Characteristics and Source Regions of Slow Alfvenic Solar Wind Observed by Parker Solar Probe

TL;DR

This study uses Parker Solar Probe data and modeling to identify the sources and characteristics of slow Alfvénic solar wind, revealing distinct populations linked to different solar structures and their acceleration behaviors.

Contribution

It introduces a classification of slow Alfvénic solar wind sources based on magnetic field strength and connects these to specific solar regions using PFSS modeling.

Findings

Low-$B_0$ SASW originates from small coronal holes and their boundaries.

High-$B_0$ SASW may emerge from non-CH structures via interchange reconnection.

Low-$B_0$ SASW shows larger expansion and stronger alpha particle acceleration.

Abstract

Using a classification scheme for solar wind type based on the heliocentric distance of the observation, we look at near perihelion observations from Parker Solar Probe Encounters Four to Fourteen to study the sources of the slow Alfv$\'e$nic solar wind (SASW). Through Potential Field Source Surface (PFSS) modeling and ballistic mapping, we connect streams to their solar source and find that a primary population of SASW comes from low magnetic field strength regions (low-$B_0$), likely small coronal holes (CHs) and their over-expanded boundaries, while a second population of high field strength (high-$B_0$) seems to emerge from non-CH structures potentially through interchange reconnection with nearby open field lines. This low-$B_0$ SASW shows larger expansion than the fast solar wind (FSW) but similar mass flux, potentially indicating additional heating below the critical point, and emergence from a cooler structure, which could lead to slower wind emerging from CH-like structures. We show that this low-$B_0$ SASW shows stronger preferential acceleration of alpha particles (similar to the FSW) than the high-$B_0$ SASW, and that this is a velocity dependent phenomenon as found in previous studies. To have additional confidence in our mapping results, we quantify the error on both the PFSS model and ballistic mapping and discuss how additional multi-point observations of plasma parameters and composition would allow us to better constrain our models and connect the solar wind to its source.