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The Cosmic Ray Nucleonic Component: The Invention and Scientific Uses of the Neutron Monitor – (Keynote Lecture)


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 nucl−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.

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Simpson, J.A. The Cosmic Ray Nucleonic Component: The Invention and Scientific Uses of the Neutron Monitor – (Keynote Lecture). Space Science Reviews 93, 11–32 (2000).

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  • Fast Neutron
  • Neutron Monitor
  • Cosmic Radiation
  • Geomagnetic Latitude
  • Forbush Decrease