Coronal Hole Detection and Open Magnetic Flux

TL;DR

This study compares different coronal hole detection methods to quantify uncertainties in open magnetic flux estimates, revealing significant underestimation in observations and minor overestimation in simulations, impacting our understanding of solar magnetic flux.

Contribution

The paper systematically evaluates multiple detection techniques and magnetic data sources, quantifying uncertainties and biases in coronal hole open flux measurements.

Findings

Uncertainty in open flux estimates is about 26%, rising to 45% with data source variability.

Observed open flux is underestimated by 2.2-4 times compared to in-situ measurements.

Detection methods overestimate area and flux in simulated coronal holes, but errors are relatively small.

Abstract

Many scientists use coronal hole (CH) detections to infer open magnetic flux. Detection techniques differ in the areas that they assign as open, and may obtain different values for the open magnetic flux. We characterize the uncertainties of these methods, by applying six different detection methods to deduce the area and open flux of a near-disk center CH observed on 9/19/2010, and applying a single method to five different EUV filtergrams for this CH. Open flux was calculated using five different magnetic maps. The standard deviation (interpreted as the uncertainty) in the open flux estimate for this CH was about 26%. However, including the variability of different magnetic data sources, this uncertainty almost doubles to 45%. We use two of the methods to characterize the area and open flux for all CHs in this time period. We find that the open flux is greatly underestimated compared to values inferred from in-situ measurements (by 2.2-4 times). We also test our detection techniques on simulated emission images from a thermodynamic MHD model of the solar corona. We find that the methods overestimate the area and open flux in the simulated CH, but the average error in the flux is only about 7%. The full-Sun detections on the simulated corona underestimate the model open flux, but by factors well below what is needed to account for the missing flux in the observations. Under-detection of open flux in coronal holes likely contributes to the recognized deficit in solar open flux, but is unlikely to resolve it.