On the Generation and Evolution of Switchbacks and the Morphology of Alfv\'enic Transition: Low Mach-number Boundary Layers

TL;DR

This paper explores the origins and evolution of solar wind switchbacks and the Alfvénic transition, revealing their connection to low Mach-number boundary layers and proposing a model involving expanding waves and turbulence.

Contribution

It introduces the concept of low Mach-number boundary layers in the solar wind and links them to the generation of switchbacks and the morphology of the Alfvénic transition.

Findings

LMBLs are associated with suppressed switchbacks and increased Alfvén radius.

Sub-Alfvénic wind is identified as an LMBL flow with a common origin.

Switchbacks are likely caused by expanding waves and turbulence, modulated by the Alfvén Mach number.

Abstract

We investigate the generation and evolution of switchbacks (SBs), the nature of the sub-Alfv\'enic wind observed by Parker Solar Probe (PSP), and the morphology of the Alfv\'enic transition, all of which are key issues in solar wind research. First we highlight a special structure in the pristine solar wind, termed a low Mach-number boundary layer (LMBL). An increased Alfv\'en radius and suppressed SBs are observed within an LMBL. A probable source on the Sun for an LMBL is the peripheral region inside a coronal hole with rapidly diverging open fields. The sub-Alfv\'enic wind detected by PSP is an LMBL flow by nature. The similar origin and similar properties of the sub-Alfv\'enic intervals favor a wrinkled surface for the morphology of the Alfv\'enic transition. We find that a larger deflection angle tends to be associated with a higher Alfv\'en Mach number. The magnetic deflections have an origin well below the Alfv\'en critical point, and deflection angles larger than $90^{\circ}$ seem to occur only when $M_{\rm A} \gtrsim 2$. The velocity enhancement in units of the local Alfv\'en speed generally increases with the deflection angle, which is explained by a simple model. A nonlinearly evolved, saturated state is revealed for SBs, where the local Alfv\'en speed is roughly an upper bound for the velocity enhancement. In the context of these results, the most promising theory on the origin of SBs is the model of expanding waves and turbulence, and the patchy distribution of SBs is attributed to modulation by reductions in the Alfv\'en Mach number. Finally, a picture on the generation and evolution of SBs is created based on the results.