The Alfv\'en Transition Zone observed by the Parker Solar Probe in Young Solar Wind -- Global Properties and Model Comparisons

TL;DR

This study uses Parker Solar Probe data to analyze the transition from subAlfvénic to superAlfvénic flow in the solar wind, revealing a turbulent, patchy Alfvén zone with an extended transition region, supported by model comparisons.

Contribution

It provides the first comprehensive analysis of the Alfvén transition zone in the young solar wind using PSP data and compares observations with global MHD models including turbulence effects.

Findings

Identification of 220 subAlfvénic periods lasting over 10 minutes.

Evidence of a turbulent, patchy Alfvén zone with extended transition region.

Consistency between observations and MHD model predictions.

Abstract

The transition from subAlfv\'enic to superAlfv\'enic flow in the solar atmosphere is examined by means of Parker Solar Probe (PSP) measurements during solar encounters 8 to 14. Around 220 subAlfv\'enic periods with a duration $\ge$ 10 minutes are identified. The distribution of their durations, heliocentric distances, and Alfv\'en Mach number are analyzed and compared with a global magnetohydrodynamic model of the solar corona and wind, which includes turbulence effects. The results are consistent with a patchy and fragmented morphology, and suggestive of a turbulent Alfv\'en zone within which the transition from subAlfv\'enic to superAlfv\'enic flow occurs over an extended range of helioradii. These results inform and establish context for detailed analyses of subAlfv\'enic coronal plasma that are expected to emerge from PSP's final mission phase, as well as for NASA's planned PUNCH mission.