When the distribution of electron velocities parallel to the magnetic field has a tail with a bump, a linear triple-wave resonance instability can cause the growth of electromagnetic waves at ωp. The bandwidth of the radiation depends on the ratio of the number of electrons in the tail to the total number of electrons; the smaller this ratio, the narrower the bandwidth. The instability occurs only when the mean tail velocity has a specific value. The radiation thus gives an indication of the tail structure.