Abstract
At present, there are some theories being able to explain the frequency characteristic of dawn chorus. In these theories, the natural thermal noise excited in a manner similar to the operation of a travelling-wave tube, the Doppler shifted proton cyclotron radiation and the Cerenkov radiation are considered as the origin of V. L. F. noise in the earth's exosphere respectively. This paper indicates that the Cerenkov radiation will be the most effective with respect to the emission energy in the above mentioned mechanisms in the earth's exosphere and that Cerenkov radio emissions generated by high speed protons (1.4×104km/sec) precipitating into the ionized exosphere along the earth's magnetic field may be excited simultaneously by the operation of the mechanism similar to a travelling-wave tube due to charged particles different from those causing the Cerenkov radiation relatively near the earth less than about 4 earth radii from the center of the earth and then propagate toward the earth in the whistler mode.
