Turbulent magnetic fields in the quiet Sun: implications of Hinode observations and small-scale dynamo simulations

TL;DR

This study combines high-resolution simulations and Hinode observations to reveal that the quiet Sun's magnetic fields are more pervasive and structured at smaller scales than previously detected, with implications for understanding solar magnetism.

Contribution

The paper introduces a fractal self-similar model of turbulent solar magnetic fields and provides new estimates of the true magnetic flux density, reconciling Zeeman and Hanle measurement discrepancies.

Findings

Most vertical magnetic flux is invisible at 200 km resolution.

Magnetic structuring extends below 200 km, following a power-law scaling.

Estimated true flux density is around 50 G, reducing previous measurement discrepancies.

Abstract

Using turbulent MHD simulations (magnetic Reynolds numbers up to 8000) and Hinode observations, we study effects of turbulence on measuring the solar magnetic field outside active regions. Firstly, from synthetic Stokes V profiles for the FeI lines at 630.1 and 630.2 nm, we show that a peaked probability distribution function (PDF) for observationally-derived field estimates is consistent with a monotonic PDF for actual vertical field strengths. Hence, the prevalence of weak fields is greater than would be naively inferred from observations. Secondly, we employ the fractal self-similar geometry of the turbulent solar magnetic field to derive two estimates (numerical and observational) of the true mean vertical unsigned flux density. We also find observational evidence that the scales of magnetic structuring in the photosphere extend at least down to an order of magnitude smaller than 200 km: the self-similar power-law scaling in the signed measure from a Hinode magnetogram ranges (over two decades in length scales and including the granulation scale) down to the 200 km resolution limit. From the self-similar scaling, we determine a lower bound for the true quiet-Sun mean vertical unsigned flux density of ~50 G. This is consistent with our numerically-based estimates that 80% or more of the vertical unsigned flux should be invisible to Stokes-V observations at a resolution of 200 km owing to the cancellation of signal from opposite magnetic polarities. Our estimates significantly reduce the order-of-magnitude discrepancy between Zeeman- and Hanle-based estimates.