Loss‐cone‐driven ion cyclotron waves in the magnetosphere

Abstract

The theoretical properties of linear ion cyclotron waves propagating in the magnetosphere at arbitrary angles to the background magnetic field are explored. It is found that in some cases the linear wave growth of modes with oblique propagation can dominate that of the parallel propagating electromagnetic ion cyclotron (EMIC) wave. In particular, when the hot ring current protons have a loss cone and their temperature anisotropy A ≡ T⊥/T∥ ‐ 1 is reduced, the parallel propagating EMIC wave becomes stable, while the obliquely propagating loss‐cone‐driven mode persists. The growth rate of the loss‐cone‐driven mode depends strongly on the depth of the loss cone. Unlike the parallel propagating EMIC wave, it can be unstable with A = 0. Other conditions that favor the loss‐cone‐driven mode in comparison to the parallel mode are stronger background magnetic field, lower density of cold hydrogen, and a lower temperature for the hot anisotropic component of hydrogen. A simple analytical theory is presented which explains the scaling of the growth rate of the oblique mode with respect to various parameters. The loss‐cone‐driven mode is an electromagnetic mode which is preferentially nearly linearly polarized. It is nearly electrostatic in the sense that the wave electric field is aligned with the perpendicular (to B0) component of the wave vector k and k⊥ > k∥. Since the electric and magnetic wave fields are perpendicular to B0, they would be difficult to distinguish from those of a linearly polarized parallel propagating electromagnetic wave with the same k∥. However, in certain parameter ranges the loss‐cone‐driven mode can have nonzero ellipticity with a significant compressional component, in contrast to the parallel mode. The loss‐cone‐driven mode may explain recent Pc 1 observations of ion cyclotron wave events which appear to be locally generated with linear polarization (Anderson et al., 1992).