Solar physics

Hidden magnetism

Observations of the Hanle effect have revealed the existence of small-scale ‘hidden’ magnetic flux on the quiet Sun. The magnetic-energy density of this hidden flux is much larger than previously thought.

Magnetic fields have occupied centre stage in solar physics for the past several decades, and have come to be regarded as the key ingredient for a unified understanding of solar phenomena. It may therefore come as a surprise that, after all these years, the magnetic-energy density in the solar atmosphere might have been seriously underestimated — as Trujillo Bueno et al.1 conclude on page 326 of this issue.

The spectrum of radiation from the Sun can be resolved into a series of lines corresponding to different atomic transitions. In the presence of a magnetic field, these spectral lines can split into multiple polarized components — this is the Zeeman effect and it has been used to diagnose the magnetic fields on the Sun since it was first introduced2 to astrophysics by George Ellery Hale in 1908. Ever smaller magnetic structures have been revealed, and there is no end in sight. In fact, the magnetic flux seems to have a fractal structure, with an almost scale-invariant, self-similar pattern3 (Fig. 1). According to theoretical predictions based on magnetoturbulence, the structuring should continue for several orders of magnitude beyond the scales that have so far been resolved.

Figure 1: The fractal-like pattern of magnetic fields on the quiet Sun.
figure1

The enlarged image on the right (courtesy of Göran Scharmer, from the Swedish Solar Telescope on La Palma) covers 1% of the area of the left panel (from the Kitt Peak Observatory). These maps represent the patterns of circular polarization in the Sun's radiation caused by the Zeeman effect. The red and blue patches represent flux of opposing magnetic polarities, separated by the grey voids of seemingly no flux. Observations made using the Hanle effect now reveal1 that these grey regions are not voids at all, but are teeming with turbulent fields that carry a significant density of magnetic energy.

But the Zeeman effect as a diagnostic tool is ‘blind’ to magnetic fields that are tangled on scales too small to be resolved. Below the scale of the achievable angular resolution, the contributions to the Zeeman-effect polarization from opposite-polarity components within a tangled magnetic field cancel each other; so an unresolved mixed-polarity field leaves no ‘footprint’ in the Zeeman-split spectral lines. For this reason, a vast amount of solar magnetic flux has possibly remained hidden from view.

Trujillo Bueno et al.1 have used a different approach. The alternative tool is the Hanle effect, a coherence phenomenon discovered4 by Wilhelm Hanle in 1924. This effect was of great significance in the early development of quantum mechanics, because it demonstrated the principle of the coherent superposition of quantum states, and the nature of decoherence when the degeneracy of the quantum states is partially lifted as weak magnetic fields are introduced5. In the context of solar physics, the Hanle effect refers to the set of polarization effects that are caused by the coherent scattering of the radiation in the Sun's atmosphere under the influence of an external magnetic field. The symmetry properties of the Hanle effect with respect to the orientation of the magnetic field are entirely different from those of the Zeeman effect. Thus an unresolved, tangled magnetic field leaves a polarimetric footprint for the Hanle effect, while being invisible to the Zeeman effect.

Although the Hanle effect was introduced decades ago as a tool to gain information on the elusive, turbulent solar magnetic field6 (and a lower limit of 10 gauss on the turbulent field strength was determined straight away), further progress was limited by the insufficient polarimetric precision of the instruments available. The polarization amplitudes sought are small (typically at the level of 0.1% or less), because the anisotropy of the radiation field in the solar atmosphere is so small. But the wealth of polarization phenomena caused by the coherent scattering of the Sun's radiation at last became fully accessible with the introduction a decade ago of the Zurich Imaging Polarimeter, ZIMPOL7, with which the polarimetric noise could be dramatically reduced. Using ZIMPOL, the electro-optically modulated polarization signal is ‘demodulated’ into four image planes, which correspond to the different polarization states needed to form the full Stokes vector (which contains the complete polarization information).

With the polarimetric accuracy of one part in 105 that is routinely reached with ZIMPOL, combined with high spectral resolution, an astounding variety of spectral structures can be seen throughout the whole solar spectrum. This linearly polarized spectrum — which has been called the ‘second solar spectrum’, because it bears so little resemblance to the ordinary intensity spectrum — is a veritable treasure trove of all kinds of coherence phenomena8. The different polarized structures in the second solar spectrum are affected to various degrees, through the Hanle effect, by the ‘hidden’ magnetic fields in the solar atmosphere. Differential effects can then be used as diagnostics.

However, the proper quantitative interpretation of the Hanle signatures in the solar spectrum is a tricky business. The polarization amplitudes observed unfortunately depend on details of how the spectral line is formed — including the details of the structure of both the atom/molecule and of the solar atmosphere, as well as the way in which the polarized radiation is transported through the atmosphere. Trujillo Bueno et al.1 have brought the theory of line formation to a new level: they have modelled the Hanle effect for spectral lines from atoms and molecules using a three-dimensional radiative-transfer code and a model of the Sun's atmosphere obtained from simulations of the star's surface convection. They find much higher magnetic-energy densities than in previous investigations, both because of their more realistic three-dimensional approach and because they introduce a more realistic probability distribution function for the turbulent field strengths, rather than using a single-value field.

This work is a pleasing example of how different areas of astrophysics can be brought together to achieve a scientific objective: Trujillo Bueno et al. have combined high-precision spectro-polarimetry, coherency effects in atomic physics, advanced techniques in radiative-transfer theory and simulations of magnetoconvection. The information that can be retrieved about the Sun's magnetism is, however, model-dependent, and will remain so for the foreseeable future, because the small-scale magnetic structures in question will not be resolved even with the next generation of telescopes. Still, with the wealth of new diagnostic information available through the second solar spectrum, we can expect to see major advances in our understanding of the Sun's hidden magnetism in the years to come.

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Stenflo, J. Hidden magnetism. Nature 430, 304–305 (2004). https://doi.org/10.1038/430304a

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