Observations of Anomalous Cosmic Rays in the Heliosphere from the SAMPEX, Ulysses, Voyager, and Pioneer Spacecraft

Abstract

The radial and latitudinal gradients of anomalous cosmic rays (ACRs) in the heliosphere are expected to depend on the phase of the solar cycle. During the last solar minimum in 1987, when positive energetic particles were entering the heliosphere along the neutral current sheet, the radial gradient of these particles in the outer heliosphere was found to depend on the tilt of the sheet and was a minimum when the tilt assumed its minimum value. The latitudinal gradient was large and negative during this same period. On a large scale, the radial gradient from l to -40 AU was found to decrease linearly with radial distance for ACR oxygen. These particle distributions are expected to be different after the polarity reversal of the solar magnetic field, which occurred -1990. By July 1994, the Voyagers 1 and 2 (Vl and V2) and Pioneer 10 (PlO) spacecraft will be positioned in the outer heliosphere in the radial range 45 to 62 AU. V2 and PlO will be at relatively low heliographic latitudes, while V l will be at 32° N. In the inner heliosphere Ulysses will be beyond 70° S at a radial distance of 2.7 AU. The SAMPEX spacecraft is in a near-polar Earth orbit. Using measurements of anomalous cosmic rays from instruments on these 5 spacecraft, we will report on the temporal behavior of the fluxes and their radial and latitudinal gradients during the current approach towards solar minimum. We will compare the results from those taken during the last solar minimum period and relate these to current models of solar modulation.