2008 marks the 100th anniversary of the discovery of astrophysical magneticfields, when George Ellery Hale recorded the Zeeman splitting of spectral linesin sunspots. With the introduction of Babcock's photoelectric magnetograph itsoon became clear that the Sun's magnetic field outside sunspots is extremelystructured. The field strengths that were measured were found to get largerwhen the spatial resolution was improved. It was therefore necessary to come upwith methods to go beyond the spatial resolution limit and diagnose theintrinsic magnetic-field properties without dependence on the quality of thetelescope used. The line-ratio technique that was developed in the early 1970srevealed a picture where most flux that we see in magnetograms originates inhighly bundled, kG fields with a tiny volume filling factor. This led tointerpretations in terms of discrete, strong-field magnetic flux tubes embeddedin a rather field-free medium, and a whole industry of flux tube models atincreasing levels of sophistication. This magnetic-field paradigm has now beenshattered with the advent of high-precision imaging polarimeters that allow usto apply the so-called "Second Solar Spectrum" to diagnose aspects of solarmagnetism that have been hidden to Zeeman diagnostics. It is found that thebulk of the photospheric volume is seething with intermediately strong, tangledfields. In the new paradigm the field behaves like a fractal with a high degreeof self-similarity, spanning about 8 orders of magnitude in scale size, down toscales of order 10 m.
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