Interrelation between VLF and ULF emissions

Observations and theoretical works so far made are reviewed in regard to the interrelation of VLF and ULF emissions. Quasi-periodic VLF emissions are one of the typical examples showing the interrelation between the two phenomena. The term modulation may be more appropriate to explain these phenomena. Tentative interpretations will be given of the VLF and ULF emissions which are closely associated through a modulation of the electron distributions.