Hybrid code simulation of electromagnetic ion cyclotron waves in curvilinear coordinates.

摘要

Electromagnetic ion cyclotron waves, a common feature in the Earth' magnetosphere, can interact with both ions and electrons, leading to an important impact on the dynamics in the radiation belts. These waves are usually driven by the anisotropic and hot ring current protons in the equatorial region, and propagate earthward along the Earth's (nearly dipolar) magnetic field lines. When the waves grow near the equator, they are dominantly left-hand polarized and have small wave normal angle. The inhomogeneity of the magnetic field stretches the wave fronts and causes refraction. While propagating toward high latitudes, the waves become linearly or right-handed polarized with a larger normal angle due to the refraction. If the waves are refracted enough, then they may encounter the bi-ion frequency and reflect; if the waves are not refracted as much, they are reflected at a cutoff frequency lower than the bi-ion frequency. Different wave modes may couple during the course of propagation due to mode conversion, which indicates that some waves may tunnel through the so called stop band to reach frequencies very close to the harmonic of the ion cyclotron frequency, leading to acceleration of the ions. This is a complex process in which a variety of phenomena, including the generation, growth, propagation, reflection, absorption, and coupling of the EMIC waves, may occur. We have developed a nonliner hybrid code using curvilinear coordinates to investigate the EMIC waves in a dipole magnetic field, capable of addressing all of the above issues self-consistently. Both single-ion and multi-ion plasmas are investigated. In the presence of cold heavy ions, such as He+ and O+, the waves are able to accelerate these cold particles at multiple harmonics of the cyclotron resonances, and in some cases the heating is significant so that the cold particles become hot.