The sun rises on Mount Wilson to reveal a clear, cobalt-blue sky over thick clouds blanketing the Los Angeles Basin below.
On top of the 60-foot tower operated by USC astronomers, two mirrors begin tracking the sun to bounce its image through a filter containing magnetized sodium vapor onto a million-pixel digital camera at the bottom of the tower.
The sodium gas absorbs a specific wavelength of the light, the same stark yellowish hue of urban streetlights. The magnetic field splits the light into two images, one shifted very slightly toward the blue end of the visible spectrum and the other toward the red.
“Each blue-shifted image is sensitive to the parts of the sun that are coming toward and going away from us in a slightly different manner than is each red-shifted one,”said Ed Rhodes, professor of physics and astronomy. A computer combines the two images into a single picture, called a dopplergram, that gives a clear view of the motions of the rising and falling areas of the sun’s surface.
“The sun’s surface looks like a big pot of boiling water. Everywhere you look, you see evidence of the convection below,” said Rhodes, a helioseismologist who studies the waves that wrack the sun. “The sun is oscillating, or vibrating, in as many as a million different modes, and we are trying to measure all of those modes.”
Every minute of every clear day, from half an hour after sunrise to half an hour before sunset, the camera methodically records another set of images. Although the process is largely automated, the equipment has to be turned on, monitored and turned off every day. Two undergraduate astronomy students, Jon Estay, a senior, and Bill Rudnisky, a junior, are key members of the astronomy team that also includes observers Andy Grubb, Shawn Irish and graduate student Perry Rose.
“We give the undergrads a lot of responsibility. They’ve had to learn quickly how to operate the equipment and do the data reduction,” Rhodes said.
Each day, the data from 700 or 800 pairs of high-resolution images are stored on digital audiotape cassettes. Each year, there are 200 to 300 clear days in which this can be done.
Rose, the physics graduate student, calculates that the project has generated about 7 terabytes of data since it began in 1984. A terabyte is a thousand gigabytes. A new personal computer today might come equipped with a 10-gigabyte hard drive to store data, so it would take 700 such computers just to store the project’s data.
Their data is first processed by Sun workstations at the tower, but it takes computers with fast parallel processors at USC and a Cray supercomputer at the Jet Propulsion Laboratory to do the real analysis.
“The dopplergrams show oscillations similar to the motion of earthquake waves,” Rhodes said. The waves are compression waves or acoustical waves that reflect off of boundaries caused by pressure and temperature differences within the sun. “It is like the sound that resonates in an organ pipe except that the waves in the sun are large. It wouldn’t be audible to people.”
Each of these resonating waves represents another oscillation mode. Rhodes said that from the ground he can detect about 700 oscillations and from space, 3,000 or more oscillations. Rhodes is also a guest investigator for NASA’s Solar and Heliospheric Observatory (SOHO), a spacecraft parked in an orbit that keeps it directly between the Earth and the sun as the Earth orbits the sun.
Space observations are not subject to atmospheric interference and can be continuous because there is no night in space. But Rhodes said NASA can’t afford to maintain the high-resolution tracking of the satellite necessary to gather data for more than two or three months per year.
“So we fill in gaps with ground-based data,” he said. “We are able to synchronize with SOHO so that we can capture images at exactly the same moments from both the satellite and the tower.”
Rhodes’ goal is to have data for a complete solar cycle, which is 11 years. Unraveling the complex solar oscillation patterns will someday enable astronomers to predict where and when sunspots will occur and how that will affect the Earth.
But back on Earth in the first gathering gloom of sunset, Rhodes is concerned with more earthly matters. He’s demonstrating the “Mount Wilson salute,” frantically waving his hands around his head in a futile effort to disperse a horde of deer flies.
“You thought astronomers only worked at night?” he said.