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The growth of electromagnetic ion cyclotron (EMIC) waves due to a ring distribution of hydrogen ions is examined. Though these distributions are more commonly implicated in the generation of equatorial noise, their potential for exciting EMIC waves is considered here. It is shown that since the ring distribution is nonmonotonic in perpendicular velocity, the amplification achieved by this instability is greater than bi‐Maxwellian distributions for typical observed anisotropies, because the waves can maintain resonance over a much longer part of its trajectory. For ring speeds ( V   r  ) close to the Alfvén speed ( V   A  ), the growth rate is largest at parallel propagation but decreases less rapidly toward oblique angles compared with a bi‐Maxwellian and can have a second peak for shorter wavelengths. The ring distribution parallel thermal speed is deduced from the observation of equatorial noise because a statistical survey of this parameter is not available. A ring density to background plasma density of a few percent is sufficient to achieve moderate gain. The gain is also shown to increase with L shell since the perpendicular wave number does not rise as rapidly along the raypath as compared with smaller L. The analysis suggests that EMIC wave activity should be closely associated with equatorial noise.

Convective amplification of electromagnetic ion cyclotron waves from ring‐distribution protons in the inner magnetosphere