Solar wind stream interfaces in corotating interaction regions: New SWICS/Ulysses results

We have analyzed data from the solar wind ion composition spectrometer (SWICS)/Ulysses instrument taken between August 1996 and May 1997. In this period the Ulysses spacecraft traveled from 28° to 11° N and encountered a highly regular pattern of high-speed streams alternating with slow solar wind. Heliocentric distance varied between 4.3 and 5.1 astronomical units (AU). Using proton and alpha-particle kinetic parameters (density, speed, and kinetic temperatures) as well as charge state and elemental composition data, we identify the stream interfaces in the corotating interaction regions (CIRs) observed in this period. As Wimmer-Schweingruber et al. [1997] previously reported for a similar period in 1992/1993, stream interfaces are the sites of compositional changes between values typical of the slow solar wind and values typical of high-speed streams. During that period, Ulysses traveled from 13° to 34° S and from 4.5 to 5.4 AU. In spite of the similarity of the heliospheric regions probed during 1992/1993 and 1996/1997 the corotating interaction regions observed in 1996/1997 are quite different from those observed in 1992/1993. We observe fewer (11) CIRs in 1996/1997 than in the previous period (15); the CIRs are less evolved, i.e., the kinetic signatures at the stream interface are less clear; and mainly, only 3 out of the 11 CIRs are bounded by forward-reverse shock pairs, whereas in 1992/1993, 13 out of 15 were. This may have important consequences for particle acceleration in CIRs in the inner heliosphere.