The Energy Spectra of Anomalous Cosmic Rays in the Outer Heliosphere During 1991-1994

Abstract

Since the launch of the Voyager spacecraft in 1977, we have monitored the energy spectra of anomalous cosmic rays (ACRs) as a function of radial and latitudinal position over the course of the solar cycle, including two reversals of the Sun's magnetic field. The energy spectrum for ACR helium observed during the last solar minimum period in 1987 at approximately 23 AU showed a rather narrow peak at approximately 22 MeV/nuc. In 1978 at approximately 2 AU, this spectrum was much broader with a peak at lower energies, in agreement with theories of the acceleration and propagation of these particles which include curvature and gradient drift in the interplanetary magnetic field. Currently, in the outer heliosphere at the position of Voyager 1 (approximately 55 AU and 32 deg N), the energy spectrum of ACR helium has a peak energy at approximately 8 MeV/nuc, considerably lower than previously observed. At Voyager 2 (42 AU and 11 deg S), the energy spectrum is quite different, having a peak energy near 20 MeV/nuc. We will report on the evolution of the spectra of ACRs as observed by the instruments on the Voyager spacecraft as the fluxes again approach solar minimum levels, expected in 1994, and relate the observations to models of their propagation.