Near-Sun Alfvenic Slow Solar Wind with Variable Helium Abundance: PSP observation and source tracing

Abstract

The solar wind, except for the coronal mass ejection events, is traditionally divided into two groups, fast wind and slow wind. However, the origin and releasing mechanism of highly variable slow winds are still a riddle. The slow wind is generally believed to have low Alfvénicity, while the fast wind shows high Alfvénicity. However, Parker Solar Probe shows that highly Alfvénic slow winds take a large proportion of the near-Sun pristine slow winds, suggesting similar sources with the Alfvénic fast winds. Helium abundance (Nα/Np) also bears information about the source region and release mechanism of the slow winds. The typical value of helium abundance in slow winds can be as low as 1% or even lower, while it stays around 4% in fast winds. During its 8th encounter, PSP observed intervals of helium-poor (Nα/Np~0.1%) and helium-normal (Nα/Np~1%) Alfvénic solar winds on two sides of the heliospheric current sheet. We calculate and compare the alpha-particle properties, plasma parameters, collisional age, and magnetic fluctuations in these intervals statistically. In addition, we check the magnetic connection from PSP to the Sun during these intervals with two-step ballistic backmapping. We conclude that the helium-poor winds originate from large quiescent magnetic loops and experience sufficient collisions before and during release. In contrast, helium-normal winds originate from low-latitude coronal holes and experience preferential acceleration and heating due to wave-particle interactions. These results suggest that Alfvénic slow solar winds likely have multiple origins.