Why a Structureless Solar Corona Is Difficult to Achieve During a Grand Solar Minimum

Abstract

The structure of the solar corona during the Maunder Minimum (~1645–1715$) remains poorly constrained due to limited direct observations. In spite of this, historical records, including the near-absence of sunspots, reduced auroral activity, elevated cosmogenic isotope levels, and eclipse descriptions, suggest a radically different solar state. In this study, we simulate the evolution of the photospheric magnetic field under idealised grand-minimum conditions, using the High-performance Flux Transport (HipFT) code, which is part of the Open-source Flux Transport (OFT) model. We consider a range of magnetic emergence scenarios, including both idealised bipolar magnetic regions (BMRs) and randomised small-scale flux injections emulating ephemeral regions, and assess their implications for large-scale coronal structure. Despite suppressing large-scale active region emergence, we find that stochastic surface transport processes naturally give rise to persistent polar fields and large-scale dipolar structure, even from zero-net-flux, randomly distributed sources. Simulations show that while the large-scale photospheric field, and by implication the associated coronal structure, may weaken and become more diffuse, a truly structureless configuration is difficult, although not impossible, to achieve. These results offer an alternative picture for interpreting historical eclipse records and suggest that weak, yet organised, photospheric magnetic fields may have persisted even during grand minima, implying some degree of persistent coronal structure. Our findings also have implications for assessing the likelihood and character of future low-activity states in the solar cycle.