A new generation regime of the magnetosphere cyclotron maser is considered, based on phase coherence effects in wave-particle systems with step-like deformations of electron velocity distribution functions. Such deformations appear during cyclotron interactions of noise-like whistler wave emissions and energetic electrons at the boundaries between resonant and nonresonant particles. The new regime is similar to the backward wave oscillator in laboratory electronic devices. This regime applies to the generation of chorus emissions and may explain the connection between chorus and hiss, their fast growth rates, and the temporal succession of chorus elements. ¿ American Geophysical Union 1995