Electromagnetic proton cyclotron instability: heating of cool magnetospheric helium ions

Abstract. The electromagnetic proton cyclotron anisotropy instability is excited if the hot proton temperature anisotropy, T_⊥h/T_{\mid\mid h}, is sufficiently large compared to unity, where the subscript h denotes the hot protons and the perpendicular and parallel symbols denote directions relative to the background magnetic field. This instability is important in the outer magnetosphere because it has been shown to lead to an upper bound on T_⊥h/T_{\mid\mid h} and to cool iron heating. Here one-dimensional initial-value hybrid simulations with spatial variations in the direction of the background magnetic field are used to study this instability in a homogeneous plasma model which represents three ionic constituents of the outer magnetosphere: hot anisotropic protons, cool, initially isotropic protons, and cool, initially isotropic singly ionized helium. These simulations show that the presence of a tenuous helium component does not significantly change the scalings of either the hot proton anisotropy upper bound or the heating of the cool protons. The simulations also show that the helium ion heating rate increases with β_\mid\midh in contrast to the cool proton energization which decreases with this parameter. The prediction of this homogeneous plasma model, therefore, for cool ions subject to heating by the proton cyclotron instability is that the observed ratio of cool helium temperature to cool proton temperature should increase as β_\mid\midh increases.