Detection of zero anisotropy at 5.2 AU during the November 1998 solar particle event: Ulysses Anisotropy Telescopes observations

For the first time during the mission, the Anisotropy Telescopes instrument on board the Ulysses spacecraft measured constant zero anisotropy of proto ns in the 1.3-2.2MeV energy range, for a period lasting more than three day s. This measurement was made during the energetic particle event taking pla ce at Ulysses between 25 November and 15 December 1998, an event characteri sed by constant high proton fluxes within a region delimited by two interpl anetary forward shocks, at a distance of 5.2 AU from the Sun and heliograph ic latitude of 17 degrees S. We present the ATs results for this event and discuss their possible interpretation and their relevance to the issue of i ntercalibration of the two telescopes.