Energy transfer between energetic ring current H+ and O+ by electromagnetic ion cyclotron waves

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Abstract
Electromagnetic ion cyclotron (EMIC) waves in the frequency range below the helium gyrofrequency can be excited in the equatorial region of the outer magnetosphere by cyclotron resonant instability with anisotropic ring current H+ ions. As the unducted waves propagate to higher latitudes, the wave normal should become highly inclined to the ambient magnetic field. Under such conditions, wave energy can be absorbed by cyclotron resonant interactions with ambient O+, leading to ion heating perpendicular to the ambient magnetic field. Resonant wave absorption peaks in the vicinity of the bi‐ion frequency and the second harmonic of the O+ gyrofrequency. This absorption should mainly occur at latitudes between 10° and 30° along auroral field lines (L ≥ 7) in the postnoon sector. The concomitant ion heating perpendicular to the ambient magnetic field can contribute to the isotropization and geomagnetic trapping of collapsed O+ ion conics (or beams) that originate from a low‐altitude ionospheric source region. During geomagnetic storms when the O+ content of the magnetosphere is significantly enhanced, the absorption of EMIC waves should become more efficient, and it may contribute to the observed acceleration of O+ ions of ionospheric origin up to ring current energies.