Current sheet control of recurrent particle increases at 4-5 AU

Abstract

We examine the relation between the current sheet as derived from observations of the Sun's neutral line, and recurrent particle increases associated with co-rotating interactions regions (CIR) at 4 to 5 AU radial distance as observed by the Ulysses spacecraft during its first out-of-ecliptic orbit around the Sun. We compare observations during the ascent to high southern latitudes, close to solar maximum, with observations during the return to the ecliptic from high northern latitudes, close to solar minimum, when the spacecraft was passing from the streamer belt to high-speed solar wind flow or vice-versa every solar rotation, and when regular recurrent particle increases were observed. Although conditions are very different, we show that the position of the Sun's neutral line, propagated out to the position of the spacecraft, can be used to explain the recurrence of the particle increases observed at 4-5 AU at Ulysses.