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

Motivated by the well‐known differences in charge state and even elemental composition of the slow solar wind as compared to high‐speed streams, we have analyzed observations made by the Solar Wind Ion Composition Spectrometer (SWICS)Ulysses mass spectrometer of the series of corotating interaction regions (CIRs) encountered by Ulysses in 1992/1993 at 4.5–5.4 AU from the Sun. Stream interfaces (SIs) are first identified within each CIR using the kinetic parameters (density and thermal speed) of the main components, protons and alpha particles. We then show that SIs are also the location of the expected compositional changes in the heavy ion components, such as the freezing‐in temperatures of C and O and the strength of FIP fractionation as given by the Mg/O abundance ratio, to within the time resolution with which these parameters may be obtained by SWICS. The changes in freezing‐in temperatures are so clear that they in turn may be used as a robust indicator of the SI location and even reveal that some of the SI crossings were in fact multiple. We conclude that the heavy ion composition of the solar wind changes abruptly at the SI and remains well preserved out to large heliocentric distances.