How Does the Structure of Rarefaction Regions Develop?

A rarefaction region (RR) occurs at the trailing edge of the fast solar wind stream. It comes from an area of small longitudinal extent on the solar surface and exhibits a fine and complex structure. In our study, we did a superposed epoch analysis of the proton and alpha parameters across the RR and observed their gradual evolution. We did not find any clear boundary between the fast and slow solar winds inside the RR because a majority of our observations show that most of the RR plasma corresponds to the fast solar wind; only the alpha-proton drift velocity decreases from the beginning of the RR. We investigate different ways of its reduction in interplanetary space and show that this feature is likely associated with the mirroring of the multicomponent solar wind. Nevertheless, considering the observed solar wind characteristics and taking into account the mutual relations between the proton and alpha parameters, we define 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. This boundary is the most probable candidate for the stream interface. Based on these findings, we speculate that the RR formation starts already near the Sun and formulate two possible scenarios.