1.2 Discovery of sunspot’s magnetic field

The key physical process that makes human be aware of the existence of magnetic fields in sunspots is the interaction between atoms and the magnetic field, i.e., the so called Zeeman effect, discovered in laboratory by the young physicist Pieter Zeeman in The Netherlands by the end of the 19th century (Zeeman, 1897). This effect describes how the electronic energy levels of an atom split in the presence of a magnetic field, giving raise to several absorption/emission spectral lines where there was only one spectral line in the absence of the magnetic field. In addition, the magnetic field modifies the polarization properties of the emitted/absorbed photons in a manner that depends on the viewing angle between the observer and the magnetic field vector.

An initial hint of the presence of the Zeeman effect in the spectra of sunspots is actually found in a historic record prior to the discovery of the Zeeman effect by Lockyer in 1866, describing “thick spectral lines in sunspots”. Cortie in 1896 mentioned a reversal (bright core) of an absorption line in sunspot spectra, which would obviously be a manifestation of the Zeeman effect under a strong magnetic field.

A more concrete evidence of the presence of magnetic field in sunspots was established by George Hale in a paper entitled “On the Probable Existence of a Magnetic Field in Sun-Spots” (Hale, 1908Jump To The Next Citation Point). He observed line splitting and polarization in sunspot spectra observed by the newly constructed 35-feet solar tower at the Mount Wilson Observatory. By comparing the separation between the spectral components in the observed lines in sunspots and in sparks in laboratory experiments, he deduced that the magnetic field strength in sunspots was about 2600 – 2900 Gauss. This was the first detection of the extraterrestrial magnetic field, which opened the way for measuring the magnetic field on the Sun and on other astronomical objects. A more detailed description of the discovery of magnetic fields in sunspots can also be found in Toro Iniesta (1996).

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