ion cyclotron (EMIC) wave growth. Many studies have calculated the EMIC growth rate assuming a bi-Maxwellian or ring distribution of energetic ions in velocity space. To test the appropriateness of these assumptions, we compute the EMIC growth rate by using the realistic ion distributions output by our Comprehensive Inner Magnetosphere-Ionosphere (CIMI) model. The CIMI model calculates energetic ion and electron distributions and many other essential quantities in the inner magnetosphere and ionosphere. We found that the ring current distributions are often very different from a bi-Maxwellian or a ring distribution. The corresponding EMIC growth rates are likewise quite distinctive from those estimated by functional fits. In the simulation of an idealized magnetic storm, we found that much stronger EMIC growth near the dayside magnetopause was attained with realistic ring current distributions. On the other hand, with the assumption of a ring distribution, EMIC growth at the peak of the ring current pressure was overestimated. Our calculation using the realistic ion distribution function reproduces the observed occurrence of EMIC waves and demonstrates the need to use realistic ring current ion distributions when modeling EMIC wave excitation.