Observations of Cosmic Radio Noise at 18 Mc/s in Hawaii

Cosmic-radio-noise measurements at 18 Mc/s made in Hawaii during the period of November 1958 to December 1959 are reported. Diurnal attenuation curves are presented for the summer, winter, and equinoctial months that show a very pronounced resemblance to the F-region critical frequency curves. It is shown that an important part of the total attenuation can be accounted for by an ionospheric window effect. The remaining attenuation is attributed to deviative absorption in the F region and nondeviative absorption in the D region. A nighttime component of attenuation is observed and it is suggested that this phenomenon is related to the cooling of the ionosphere. Finally, it is observed that on numerous occasions when the F-region critical frequency exceeds the operating frequency, considerable flux continues to arrive at the antenna. It is suggested that this can be accounted for in terms of cosmic radio radiation trapped and propagated underneath the ionosphere. It is also possible that thermal radiation from the ionosphere plays a secondary role in this phenomenon.