The Ulysses north polar pass: Latitudinal gradients of anomalous cosmic ray O, N And Ne

Abstract

A key finding of the Ulysses mission has been the absence of large latitudinal gradients in the Anomalous Cosmic Ray (ACR) component. Studies using data from several satellites have consistently shown a small (∼ 2%/deg.) positive latitudinal gradient for anomalous O, N and Ne in the energy range 4 to 30 MeV/n, and even smaller gradients for ACR He. In this paper we use data from the COSPIN/LET experiment on board Ulysses to investigate (a) the behaviour of the ACR latitudinal gradients during the passage over the northern polar regions, and (b) possible north-south asymmetries. The descent from the north pole has taken place under solar minimum conditions, providing a unique opportunity to study fully developed latitudinal effects. Our data survey covers three ACR species, O (4 to 20 MeV/n), N (4 to 20 MeV/n) and Ne (4 to 30 MeV/n). ACR gradients have been calculated for three different regions: 0°-20° (∼ 5%/deg.), 20°-60° (∼ 3%/deg.) and 60°-80.2° (∼ 1%/deg.). Comparison of data from the northern and southern hemishere at a given latitude shows an asymmetry with higher fluxes in the north than in the south at a given latitude.