The cosmic ray nucleonic component: The invention and scientific uses of the neutron monitor

Abstract

The invention of the neutron monitor pile for the study of cosmic-ray intensity-time and energy changes began with the discovery in 1948 that the nucleonic component cascade in the atmosphere had a huge geomagnetic latitude dependence. For example, between 0°and 60°this dependence was a ~ 200-400% effect - depending on altitude - thus opening the opportunity to measure the intensity changes in the arriving cosmic-ray nuclei down to ~ 1-2 GeV nuc1-1 for the first time. In these measurements the fast (high energy) neutron intensity was shown to be a surrogate for the nuclear cascade intensity in the atmosphere. The development of the neutron monitor in 1948-1951 and the first geomagnetic latitude network will be discussed. Among its early applications were: (1) to prove that there exists interplanetary solar modulation of galactic cosmic-rays (1952), and; (2) to provide the evidence for a dynamical heliosphere (1956). With the world-wide distribution of neutron monitor stations that are presently operating (~ 50) many novel investigations are still to be carried out, especially in collaborations with spacecraft experiments.