Magnetic Topology and Loop Statistics in Observed Coronal Holes Using Potential Field Modeling

TL;DR

This study analyzes the magnetic topology and loop structures of observed coronal holes using PFSS models, revealing key differences from quiet Sun regions and highlighting modeling limitations.

Contribution

It provides a detailed comparison of coronal hole magnetic structures with quiet Sun regions using PFSS models, identifying the influence of flux density and loop extension.

Findings

Low-lying loops in coronal holes are smaller and narrower than in quiet Sun regions.

Coronal holes contain higher-reaching loops, possibly affected by source surface height.

Discrepancies in models reflect limitations, not absence of coronal holes.

Abstract

Potential Field Source Surface (PFSS) models are widely used to study the solar corona and form the basis for solar wind forecasting, yet often fail to reproduce observed properties of coronal holes. We analyze 702 observed coronal holes between 2010 and 2019 and compute corresponding PFSS magnetic field extrapolations to examine their magnetic topology and loop statistics, comparing them with quiet Sun regions. Our goal is to determine how observed coronal holes are represented in a PFSS model and to identify sources of known discrepancies. We find that low-lying loops covering the weak, balanced background field in coronal holes are statistically smaller and narrower than in quiet Sun regions, with a median height strongly correlated to the coronal holes mean magnetic flux density (cc_Pearson = 0.81). This suggests that at low altitudes, the coronal hole magnetic topology is primarily governed by its flux density, unlike in quiet Sun regions. Coronal holes also contain loops extending much higher into the corona than typical quiet Sun loops, although it is unclear if these are truly closed or reflect source surface height limitations. Overall, differences in modeled magnetic structures of coronal holes and quiet Sun regions are evident, even when the PFSS model does not indicate any open fields. These results suggest that observed coronal holes correspond to distinct photospheric magnetic structures, and that discrepancies with PFSS models reflect modeling limitations rather than the absence of coronal holes.