Sub-Alfvenic Solar Wind observed by PSP: Characterization of Turbulence, Anisotropy, Intermittency, and Switchback

TL;DR

This study reports the first in-situ observations of sub-Alfvenic solar wind by PSP, analyzing turbulence, anisotropy, intermittency, and switchbacks to understand the transition region near the Sun.

Contribution

It provides novel in-situ measurements of sub-Alfvenic solar wind and characterizes its turbulence and magnetic properties, advancing understanding of the Alfven critical region.

Findings

First in-situ detection of sub-Alfvenic solar wind by PSP.

Distinct turbulence and anisotropy features in sub-Alfvenic vs super-Alfvenic wind.

Insights into the physical processes at the transition between coronal and solar wind regimes.

Abstract

In the lower solar coronal regions where the magnetic field is dominant, the Alfven speed is much higher than the wind speed. In contrast, the near-Earth solar wind is strongly super-Alfvenic, i.e., the wind speed greatly exceeds the Alfven speed. The transition between these regimes is classically described as the "Alfven point" but may in fact occur in a distributed Alfven critical region. NASA's Parker Solar Probe (PSP) mission has entered this region, as it follows a series of orbits that gradually approach more closely to the sun. During its 8th and 9th solar encounters, at a distance of 16 solar radii from the Sun, PSP sampled four extended periods in which the solar wind speed was measured to be smaller than the local Alfven speed. These are the first in-situ detections of sub-Alfvenic solar wind in the inner heliosphere by PSP. Here we explore properties of these samples of sub-Alfvenic solar wind, which may provide important previews of the physical processes operating at lower altitude. Specifically, we characterize the turbulence, anisotropy, intermittency, and directional switchback properties of these sub-Alfvenic winds and contrast these with the neighboring super-Alfvenic periods.