Abstract
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 2010 September 19, 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 ≈ 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.
Highlights
The solar wind is a magnetized plasma that expands outward from the solar corona to fill the interplanetary space
We have investigated coronal hole (CH) detection techniques to characterize the uncertainty in characterizing CH area and open flux from observational EUV data
We applied a single method to five different EUV filtergrams for this CH
Summary
The solar wind is a magnetized plasma that expands outward from the solar corona to fill the interplanetary space. Fast solar wind streams are associated with recurrent geomagnetic activity (Neupert & Pizzo 1974) and are of increased research interest. They have been identified to originate from deep within coronal holes (Krieger et al 1973), where the predominantly open magnetic field allows plasma to escape (Altschuler et al 1972). Interchange reconnection (reconnection between open and closed fields, Crooker et al 2002) has been suggested as the source of a dynamic slow solar wind (Fisk et al 1998; Antiochos et al 2011) and would most occur near CH boundaries. Identifying and characterizing CHs and their boundaries is crucial to understand the origins of the solar wind and to assess the uncertainties in the quantification of open magnetic flux
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