Slide Show page 8 for
The Sun in Time

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While studying the visible spectrum of the Sun, seen above, scientists noticed dark bands spaced at specific intervals.  These are called absorption lines and are formed when a cooler gas is between the light emitting gas and the observer. 

Now, if there is a magnetic field imbedded within that hot gas, as in the case of sunspots on the solar surface, individual absorption lines may be split into several parts (an effect called Zeeman splitting).  The figure below was obtained by scanning across the sunspot, on the right half of the image, with a spectroscope.  The left half of the image shows that the absorption line splitting is largest in the center of the sunspot, where the magnetic field is strongest.  (The single line to the right of the Zeeman split line is not magnetically sensitive.)  To determine the strength of the magnetic field at that point, we need only measure the distance between the split absorption line.  We can then map the magnetic field of the sunspot by measuring the distance between the splitting of the absorption line at each point on the spectral image and matching that point with the white light image.

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Measuring the direction of the magnetic field, to get the full vector, is a bit more difficult than just finding the magnitude.  Scientists must remember that light (or photons) has a wave nature as well as a particle nature.  Waves have a preferred direction of travel and a particular vibration.  Vector magnetographs analyze the directions of vibration of the light coming from sunspots and infer from that the direction of the magnetic field. 

Pictured here, is the Marshall Space Flight Center's vector magnetograph.  The co-aligned gold telescope (which views the Sun in the light of  H-alpha*), allows observers to see solar flares within the magnetograph's field-of-view (solar flares are usually not detected in visible light). 

 

*H-alpha light has a wavelength of 656 nm and is created within the chromospheric layer of the solar atmosphere.

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MSFC's prime observer James Smith, is picured here with MSFC's vector magnetograph

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