Radial evolution of the three‐dimensional structure in CIRs between Earth and Ulysses

[1] The radial alignment of ACE and Ulysses in February 2004 and August 2007 provided us with unprecedented opportunities to study the radial evolution of planar magnetic structures (PMSs) in three corotating interaction regions (CIRs). The in situ observations are compared with results from an analytical and a numerical model of CIRs. We find that (1) all three CIRs' meridional tilt retained its North/South orientation at ACE and Ulysses, but the evolution was not systematic. Further, the model results of CIR meridional tilt do not agree with observations. (2) All three CIRs rotated azimuthally with the Parker spiral as expected; however, model results only describe this behavior quantitatively for one CIR. (3) For all three CIRs, the solar wind deflection angles were predicted by the coupled solar corona-solar wind models, Wang-Sheely-Arge (WSA)-Enlil and MHD Around a Sphere (MAS)-Enlil, but neither model reproduced the observed planar magnetic structures. (4) The WSA-Enlil results of azimuthal magnetic field orientation are in better agreement with observations than those based on the MAS-Enlil. We suggest that the evolution of meridional tilt from ACE to Ulysses does not agree with projections because the parent coronal holes were highly structured. We also suggest that observations of azimuthal tilt do not agree with the model results because the models may underestimate transverse flows, whereas in reality, these flows could affect the observed azimuthal tilt of the CIR. The local orientation of PMSs within CIRs may also be distorted by transients, but their effect is unclear.