Addressing the open flux problem with a non-spherical solar coronal magnetic field model

TL;DR

This paper introduces a non-spherical magnetic field model for the solar corona that better matches observed open flux levels and complex magnetic topologies.

Contribution

The paper develops the NSPF model with a non-spherical source surface, improving open flux estimation and magnetic topology representation.

Findings

The NSPF model produces more open magnetic flux than PFSS.

It reproduces complex coronal magnetic structures.

The model aligns well with in-situ measurements and solar wind data.

Abstract

The coronal magnetic field plays a fundamental role in governing coronal activities, driving space-weather events, and shaping the heliosphere. Due to a lack of direct observations, extrapolation models such as the Potential Field Source Surface (PFSS) model become the primary method to obtain the three-dimensional magnetic field distribution in the corona. However, the PFSS model cannot solve the long-standing open-flux problem, in which the extrapolated open magnetic flux is significantly lower than that inferred from in-situ measurements. To address this issue, we develop an innovative Non-Spherical Potential Field (NSPF) model. The model introduces a Non-Spherical Source Surface (NSSS) defined as an isosurface of the total magnetic field. The NSSS naturally forms concave structures beneath external current sheets, enabling the model to generate substantially more open magnetic flux while yielding a physically plausible distribution of open field regions. As a result, the NSPF model successfully reproduces complex coronal magnetic topologies, interplanetary magnetic field properties, and solar wind source mappings. Our refined coronal magnetic model provides a proper foundation for future research on solar and heliospheric magnetic coupling.