The fine-scale structure of polar coronal holes

TL;DR

This study reveals that polar coronal holes contain fine-scale filamentary structures, called plumelets, which extend to the edge of the observed field and follow specific spatial distributions, impacting our understanding of solar wind acceleration.

Contribution

The paper demonstrates that polar coronal hole plumes are composed of fine filamentary structures, expanding super-radially, and analyzes their spatial distribution and implications for solar wind models.

Findings

Fine structures have scales of 2-10 arcseconds.

Spatial distribution follows a k^{-1} power law.

Some structures expand super-radially with height.

Abstract

Coronal holes are thought to be composed of relatively broad columnar structures known as plumes. Here we demonstrate that the plumes (and inter-plumes) in polar coronal holes are composed of fine-scale filamentary structure, with average scales of 2-10$^{\arcsec}$. The fine structure is the off-limb analogue of the previously found 'plumelets' of \cite{Uritsky_2021}. The off-limb observations enable an examination of the fine-structure without the influence of the underlying atmosphere along the line of sight. Hence, we show that the fine-scale structure is present at least until the edge of the field of view of the Solar Dynamics Observatory. The fine structure is found to have spatial distribution that follows a $k^{-1}$ power law perpendicular to the inferred magnetic field direction. For a small sample of the fine structure, the cross-sectional profiles are measured as a function of height. In some cases, the measurements indicate that the fine structure expands super-radially, consistent with existing models of polar field expansion and the expansion of the plumes. We discuss the implications of the presence of the fine structure with respect to understanding wave propagation in the coronal holes and their contribution to powering the solar wind.