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Quasielectrostatic instabilities driven by anisotropic oxygen ions are investigated during storms/substorms in three-component ring current plasma consisting of electrons, protons, and energetic oxygen ions having loss-cone distribution. The response of the electrons is fully electromagnetic and that of the protons and oxygen ions is electrostatic. The electrons, protons, and oxygen ions are treated as magnetized. Obliquely propagating modes with frequencies near the harmonics of the oxygen ion cyclotron frequency are found to become unstable due to pressure anisotropy of the energetic oxygen ions. The results are applied to storm-time ring-current region parameters. The loss of energetic particles resulting from wave-particle interaction may provide an alternate mechanism, complimentary to and independent of the charge exchange process for the ring-current decay. [ABSTRACT FROM AUTHOR]

A theoretical and experimental investigation is made of a wave resonator based on the concept of wave reflection along the confinement magnetic field at a spatial location where the wave frequency matches the local value of the ion-ion hybrid frequency. Such a situation can be realized by shear Alfvén waves in a magnetized plasma with two ion species because this mode has zero parallel group velocity and experiences a cut-off at the ion-ion hybrid frequency. Since the ion-ion hybrid frequency is proportional to the magnetic field, it is expected that a magnetic well configuration in a two-ion plasma can result in an Alfvén wave resonator. Such a concept has been proposed in various space plasma studies and could have relevance to mirror and tokamak fusion devices. This study demonstrates such a resonator in a controlled laboratory experiment using a H+-He+ mixture. The resonator response is investigated by launching monochromatic waves and impulses from a magnetic loop antenna. The observed frequency spectra are found to agree with predictions of a theoretical model of trapped eigenmodes. [ABSTRACT FROM AUTHOR]