Measuring the Hidden Aspects of Solar Magnetism

TL;DR

This paper reviews the evolution of solar magnetic field measurement techniques, highlighting how high-precision polarimetry reveals a complex, fractal magnetic structure in the Sun's photosphere that challenges earlier flux tube models.

Contribution

It introduces a new paradigm of solar magnetism based on high-resolution polarimetric observations, showing the magnetic field is highly tangled and fractal across multiple scales.

Findings

Magnetic fields in the photosphere are highly tangled and fractal.

Most magnetic flux originates from intermediately strong, tangled fields.

The magnetic field spans about 8 orders of magnitude in scale.

Abstract

2008 marks the 100th anniversary of the discovery of astrophysical magnetic fields, when George Ellery Hale recorded the Zeeman splitting of spectral lines in sunspots. With the introduction of Babcock's photoelectric magnetograph it soon became clear that the Sun's magnetic field outside sunspots is extremely structured. The field strengths that were measured were found to get larger when the spatial resolution was improved. It was therefore necessary to come up with methods to go beyond the spatial resolution limit and diagnose the intrinsic magnetic-field properties without dependence on the quality of the telescope used. The line-ratio technique that was developed in the early 1970s revealed a picture where most flux that we see in magnetograms originates in highly bundled, kG fields with a tiny volume filling factor. This led to interpretations in terms of discrete, strong-field magnetic flux tubes embedded in a rather field-free medium, and a whole industry of flux tube models at increasing levels of sophistication. This magnetic-field paradigm has now been shattered with the advent of high-precision imaging polarimeters that allow us to apply the so-called "Second Solar Spectrum" to diagnose aspects of solar magnetism that have been hidden to Zeeman diagnostics. It is found that the bulk of the photospheric volume is seething with intermediately strong, tangled fields. In the new paradigm the field behaves like a fractal with a high degree of self-similarity, spanning about 8 orders of magnitude in scale size, down to scales of order 10 m.