From Berkeley’s Science Matters: Probing the Solar Plasma by Eng-Tips

 

The sun is an orb of shimmering energy. It pours out life-giving light and withering heat, but also an invisible torrent of charged particles called the solar wind. Made of plasma—gases heated until their atoms disintegrate into electrons and ions—the solar wind blasts past every planet in the solar system at supersonic speeds.

“From just outside earth’s atmosphere all the way to the part of the sun you see during the day, is all plasma,” says Stuart Bale. A Berkeley professor of physics and director of the university’s Space Sciences Laboratory, Bale studies the plasmas that stream from stars and suffuse entire galaxies.

Of the countless sources of plasma in the universe, the sun is the nearest and easiest to observe. That makes the solar atmosphere “a great laboratory for studying plasma physics,” Bale says.

Bale hopes to get his best view of solar plasma to date on NASA’s Solar Probe Plus mission. Scheduled to launch in 2018, the unmanned spacecraft will hurtle into the hellish environment of the sun more than 20 times over six years, flying closer to the star’s surface with every pass.

Bale has submitted a proposal for an experiment that will study the acceleration and heating of the solar wind. The wind originates as plasma evaporating from the sun’s relatively cool 5,000 to 6,000 degree Celsius surface. As it expands and rises into the corona, the radiant solar “atmosphere” only visible during a total solar eclipse, this plasma grows hotter and faster until it is roaring through the solar system at more than a million kilometers per hour.

The process can be likened to the physics of a rocket engine. The fuel within a rocket gets ignited within one chamber of an hourglass-shaped nozzle. The heated gas expands with constant force. But as it passes through the constriction of the hourglass, the change in volume increases its pressure enough to accelerate the gas to supersonic speeds. On the sun, Bale explains, “the rocket nozzle is not spatial. It’s the gravity of the sun that’s holding back the atmosphere at the same time as the gas pressure is trying to push outward. This forms a kind of rocket engine that the solar wind expands through.”

Yet how the plasma of the corona achieves such a blistering velocity, and continues to increase in temperature and acceleration millions of kilometers after leaving the sun, has scientists scratching their heads. “We’re looking for a way to keep the solar wind hot as it expands,” Bale says.

On earth, that kind of heat transfer can be observed in water waves in a bathtub. The larger waves decay into smaller and smaller eddies until the friction of one molecule rubbing against another becomes an important factor. The energy of the waves is then transferred into heat. On the sun, magnetic fields are shaken into waves by the rotation of the sun and larger scale magnetic features. But one plasma particle cannot rub against another to dissipate the extra wave energy. Solar plasma is collisionless—one particle isn’t likely to encounter another within the 150 million kilometer distance between the sun and the earth.

“There has to be some collisionless process that’s heating the plasma. There has to be some place for that energy cascading into smaller and smaller scales to go,” Bale says. To observe that transfer of heat, Bale has proposed to measure the electromagnetic fields in the corona and the temperature at the same time. In this way, he can link the behavior of the fields to the transfer of heat.

Though the theory seems to fit, details of the process aren’t known. The information gleaned from the experiment will then help characterize the physics of black hole accretion disks, galaxy clusters, and other stars. “Heating coronas is a universal astrophysical problem,” Bale says.

The rigors of space travel will impose strict limits on the experiment’s design. “The power available for all of the experiments is about the same as for a lightbulb,” Bale says. “Once you put it into space, you can’t get it back, so it must be autonomous. You have to think through all possible failures, and use parts and materials that are well characterized and resistant to radiation, that will work after being shaken up on a rocket, heating up, and cooling down again.”

The Space Sciences Laboratory has a long history of developing just such independent instruments, having launched more than 75 into space over its fifty-one year history. In fact, more than a dozen are still operating in space. Bale’s experiment may be sailing among them in the near future, en route to a rendezvous with the sun.

 

by Kathleen M. Wong

Related Web Sites

• Stuart D. Bale, Research Interests, Department of Physics
• Stuart Bale, Department of Physics
• Space Sciences Laboratory, UC Berkeley
• Solar Probe Plus—NASA

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