An interpretation is given for the high fluxes of 0+ ions with energies to 12 kev observed during the magnetic storm of 17 December 1971. It is suggested that the ions were accelerated initially by the absorption, near the altitude of the 0+ resonance (ω = Ω0+), of ion cyclotron waves amplified at a slightly lower altitude by ring current protons moving downward along magnetic field lines. The energized ions then moved toward higher altitudes along the field lines where they diffused in pitch angle and energy due to resonant interactions with ion cyclotron turbulence. The gradient of the geomagnetic field plays an important role in the amplification of the ion cyclotron waves at low altitudes. It is shown that ring current protons may lose considerable energy while resonating with ion cyclotron waves propagating upward. Conversely, ions may gain substantial energy while resonating with waves propagating downward. The observations of the 0+ ions are correlated with simultaneous measurements of the ring current protons, and it is argued that the observed energy loss of the protons near the plasmapause may be due to off-equatorial resonant interactions between the protons and ion cyclotron waves.
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