Rocket observations of the intensity of very low frequency noise above the ionosphere

Abstract

Abstract The intensity of radio signals in the frequency range 0.5–12 kc s was measured during two rocket flights, one near midnight and one near noon. On both flights, whistlers were observed—22 at night and 122 in the day. Hiss also was recorded on both flights—at 2.8 kc s at night and at 800 c s in the day. No hiss or whistlers were observed on the ground on either occasion. At night the intensity of whistlers in the ionosphere was at least 750 times that on the ground and the hiss intensity was at least 180 times that on the ground. During the day the corresponding factors were 8000 and 10,000. These ratios are much greater than could be caused by absorption. They are probably due to total internal reflection in the low ionosphere, since VLF waves penetrate to the ground only if their wave normals are within a small cone about the vertical. It is concluded that it is impossible to deduce the intensity of VLF waves in the ionosphere from measurements made on the ground.