A comparison of recurrent energetic ion enhancements observed at Ulysses and at 1 AU by IMP 8 and SAMPEX: Ulysses launch until following the first north polar passage

During the Ulysses mission, the Goddard Space Flight Center instrument on IMP 8 and the University of Maryland low energy ion composition analyzer instrument on SAMPEX observed recurrent MeV/amu ion enhancements in the ecliptic plane at 1 AU. We identify those IMP 8/SAMPEX events related to the recurrent events observed at Ulysses from its launch until mid‐1996, following the first passage over the north solar pole, and compare their properties. Two types of recurrent events were present at Earth. Recurrent enhancements of particles streaming sunward from CIRs in the outer heliosphere were observed most clearly at Earth after the Ulysses Jupiter encounter. Most of the CIRs/particle enhancements at Ulysses, even at high heliolatitudes, can be associated with recurrent events/high‐speed streams at Earth by considering simple corotation delays. This suggests that the events at high latitudes occurred poleward of equatorward extensions of the polar coronal holes which produced corotating high‐speed streams in the ecliptic. The association between events at Ulysses and Earth was less clear in 1996 when there were few major streams from polar coronal hole extensions present near the ecliptic. At the furthest distance of Ulysses from the Sun (5.4 AU), the intensity of ∼1 MeV/amu ions in CIR‐associated events was 1–2 orders of magnitude higher than at Earth. The intensity at Ulysses then declined as the spacecraft moved closer to the Sun and to high latitudes to reach < 0.1 % of that at Earth near polar passage. The observations confirm that the decrease in the intensity of corotating events at high latitudes was predominantly due to the changing spacecraft latitude rather than to a temporal decline in the intensity of corotating events throughout the heliosphere. We suggest that some of the north‐south asymmetries in the particle events observed by Ulysses at high latitudes may be associated with differences in the solar wind structure and energetic particle events at low latitudes during north and south polar passage. In contrast to these events, the prominent recurrent events observed at Ulysses and at Earth during the second half of 1991 were associated with solar activity which, over a period of several solar rotations, was largely confined to a limited range in Carrington longitude.