Solar astronomy at Mt. Wilson Observatory has a rich history. There is probably no historical event that can be singled out as the most important solar discovery of the 20th Century, but Mt. Wilson has been home to some of the greatest scientific achievements. In addition to having the largest photographic archive of white-light images of the sun, and having the distinction of being the first solar telescope to be built on a tower, the 60- Foot Solar Tower is home to two of the most important discoveries in the field of solar research ....

Magnetic Fields Discovered on the Sun!

It wasn't quite front page news, but to George Ellery Hale, he thought he had discovered the key to understanding solar activity!

Shortly after the first solar telescope was installed on the mountain, the Snow telescope, Hale wanted to improve the quality of his data by building a solar tower. The idea was to put the light collecting mirrors on a tall tower to minimize the amount of heat that would rise off of the ground to distort the solar image. The principle worked and Hale immediately started taking spectroheliograms once the 60-Foot Tower was in a semi-completed state in 1907. In some of the Hydrogen-alpha spectroheliograms, Hale noticed swirls around sunspot groups. This reminded him a lot of how lines of force appeared around the poles of magnets. Hale also observed that the direction of the swirls, or fibrils, were opposite around adjacent spots; another indication of magnetic fields. This latter point would later be regarded as due to the coriolis effect discovered by Robert Richardson in 1941. Nevertheless, Hale continued his investigation, and on June 25, 1908, he photographed the Zeeman effect from a sunspot spectrum using the 60-Foot Solar Tower and its 9meter Littrow spectrograph. (The Zeeman effect is a phenomenon whereby a spectral line is either broadened or split into two or more components in the presence of a magnetic field.) Hale's photograph represented the first time magnetic fields were discovered outside of earth. Hale thought he had discovered the mystery of solar activity, where in fact the discovery prompted further investigation and asked more questions about the sun.

The Sun is oscillating!

Helioseismology got its start in the early 1960s when CalTech physicist Robert Leighton discovered the 5-minute oscillation on the surface of the sun.

Before his pioneering work in solar physics, Robert Leighton had modeled Jupiter's atmosphere, studied cosmic rays at Mt. Wilson, and developed a tilt meter to measure slight changes in ground motion around active volcanoes. It was his work at Mt. Wilson that led him to the study of the solar atmosphere at the 60-Foot Solar Tower. He developed a technique to obtain the highest angular-resolution magnetograms available at the time. Suggested by George E. Hale years earlier, Leighton created difference photos from spectroheliograms taken in Zeeman-sensitive spectral lines. The Zeeman effect splits a spectral line into a red and blue component which indicates motion away from and toward an observer, respectively. Robert Leighton and his collegues (Robert Noyes and George Simon) took several spectroheliograms in "red" and "blue" shifted light of specific solar lines, like Calcium at 6103Å. By combining the two separate plates velocity maps were obtained which showed up and down motion of the "supergranulation". When Leighton analyzed several of these difference images, he noticed that both large scale and small scale features on the solar surface were oscillating in a period of 296 seconds (or roughly 5 minutes), and at a speed of 500m/s. These solar oscillations quickly became known as solar quakes, hence the study of helioseismology was born. Later, studies would show that these oscillations were due to standing accoustic waves trapped between regions of the solar interior of different densities.

Today, the 60-Foot Solar Tower and many other groups like, GONG and BiSON study these oscillations to determine the internal structure of the sun.


Hale,G.E. : 1908b, Astrophysical Journal 28,315
"On The Probable Existence of a Magetic Field in Sun-Spots"

Leighton,R.;Noyes,R.;Simon,G.: 1961, Astrophysical Journal 135,474
"Velocity Fields in the Solar Atmosphere"

Howard, R.: 1985, Solar Physics,100,171
"Eight Decades of Solar Research at Mount Wilson"

Photo Credits

  1. Montage of 3 images of the 60' Tower in 1907,1914,& 1915
  2. Andrew Carnegie and George Ellery Hale
  3. Robert Leighton

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