Differentiating the acceleration mechanisms in the slow and Alfv\'enic slow solar wind

TL;DR

This study compares the acceleration mechanisms of different types of slow solar wind, revealing that Alfvénic slow wind behaves like fast wind with wave pressure-driven acceleration, unlike non-Alfvénic slow wind.

Contribution

It distinguishes the acceleration processes of Alfvénic and non-Alfvénic slow solar wind using combined remote sensing and in-situ data, highlighting the role of wave pressure gradients.

Findings

Alfvénic slow wind requires wave pressure for acceleration.

Non-Alfvénic slow wind driven by thermal pressure gradients.

Coronal source conditions support model consistency.

Abstract

In the corona, plasma is accelerated to hundreds of kilometers per second, and heated to temperatures hundreds of times hotter than the Sun's surface, before it escapes to form the solar wind. Decades of space-based experiments have shown that the energization process does not stop after it escapes. Instead, the solar wind continues to accelerate and it cools far more slowly than a freely-expanding adiabatic gas. Recent work suggests that fast solar wind requires additional momentum beyond what can be provided by the observed thermal pressure gradients alone whereas it is sufficient for the slowest wind. The additional acceleration for fast wind can be provided through an Alfv\'en wave pressure gradient. Beyond this fast-slow categorization, however, a subset of slow solar wind exhibits high Alfv\'enicity that suggest Alfv\'en waves could play a larger role in its acceleration compared to conventional slow wind outflows. Through a well-timed conjunction between Solar Orbiter and Parker Solar Probe, we trace the energetics of slow wind to compare with a neighboring Alfv\'enic slow solar wind stream. An analysis that integrates remote and heliospheric properties and modeling of the two distinct solar wind streams finds Alfv\'enic slow solar wind behaves like fast wind, where a wave pressure gradient is required to reconcile its full acceleration, while non-Alfv\'enic slow wind can be driven by its non-adiabatic electron and proton thermal pressure gradients. Derived coronal conditions of the source region indicate good model compatibility but extended coronal observations are required to effectively trace solar wind energetics below Parker's orbit.