Parker Solar Probe Observations of High Plasma Beta Solar Wind from Streamer Belt

TL;DR

This study uses Parker Solar Probe data to distinguish between different types of streamer belt solar wind near the Sun, revealing unique properties of partial current sheet crossings and their origins.

Contribution

It identifies and characterizes partial current sheet solar wind, highlighting their distinct features and potential origins inside the streamer belt, advancing understanding of slow solar wind.

Findings

PCS solar wind can be non-pressure-balanced structures.

Some PCS solar wind are mirror unstable.

PCS solar wind has low helium abundance and varied alpha-proton speeds.

Abstract

In general, slow solar wind from the streamer belt forms a high plasma beta 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 crossing (PCS), and they share some common features but also have different properties. In this work, using the PSP observations from encounters 4 to 10, we identify streamer belt solar wind from enhancements in plasma beta, and we further use electron pitch angle distributions to separate it into HPS solar wind that around the full HCS crossings and PCS solar wind that 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) 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 are mirror unstable; c) PCS solar wind is dominated by very low helium abundance but varied alpha-proton differential speed. We suggest 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 on the heating and acceleration of slow solar wind.