What the Rankine-Hugoniot relations tell us about co-rotating interaction regions in the solar wind, and what they don't

Co-rotating interaction regions (CIRs) are the result of a fast solar wind streat catching up with a slow stream ahead. The simplest description of a CIR portrays it as a compression region bounded by two fast shock waves. A tangential discontinuity within the CIR separates the shocked fast and slow solar wind streams. In this paper we assume that the Rankine-Hugoniot relations apply across both shocks. These relations are then used to predict the properties of the two shocked plasma states within a set of CIRs observed by the Ulysses spacecraft. The obtained properties are examined for consistency with the simple description of a CIR outlined above and the possible reasons for the observed discrepancies are discussed.