A model for the generation of pearl‐type micropulsations in the 0.1–5 cps range is presented. Under a different set of conditions, the model is also applicable to the generation of VLF emissions in the whistler mode. It is proposed that a beam of trapped protons atL≥ 4, supersonic with respect to the local Alfvén speed, emits hydromagnetic waves at a frequency corresponding to the occurrence of a cyclotron instability. These hydromagnetic waves propagate along the earth's field lines and are reflected back along the line at the ionosphere. On each pass of a wave packet through the proton beam, energy is supplied to the wave by the particles via the instability. Certain features of the observed fine structure of micropulsations are explained with the concept of bouncing hydromagnetic wave packets. A possible candidate for the necessary proton beam is the belt of 200–500 kev protons observed by Davis and Williamson, but there are other candidates as well, including low‐energy ring currents atL≃ 6. VLF emission in the 1–10 kc/s range may be produced by a similar instability if a beam of energetic electr