Large amplitude IMF fluctuations in corotating interaction regions: Ulysses at midlatitudes

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Abstract
Corotating Interaction Regions (CIRs), formed by high‐speed corotating streams interacting with slow speed streams, have been examined from −20° to −36° heliolatitudes. The high‐speed streams emanate from a polar coronal hole that Ulysses eventually becomes fully embedded in as it travels towards the south pole. We find that the trailing portion of the CIR, from the interface surface (IF) to the reverse shock (RS), contains both large amplitude transverse fluctuations and magnitude fluctuations. Similar fluctuations have been previously noted to exist within CIRs detected in the ecliptic plane, but their existence has not been explained. The normalized magnetic field component variances within this portion of the CIR and in the trailing high‐speed stream are approximately the same, indicating that the fluctuations in the CIR are compressed Alfvén waves. Mirror mode structures with lower intensities are also observed in the trailing portion of the CIR, presumably generated from a local instability driven by free energy associated with compression of the high‐speed solar wind plasma. The mixture of these two modes (compressed Alfvén waves and mirror modes) plus other modes generated by three wave processes (wave‐shock interactions) lead to a lower Alfvénicity within the trailing portion of the CIR than in the high‐speed stream proper. The results presented in this paper suggest a mechanism for generation of large amplitude Bz fluctuations within CIRs. Such phenomena have been noted to be responsible for the generation of moderate geomagnetic storms during the declining phase of the solar cycle.