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.