Mapping the hidden magnetic field of the quiet Sun

TL;DR

This study uses multi-line Zeeman effect analysis and magnetohydrodynamical simulations to estimate the average magnetic field strength of the Sun's quiet surface, revealing a value around 46 G and its correlation with convection.

Contribution

It introduces a novel multi-line inversion technique to measure the hidden magnetic fields in the quiet Sun using intensity profiles, bridging a gap between previous Hanle effect studies and direct measurements.

Findings

Average hidden magnetic field strength is about 46 G.

The hidden magnetic fields are strongly correlated with convection motions.

Reconciliation with Hanle effect results remains challenging.

Abstract

The Sun is the only star where we can resolve the intricate magnetism that all convective stars harbor. Yet, more than 99% of its visible surface along the solar cycle (the so-called quiet Sun) is filled with a tangled, unresolved magnetism. These "hidden" fields are thought to store enough magnetic energy to play a role in the heating of the Sun's outer atmosphere, but its field strength is still not constrained. Previous investigations based on the Hanle effect in atomic lines claim a strong magnetization of about 100 G, while the same effect in molecules show a factor of 10 weaker fields. The discrepancy disappears if the magnetic field strength of the hidden is not homogeneous in the solar surface. In this letter, we prove using magnetohydrodynamical simulations that it is possible to infer the average field strength of the hidden quiet Sun magnetic fields using multi-line inversions of intensity profiles in the Zeeman regime. Using this technique with 15 spectral lines in the 1.5 $\mu$m spectral range, we reveal that the spatial distribution of the hidden field is strongly correlated with convection motions, and that the average magnetization is about 46 G. Reconciling our findings with the Hanle ones is not obvious and will require future work on both sides, since it implies an increase of the field strength with height, something that is physically questionable.