BOX 1.1

LONG-TERM RADIATION STUDIES: THE ICE CORE EVIDENCE

The variability of both galactic cosmic radiation (GCR) and solar energetic particles (SEPs) with the Sun’s ~11 year activity cycle has been well established on the basis of direct measurements made over the past 70 years, first from the ground and later with ground-based and space-based instruments. It has been known since the early 1960s that GCR fluxes are modulated by the interplanetary magnetic field (IMF) and are anticorrelated with solar activity, with the lowest fluxes occurring when solar activity is highest and vice versa (Figure 1.1.1).

FIGURE 1.1.1 Representative galactic cosmic ray ion spectra depicting the intensity variations between solar maximum and solar minimum conditions. The upper curve for each species is for solar minimum, when cosmic rays can penetrate into the inner heliosphere more easily. SOURCE: Courtesy of R.A. Mewaldt, California Institute of Technology.



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