Parker Solar Probe Observations of High Plasma <i>β</i> Solar Wind from the Streamer Belt

Abstract

In general, slow solar wind from the streamer belt forms a high plasma β equatorial plasma sheet around the heliospheric current sheet (HCS) crossing, namely, the heliospheric plasma sheet (HPS). Current Parker Solar Probe (PSP) observations show that the HCS crossings near the Sun could be full or partial current sheet (PCS) crossings, and they share some common features but also have different properties. In this work, using the PSP observations from encounters 4–10, we identify streamer belt solar wind from enhancements in plasma β, and we further use electron pitch angle distributions to separate it into HPS solar wind around the full HCS crossings and PCS solar wind in the vicinity of PCS crossings. Based on our analysis, we find that the PCS solar wind has different characteristics as compared with HPS solar wind: (a) the PCS solar wind could be non-pressure-balanced structures rather than magnetic holes, and the total pressure enhancement mainly results from the less reduced magnetic pressure; (b) some of the PCS solar wind is mirror-unstable; and (c) the PCS solar wind is dominated by very low helium abundance but varied alpha–proton differential speed. We suggest that the PCS solar wind could originate from coronal loops deep inside the streamer belt, and it is pristine solar wind that still actively interacts with ambient solar wind; thus, it is valuable for further investigations of the heating and acceleration of slow solar wind.