Cosmic-ray electron spectrum and its modulation near solar maximum

We have used a balloon-borne ionization spectrometer to measure the energy spectrum of cosmic-ray electrons in the range 20 Mev to 15 Gev. Balloon flights were made from Fort Churchill, Manitoba, Canada, in the summers of 1968, 1969, and 1970. Primary electron spectra were obtained for these three periods that show the effect of differing levels of solar modulation. In particular, a relatively strong decrease in the electron intensity between 300 Mev and 2 Gev was observed between 1969 and 1970, whereas neutron-monitor counting rates increased. It is shown that the data can be explained quantitatively by using the force-field approximation to the cosmic-ray transport equation if the rigidity dependence of the diffusion coefficient is generalized to be time-dependent. The parameters obtained in fitting the electron measurements, when they are applied to the 1968 proton energy spectrum, lead to general agreement with measurements under the assumption that the unmodulated proton spectrum is a power law in total energy.