How the structure of rarefaction regions develops?

As the solar wind streams propagate with different speeds through the interplanetary space, they meet and interact. This leads to the formation of large interaction regions, one of which is the rarefaction regions (RRs). RRs develop at the trailing edge of the fast solar wind stream and come from an area of small longitudinal extent on the Sun. They exhibit a fine and complex structure, and the stream interface position is usually unclear. Superposed epoch analysis of the proton and alpha parameters for different regions within RRs reveals gradual transitions in many of them. Moreover, majority of our observations show that most of the RR plasma parameters correspond to the fast solar wind characteristics, only the alpha-proton drift velocity decreases from the beginning of RR. We investigate different ways of its reduction in the interplanetary space and show that this feature is likely associated with the mirroring of the multi-component solar wind. We identify the composition boundary where the alpha relative abundance and alpha-proton temperature ratio change abruptly from the values typical for the fast wind toward slow wind values. We suggest that this boundary is the most probable candidate for the stream interface. Based on these findings, we speculate that the RR formation occurs near the Sun and formulate two possible scenarios.