The Open Flux Problem

TL;DR

This study investigates the discrepancy between modeled and observed open magnetic flux in the heliosphere, revealing that models either underestimate flux or overestimate open field regions compared to coronal hole observations.

Contribution

It systematically compares multiple magnetic field models with observations, highlighting a persistent open flux problem and suggesting potential underestimation of solar magnetic flux or hidden open flux regions.

Findings

Models underestimate interplanetary magnetic flux when coronal hole areas match observations.

Matching interplanetary flux requires larger open field regions than observed coronal holes.

Estimated open flux from solar observations also underestimates interplanetary magnetic flux.

Abstract

The heliospheric magnetic field is of pivotal importance in solar and space physics. The field is rooted in the Sun's photosphere, where it has been observed for many years. Global maps of the solar magnetic field based on full disk magnetograms are commonly used as boundary conditions for coronal and solar wind models. Two primary observational constraints on the models are (1) the open field regions in the model should approximately correspond to coronal holes observed in emission, and (2) the magnitude of the open magnetic flux in the model should match that inferred from in situ spacecraft measurements. In this study, we calculate both MHD and PFSS solutions using fourteen different magnetic maps produced from five different types of observatory magnetograms, for the time period surrounding July, 2010. We have found that for all of the model/map combinations, models that have coronal hole areas close to observations underestimate the interplanetary magnetic flux, or, conversely, for models to match the interplanetary flux, the modeled open field regions are larger than coronal holes observed in EUV emission. In an alternative approach, we estimate the open magnetic flux entirely from solar observations by combining automatically detected coronal holes for Carrington rotation 2098 with observatory synoptic magnetic maps. This approach also underestimates the interplanetary magnetic flux. Our results imply that either typical observatory maps underestimate the Sun's magnetic flux, or a significant portion of the open magnetic flux is not rooted in regions that are obviously dark in EUV and X-ray emission.