Nonlinear evolution of EMIC waves in a uniform magnetic field: 1. Hybrid simulations

Abstract

We employ 2.5‐D electromagnetic, hybrid simulations that treat ions kinetically via particle‐in‐cell methods and electrons as a massless fluid to study the long‐term (hours) nonlinear evolution of electromagnetic ion cyclotron waves in the presence of multiple ion species in the outer magnetosphere. The source of the instability is a small population of hot (few keV) protons, representing plasma sheet ions at L = 9 with perpendicular temperature four times the parallel. In addition to the background population of cold protons the presence of minority, cold and heavy ions such as helium are also included. Both local (homogeneous) and nonlocal (finite‐sized source region) simulations with uniform magnetic field have been carried out. In the case of a homogeneous system, parallel propagating ion cyclotron waves are generated with amplitudes of a few percent of the background magnetic field (a few nT). After about 1000 ion gyroperiods (∼4 min) the wave amplitudes decrease by about 50% and remain nearly constant thereafter. The nonlinear evolution of the waves is associated with the gyrophase bunching of the cold protons and helium ions and reduction in the temperature anisotropy of the hot protons. In addition, parallel electrostatic waves with a wavelength half of that of the ion cyclotron waves are generated. These waves are subsequently absorbed and result in parallel heating of the cold ions. Throughout the run, both cold protons and heavy ions continue to gain energy primarily in the perpendicular direction while the hot protons reach isotropy followed by cooling in both parallel and perpendicular directions. In effect, the ion cyclotron waves continue to facilitate transfer of energy from the hot to cold ions. The results of simulations with localized hot protons show the simultaneous generation of ion cyclotron waves and expansion of the source region along the magnetic field. The nonlinear evolution of the waves is found to be similar to that seen in the homogeneous runs.