Sun Seekers

College scientists reveal a whole new face to the photosphere

Every day, the sun bathes the planet in a steady stream of heat and light. Or so most of us think. New research paints a very different portrait, revealing a sun plagued by ebbs and flows, sputters, spots, explosions, shaking and wobbling. The sun, it seems, is more dynamic and variable than anyone had guessed.

The work of three USC College professors of astronomy and physics—Edward Rhodes Jr., Darrell Judge and Werner Däppen—has been crucial to the development of the new vision of the sun. Perhaps most important, their work is providing new insights—and as many new questions—about how its wild ways impact the climate and lives of people on Earth.

The Inside Story

Werner Däppen considers physics his true calling. It just happens that the physics problems that most intrigue him occur at the very core of the sun.

The sun’s size and its enormous inward gravitational pull make the sun’s center an extremely hot and dense place. At temperatures of nearly 29 million degrees Fahrenheit, even the hydrogen and helium gases that make up the sun become 150 times as dense as water and eight times as dense as gold. The conditions are so intense that atoms regularly fuse in a nuclear reaction that powers the sun and, by extension, the entire solar system.

A theoretician, Däppen develops sophisticated computer models of the sun’s core. He has shown how the action of very small forces acting deep within the sun can lead to significant changes in the sun’s structure, and thus, its activity.

“We know like particles repel and opposites attract. But, if you’ve got a soup of negatively and positively charged particles, as we see inside the sun, what happens?” Däppen says. “We discovered that, added up, these very small pushes and pulls can change the overall density of the sun’s core by about 5 percent.” By taking this effect into account, Däppen has created more accurate models of the sun’s interior.

Däppen’s work is ultimately important to understanding the connection between sun and Earth. “What happens in the solar interior affects events in the outer layers and surface of the sun,” Rhodes says. “In turn, changes in solar surface dynamics lead to the kinds of changes in solar output that have come under intense scrutiny in terms of climate.”

Sun Spots, Quakes & Flares

Rhodes has the work habits most would associate with an astronomer, except that he works by the light of day. He does much of his research high atop a mountain, where he operates the 60-Foot Solar Tower of the Mt. Wilson Observatory. Rhodes studies the sun from space, too, as part of the joint NASA/ESA Solar and Heliospheric Observatory (SOHO) Mission.

“I try to understand what’s going on at the sun’s surface, and below that in the sun’s convection zone,” says Rhodes, who is also a part-time astronomer at Jet Propulsion Laboratory.

Rhodes studies the shaking and wobbling caused by solar “quakes” called solar oscillations, the cyclical appearance of magnetic storms that mar the sun’s fiery surface with darkened sun spots as well as the eruption of solar flares that spew electrically charged gases into space.

Since 1987, Rhodes’ group has collected high-resolution images from Mt. Wilson, including the only complete set of high-resolution images that record the last solar cycle from start to finish. The 11-year solar cycle impacts a number of the phenomena that most interest Rhodes. Sunspot activity is highest during the solar maximum and lowest at the minimum of the solar cycle. Counter-intuitively, the more spots, the greater the amount of light the sun emits.

The sun’s cycles, including the low sunspot activity that occurs at the solar minimum, might explain at least one period of unusual cold, often called the Little Ice Age, that hit northern Europe in the late 17th and early 18th centuries.

“The sun shone less powerfully over this time, and at the same time people were ice skating on the Thames in London,” Rhodes says. “We’re trying to help improve understanding of how the sun varies with time to see whether or not we can ultimately show exactly how the sun affects our climate,” says Rhodes, who is most known for his trailblazing research pinpointing the cause of solar oscillations and figuring out how the sun rotates below the surface.

Sunspots also have been linked with increases in solar flares and the electrical solar storms that can temporarily take out satellites and communication networks, threaten astronauts aboard the International Space Station and render Earth-based power systems lifeless, like the intense solar storm that took out Quebec’s power grid in 1989.

Many researchers are studying the solar cycle with an aim of better predicting damaging solar storms. Rhodes’ extensive data will be key to their studies.

Seeing the (Invisible) Light

Trained in the early days of the U.S. space program, Darrell Judge is a physicist at heart. “I enjoy figuring out how things work,” says Judge, who tinkers with old tractors in his spare time. At work, he turns his mechanical abilities to building state-of-the-art UV light detectors. His instruments are designed to stare, 24 hours a day, seven days a week, straight at the sun.

Right now, a detector designed by Judge and his colleagues is aboard the sun-orbiting spacecraft SOHO, measuring the sun’s emission of light in the "deep, deep blue region of the UV spectrum,” he says. The sunlight he studies, in the extreme ultraviolet (EUV), is invisible to the human eye. It’s also blocked by Earth’s atmosphere. EUV light represents a tiny fraction of the sun’s total output, but is incredibly important to life on Earth. Considered the most energetic portion of sunlight, EUV drives photochemical reactions in the planet’s upper atmosphere and creates a layer of charged particles called the ionosphere. The EUV light also heats the Earth’s thermosphere and in so doing influences climate and weather.

Until SOHO, EUV light had only been studied sporadically. SOHO’s position well above the planet allowed Judge to discover that the output of EUV light regularly fluctuates by factors of two to four, and even more during the solar maximum.

“It’s a steady stream, punctuated by highly variable solar eruptions,” says Judge, director of the USC Space Sciences Center.

To figure out how this affects Earth, Judge needs more detailed data about the total energy the sun emits in the EUV. He and colleagues at the University of Colorado are building a set of next-generation space-faring light detectors, to be carried on NASA’s 2007 solar mission, the Solar Dynamics Observatory (SDO).

Hopefully, the SDO mission will also help to answer key questions about the sun’s role in global climate change, he says. “Current climate models probably underestimate the influence of the sun’s changing output,” says Rhodes, who will also take part in the SDO mission.

Däppen adds: “What some of us are asking is, What if the sun is one of the culprits behind the warming of the Earth?” The work of all three scientists will contribute, directly or indirectly, to the answer.

“This is one of the big, open questions in solar science today,” says Däppen.