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Monday, June 5, 2000


New way
found to measure
sun’s brightness

The peak activity in about
10 years could seriously affect
the Earth's climate

By Helen Altonn


Scientists have a new tool to use in trying to determine whether the sun will be brighter during its peak activity in 10 or 11 years, which could seriously affect the Earth's climate.

Jeffrey Kuhn, University of Hawaii Institute for Astronomy researcher, and associates have discovered a way to learn how and why the sun changes.

Their findings are reported in the June 1 issue of Nature magazine.

Using data from the NASA spacecraft SOHO, the scientists identified features on the sun's surface similar to those in the Earth's ocean.

The Solar and Heliospheric Observatory is about 92 million miles from the sun and looks at it continuously. It's positioned above the atmosphere "so it has exquisite sensitivity to measuring very small changes in the sun," Kuhn said.

University of Hawaii
NASA has learned that the sun is covered with bumps
and dips. The solar hills would appear like this if their
relative height were magnified by a factor of 1 million.

It has revealed that the sun's surface is covered with long-lasting depressions and bumps or hills similar to those satellites have identified on the Earth's oceans, he said.

The depressions on the ocean are very small, only a few inches high, but they're about 100 miles or more across, Kuhn said.

"They move across the ocean very slowly, almost as though fixed on the ocean like mountains, very tiny ones. They can take months or years to travel from one coast to the next."

The same phenomenon is believed to occur on the sun because of the sun's interior rotation, he said. "It's a new kind of wave -- a wave we've been looking for a number of years."

Called Rossby waves, they produce weak cyclones that create hills and valleys on the solar surface, Kuhn said. The hills are about 330 feet high and about 55,800 miles apart.

The solar features were measured for the first time with a SOHO experiment called the Michelson Doppler Imager.

Tiny changes in the sun's edge, or limb, were measured for nearly three years as Rossby wave hills moved around it with the sun's rotation.

Kuhn, senior scientist for the Haleakala Observatory on Maui, said three kinds of waves occur on the sun: Acoustic or sound waves; gravity waves, almost like surface waves in the ocean, and Rossby waves.

Only acoustic waves have been seen until now, he said.

Discovery of the Rossby waves is a new tool for looking inside the sun, Kuhn said.

"The biggest problem is trying to understand how the sun changes its brightness on a time scale of 11 years."

Based on measurements of changes in solar brightness during the past 25 years, it's possible that the sun could be brighter by one-half percent or more in the next cycle, he said.

The sun's activity reaches a peak about every 11 years in what is called solar maximum. The current cycle should reach maximum in early 2001.

Scientists know from the past that the sun's brightness changes enough to affect the Earth's climate, Kuhn said.

"To understand what's going to happen in the future, we've got to understand how the solar cycle works. It's our dark matter problem in solar physics."

To figure out how it works, he said, they must have a tool to look inside the sun, just as they've learned about the Earth's interior from studying earthquakes.

"The Rossby wave is going to tell us how and why the sun rotates as it does. It turns out to be one of the big puzzles."

Kuhn's colleagues in the NASA-supported project are UH graduate student J.D. Armstrong, Phil Scherrer of Stanford University, and Alan Title of Lockheed Corp., Palo Alto, Calif.

Scherrer led the Michelson Doppler Imager experiment, which was built under Title's direction.

Ka Leo O Hawaii

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