Solar modulation of galactic cosmic ray electrons, protons, and alphas

Abstract

Over the 5-year period 1968–1972 we have measured the energy spectrum of primary cosmic ray electrons from 20 MeV to 20 GeV using a balloon-borne absorption spectrometer. During the same period, other investigators have inferred the interstellar electron spectrum mainly from radio observations and have directly measured the electron spectrum at low energy, proton and alpha energy spectra, neutron monitor counting rates, radial particle gradients, the velocity of the solar wind, and the power spectrum of interplanetary magnetic field irregularities. Taken together these measurements constitute a wealth of data applicable to the problem of solar modulation. In this paper we present observations of the electron spectrum, examine the simple spherically symmetric model of modulation involving convection, diffusion, and adiabatic deceleration, and compare the model with the various measurements. Starting with electrons because some knowledge of the interstellar electron spectrum exists, we determine the rigidity dependence of the diffusion coefficient for each year, which will produce modulated electron spectra in agreement with observations. Then using the diffusion coefficient for 1 year to demodulate the observed proton and alpha spectra, we calculate modulated spectra for the other years and compare with observations. We find that the model is substantially capable of reproducing these observations above 40 MeV/nucleon.