Theoretical study of cosmic noise absorption due to solar cosmic radiation

Abstract

Calculations have been made of the riometer absorption caused by solar protons and α particles in monoenergetic intensities and in spectral distributions. These calculations are an improvement over previous works in four respects; the geomagnetic cutoffs considered, the ionospheric parameters used, the treatment of monoenergetic intensities of particles, and the consideration of different riometer frequencies. Good agreement between calculated absorption and measured absorption has been obtained. The studies of monoenergetic intensities show that the most efficient energy for protons is a strong function of the riometer frequency, and is ~ 50 MeV for 30 Mc/s. The most effective, or dominant, energy during solar cosmic ray events is lower, due to the relative abundance of low energy particles, and for 30 Mc/s is in the 10–25 MeV region for most events. The minimum detectable intensity for riometers is ~10 protons/ cm 2-sec-ster for typical events at 30 Mc/s, although this frequency could detect a monoenergetic (50 MeV) intensity of < 1/cm 2-sec-ster. It was also found that several solar cosmic ray events near solar minimum exhibited spectra which acted as monoenergetic intensities of 10–15 MeV protons. Curves relating absorption to particle intensity for different energies are shown, which permit a quick determination of particle intensities from riometer measurements during events. All the calculations discussed above have also been carried out for α particles. In addition, it is shown that α particles in spectra act similarly to protons; the dominant terms are the total intensity and the spectral shape, while the species is relatively unimportant. Calculated absorption verus frequency profiles indicate that considerable spectral information can be obtained by multifrequency riometers.