Studies of Energetic Ions in the Inner Heliosphere

Abstract

Early in the last century high energy cosmic rays were discovered, which were initially only identified as ions. It was first recognised in 1938 (Forbush, Phys Rev 54:975, 1938) that the incoming cosmic ray intensity was modulated by the solar rotation. This was discovered from observations made with ionization chambers at various locations world-wide. From these same measurements it was also recognised that there were transient solar events, involving relativistic protons like the cosmic rays, seen at ground level associated with major solar flares. Shortly after this the neutron monitor was developed (Simpson et al., Phys Rev 90:934, 1953) and in 1956 the largest high energy solar proton event yet recorded was observed in Chicago (Meyer et al., Phys Rev 104:768, 1956). Figure 5.1 shows the intensity-time history of the neutron monitor record from that event.