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Evolution of the α-proton Differential Motion across Stream Interaction Regions

A corotating interaction region (CIR) develops between the solar wind streams with different bulk speeds emanating from distinct coronal sources. The arising pressure perturbations redistribute momentum between adjacent streams forming the regions of the compressed solar wind around the stream interface. We focus on properties of α-particles with respect to protons in CIRs using measurements of the Wind and Helios spacecraft. In the slow solar wind in front of CIRs, the relative helium abundance AHe is usually low (about 1%) and the α-proton differential drift, Vαp is close to zero. In the high-speed stream behind CIRs, both of these characteristics are significantly higher. Inside CIRs, a large enhancement of AHe accompanied by a decrease in Vαp is often observed in the compressed and slowed down fast solar wind close to the CIR leading edge. On the other hand, a depletion of AHe is sometimes present in the compressed and accelerated slow solar winds. We explain these observations in terms of magnetic mirroring of the multicomponent solar wind in a converging magnetic field that develops within CIRs.