The tilts of corotating interaction regions at midheliographic latitudes

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Abstract

Previous analysis of midlatitude corotating interaction regions (CIRs) observed by Ulysses in the southern heliosphere has revealed that the flow downstream of the forward (F) shock (or wave) on the leading edge of a CIR generally turns northward and into the direction of planetary motion (westward), while the flow downstream of the reverse (R) shock (or wave) on the trailing edge generally turns southward and eastward. These systematic flow deflections are a natural consequence of large‐scale pressure gradients associated with the CIRs and indicate that the F shocks tend to propagate toward and across the equator with increasing heliocentric distance, while the R shocks tend to propagate toward the pole. Numerical simulations indicate that these effects are a natural consequence of the tilt of the solar magnetic‐dipole axis relative to the solar rotation axis. The present work utilizes a variety of techniques to analyze the flow deflections observed within midlatitude CIRs from which we can infer the overall orientations of the CIRs and the speeds and directions of propagation of the waves. Notable results include the following: (1) On the whole, F shocks do propagate equatorward and westward, and R shocks propagate poleward and eastward; (2) shock parameters show a modulation in amplitude, peaking at latitudes roughly equivalent to the inferred dipole‐tilt angle; (3) R shocks tend to propagate faster (in the upstream solar wind frame) than their counterpart F shocks; and (4) meridional deflections tend to be larger than azimuth deflections for both F and R shocks. While it is reassuring that most of the CIRs analyzed fit the paradigm, there are a significant number of anomalies. We discuss several mechanisms which could, in principle, cause these apparent contradictions. For example, the present analysis necessarily emphasizes the local structure at the shock front. The agreement improves significantly, however, when the large‐scale flow deflections throughout the CIR are taken into account.

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