Intensity Gradients of Anomalous Cosmic Ray Oxygen Throughout the Heliosphere

Abstract

We use anomalous cosmic ray oxygen energy spectra collected from five different locations in the heliosphere during three time periods to estimate the radial and latitudinal gradients of the particle intensities at three energies. The three periods include the two high-latitude passes of Ulysses over the solar poles and the last few months of the cosmic ray oxygen data from Pioneer 10. The radial gradient is modeled as a power law in radius and the latitudinal gradient is assumed to be constant. The gradients are analyzed in two ways: the first uses the actual average spacecraft latitudes and the second assumes the symmetry plane of the heliosphere is at 10° S in heliolatitude. Reasonable fits are obtained under either assumption concerning the location of the symmetry plane, although the latitudinal gradients are smaller by a factor of~ 2 if the symmetry plane is offset by l0° S. The radial gradient exhibits a radial dependence of ~r^(-1) or r^(-2) depending on whether the symmetry plane is the helioequator or not, respectively. The r^(-2) dependence is not consistent with the gradient measured in a similar part of the solar cycle ~20 years ago, suggesting that the helioequator is the likely plane of symmetry for these particles. The only significant difference in oxygen flux between polar passes occurs at < 10 Me V /nuc and is similar to that observed one year earlier in the outer heliosphere due to decreasing solar modulation.