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Mount Wilson Solar Tower Turns 100

USC professor Edward Rhodes and his team began to probe the sun’s internal structure after USC took over the tower.Photo/Carl Marziali

The mythical Icarus tried to rise to the sun on wings of wax and perished, but scientist George Ellery Hale, a practical man, built a 60-foot tower from battleship steel to divine the orb’s secrets.

The sturdy structure served its builder well. Overlooking Los Angeles from his perch on Mt. Wilson, Hale detected the sun’s magnetic field on June 25, 1908.

On the same date this year, the USC astronomers who now run the tower celebrated both the tower’s 100 years of continuous observations and the 100th anniversary of Hale’s breakthrough

“It was the first discovery of magnetic fields on the sun. It was the first discovery of magnetic fields anywhere off the earth,” said Edward Rhodes, a USC College professor of astronomy, who has been directing observations at the tower for more than two decades.

Hale’s discovery opened a whole field of solar research. His measurements revealed the magnetic origin of sunspot activity, which may have a significant influence on Earth’s temperature.

California Institute of Technology astronomers also used the 60-Foot Solar Tower Telescope, as it is now called, to discover that the sun pulses at regular five-minute intervals.

When Rhodes began his career in solar astronomy as a graduate student in 1975, the internal workings of the sun remained a tantalizing mystery.

Working independently, he and a German competitor realized that the throbbing oscillations inside the sun were resonating sound waves, like notes trapped in an organ pipe (the music of the sun is too low for humans ears).

Rhodes then began thinking about using the Mt. Wilson solar tower to observe the sun’s oscillations over the long haul.

After USC took over the tower in 1984, Rhodes and his team began to probe the sun’s internal structure. Their discoveries include the identification of a sub-surface layer of gas that appears to play a distinct role in solar activity and the confirmation that the rotation pattern visible at the sun’s surface extends into much of its interior.

The group’s most important contribution may be its painstaking record of the sun’s weather. Every day, a member of the group � which includes undergraduate students as well as professional astronomers � captures and saves a seven-inch photograph of the sun, with sunspots clearly visible.

Then, every minute for the rest of the day, the group studies the light emitted by sodium atoms in the sun. Because the light’s color shifts depending on whether the atoms are moving into the sun or away from it, the data is as good as a map of the sun’s boiling peaks and troughs.

Rhodes and his group now have photographs and maps for almost two complete 11-year solar cycles. No other telescope has such detailed information, Rhodes said, and his group is analyzing the mountains of data from one cycle with the help of a NASA grant.

The analysis will help future scientists understand the sunspot cycle and the extent of its effect on Earth’s temperature.

The tower that Hale built is now the oldest operating telescope at the Mt. Wilson Observatory. And with so much of the planet’s future depending on whether it can avoid extreme temperature swings, the tower may have another important century ahead of it.

 

Mount Wilson Solar Tower Turns 100

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