Recurrence in MeV proton fluxes and anisotropies at 5 AU from the Sun

Abstract

We report the observation of a recurrent pattern in proton fluxes and anisotropies at 5.2 AU from the Sun, made by the Anisotropy Telescopes on board the Ulysses spacecraft between April 1998 and May 1999, during the rising phase of solar cycle 23. The time delay between recurrent features of the pattern is not constant and increases from 122 to 155 days over the three recurrences observed. We discuss a possible connection between these observations and the well documented existence of a 154-day periodicity in the rate of occurrence of solar flares.