Variations of alpha particle parameters across corotating rarefaction regions

A corotating rarefaction region (CRR) is formed between the slow solar wind stream and the fast stream in front of it. It is associated with a region of small longitudinal extent on the solar surface and plasma parameters would correspond to a smooth transition between the interacting streams. However, previous studies have shown that the stream interaction can change parameters of individual solar wind components in an unexpected way. In our study, we focused on behavior of the second most abundant ionic component, alpha particles. Preliminary analysis of measurements at 1 AU revealed large variations of the alpha relative abundance (AHe) that did not correlate with the solar wind speed and alpha-proton relative drift changes. To determine the global profile of alpha properties across CRR at 1 AU, we performed the superposed epoch analysis of identified CRRs. We found continuous but spatially separated transitions of the alpha-proton relative abundance, relative drift, and alpha-proton temperature ratio from values corresponding to fast streams to those typical for slow streams. Despite the expectation that AHe would decrease from the beginning of the rarefaction, it corresponds to values usually observed in the fast solar wind for a large part of the CRR. Moreover, AHe is often enhanced near the expected stream interface. Therefore, we propose that a major part of the CRR is filled by the wind from a coronal hole and discuss various scenarios that could explain the obtained profiles of alpha particle parameters across CRRs. Finally, we use CRR observations at different radial distances from the Sun (Solar Orbiter, Ulysses, etc.) to identify effects connected with a radial evolution of these large-scale structures.