Low Energy Anomalous Cosmic Rays in the Ecliptic Plane: 1-5 AU
L.J. Lanzerotti and C.G. Maclennan
Bell Laboratories, Lucent Technologies, Murray Hill, NJ USA
R.E. Gold
Johns Hopkins University/Applied Physics Lab, Laurel, MD USA
Anomalous cosmic ray (ACR) oxygen (O) in the energy range 0.5-5.0 MeV/nucl was measured
in the ecliptic plane at 1 AU and at 5 AU by instruments on the ACE (EPAM) and Ulysses
(HI-SCALE) spacecraft, respectively, from 1997 to early 1998. The change in the radial
gradient of the ACR O fluxes in this energy range could be followed as the increasing
solar activity reduced the ACR O fluxes in the inner heliosphere. In late 1997, for
E ~ 2-5 MeV/nucl, the ACR O radial gradient was very large, ~40%/AU. By early 1998,
the gradient had decreased to ~24%/AU, largely because of a decrease in the 5 AU fluxes.
By mid-1998, ACR O was essentially unmeasureable at both locations. Using the time
dependence of the ACR O as measured at 5 AU, we have determined the e-folding time for
sweeping this energy oxygen out of the inner heliosphere in the near ecliptic plane to
be about 140+-40 days.