Observed Fluctuation Enhancement and Departure from WKB Theory in Sub-Alfv\'enic Solar Wind

TL;DR

This study analyzes Parker Solar Probe data to investigate fluctuation behaviors in the sub-Alfvénic solar wind, revealing significant deviations from WKB theory caused by turbulence and energy input near the Alfvén critical point.

Contribution

It demonstrates the breakdown of WKB theory in the critical solar wind region due to turbulence and energy input, supported by in-situ measurements.

Findings

Magnetic and kinetic energies do not decrease monotonically as WKB predicts.

Standard WKB wave action conservation is violated near the Alfvén critical region.

Strong fluctuation energy input likely driven by large-scale coronal shear flows.

Abstract

Using Parker Solar Probe data from orbits 8 through 17, we examine fluctuation amplitudes throughout the critical region where the solar wind flow speed approaches and then exceeds the Alfv\'en wave speed, taking account of various exigencies of the plasma data. In contrast to WKB theory for non-interacting Alfv\'en waves streaming away from the Sun, the magnetic and kinetic fluctuation energies per unit volume are not monotonically decreasing. Instead, there is clear violation of conservation of standard WKB wave action, which is consistent with previous indications of strong in-situ fluctuation energy input in the solar wind near the Alfv\'en critical region. This points to strong violations of WKB theory due to nonlinearity (turbulence) and major energy input near the critical region, which we interpret as likely due to driving by large-scale coronal shear flows.