Impact of cold O+ ions on the generation and evolution of EMIC waves

摘要

We explore the effect of cool O+ ions (～11 eV) on the generation and nonlinear evolution of electromagnetic ion cyclotron (EMIC) waves in the magnetosphere, using hybrid (kinetic ions, fluid electrons) simulations with a dipolar magnetic field. The instability is driven by the temperature anisotropy of the hot (keV), ring current protons whose density is a few percent of the background plasma consisting of cold (eV) protons and O+ ions with concentrations varying between 0 and 30% of the cold protons. The results show that for O+ ion concentration of 7% or less, the properties of the EMIC waves are similar to those in the absence of O+ ions where waves are generated at low latitudes and propagate all the way to the ionospheric boundary. Further increases in O+ density (～15%) result in waves being confined in latitude due to processes explored in this paper. At O~+ densities of 30% and higher, the growth of EMIC waves is weak or non-existent due to cyclotron resonant damping by the O+ ions. Comparing the results of the runs with no and 15% O~+ ions shows that the propagation properties of the EMIC waves change dramatically in the presence of O~+ ions. Specifically, they show that waves are propagating parallel and anti-parallel to the magnetic field in both hemispheres due to reflection at points that move to higher latitudes with time. This in turn results in the nonlinear generation of field aligned electrostatic waves with large perturbations in the density of the cold protons and O~+ ions and also local heating of these ions. Examination of the wave properties also shows that EMIC waves are only present in regions of space where the density of the hot protons is larger than 1% of the background level. In other words, the propagation properties of the EMIC waves are controlled by the density of the hot protons. This finding is further confirmed by performing a test hybrid simulation in which hot protons reaching latitude of 22° are removed from the run resulting in the confinement of the waves to this and lower latitudes.