Instability of ring current protons beyond the plasmapause during injection events

The stability of ring current protons with an injection spectrum modeled by a loss cone distribution function is examined for typical ring current parameters. We find that a quasi-electrostatic ion loss cone mode can be excited with frequencies just below and growth rates of the order of 10−2 times the ion plasma frequency. The instability is strongest in the moderate β ∼ 1, low-density region just outside the plasmapause; for the β ≫ 1 auroral regions and the high-density plasmasphere the mode is nearly stable. For the same ring current parameters the electromagnetic ion cyclotron wave is almost nonconvectively unstable, with growth rates of the order of 10−1 times the ion cyclotron frequency. The combination of the two unstable modes results in a large quasi-linear diffusion coefficient throughout most of the proton velocity space. Unless it is maintained by rapid inward convection, the ring current injection anisotropy will be reduced by diffusion toward the loss cone on time scales short in comparison to the minimum precipitation lifetime.