A measurement of the spectrum of cosmic‐ray electrons between 20 Mev and 3 bev in 1968—Further evidence for extensive time variations of this component

The intensity and spectrum of cosmic‐ray electrons from 20 Mev to 3 bev has been measured at a depth of 2.5 g/cm² at Fort Churchill in the summer of 1968 at a time when solar modulation effects had reduced the sea‐level neutron monitor rate by ∼12% below its sunspot minimum value. The instrument used was identical to that used in previous flights to study electrons at this location in 1965 and 1966. The total electron intensity at 2.5 g/cm² atmospheric depth at energies ≳1.5 bev in 1968 was essentially the same as that observed in 1966, however, at lower energies the intensity was greatly reduced in 1968. This reduction was a factor of 4 in the 200–400 Mev range. Atmospheric growth curves were used to determine the contribution of atmospheric secondary electrons. The deduced extraterrestrial electron intensity appears to be a factor ∼5 lower in 1968 than in 1966 at energies just above 200 Mev. At higher and lower energies the fractional decrease becomes less. These results confirm and extend earlier measurements of the solar modulation of electrons made in 1965–1966. The energy dependence of the electron modulation is essentially the same in the 1966–1968 period as was measured in 1965–1966.