A Non-potential Model for the Sun's Open Magnetic Flux

TL;DR

This paper introduces a non-potential magnetic field model for the Sun's corona that accounts for electric currents, resolving discrepancies between observed interplanetary magnetic field measurements and previous potential field models, especially during solar maximum.

Contribution

The study presents a quasi-static numerical model incorporating electric currents in the low corona, significantly improving open magnetic flux estimates during solar maximum compared to potential field models.

Findings

Open flux increases by 75%-85% at solar maximum

Model aligns with IMF measurements across solar cycle

Includes self-consistent coronal mass ejection modeling

Abstract

Measurements of the interplanetary magnetic field (IMF) over several solar cycles do not agree with computed values of open magnetic flux from potential field extrapolations. The discrepancy becomes greater around solar maximum in each cycle, when the IMF can be twice as strong as predicted by the potential field model. Here we demonstrate that this discrepancy may be resolved by allowing for electric currents in the low corona (below 2.5 solar radii). We present a quasi-static numerical model of the large-scale coronal magnetic evolution, which systematically produces these currents through flux emergence and shearing by surface motions. The open flux is increased by 75%-85% at solar maximum, but only 25% at solar minimum, bringing it in line with estimates from IMF measurements. The additional open flux in the non-potential model arises through inflation of the magnetic field by electric currents, with super-imposed fluctuations due to coronal mass ejections. The latter are modelled by the self-consistent ejection of twisted magnetic flux ropes.