Closer to the Sun

It’s only everywhere. Plasma, that is. The fourth, less showy state of matter unless you count, our sun, the stars and lightening. 

And Michael Brown, Morris L. Clothier Professor of Physics, was awarded a National Science Foundation and Department of Energy grant in 2017 to help him continue to tease apart the turbulent characteristics of hot, magnetically-dynamic laboratory plasma.

 “The grant provides opportunities to use high-tech diagnostics to answer fundamental questions in plasma physics,” says Brown, standing in front of the Swarthmore Spheromak Experiment machine he designed and created over 20 years ago. One of the only instruments like it in the world, the SSX is rare. The chance for students to participate in such cutting-edge research is also unique. “The SSX lab is one of the few places in the U.S. where undergraduate physicists have access to astrophysically-relevant laboratory plasma," says Brown.

Studying the chaos of the solar system’s solar wind takes place in a large, neat laboratory lined with hundreds of tools and a glowing tube of plasma at center stage. Creating a lab version of the high velocity, turbulent stream of plasma, protons and electrons streaming from the Sun, means Brown must generate plasma turbulence identical to solar wind. While making the turbulence, Brown’s research team will simultaneously measure correlations of magnetic field and density, he says. 

“Normally when a physicist wants to study waves and oscillations in some medium, for example water waves or sound waves, they would disturb the medium in a lab at a fixed frequency (say with a tiny speaker in a gas column), then measure the wave velocity, the wave length. In nature, particularly in turbulent media, there are waves of many frequencies, speeds, and wavelengths all mixed together.”

The grant supports Brown, and his colleagues from Bryn Mawr College and Princeton University, in applying a technique until recently only used by satellites. In the satellite experiments, two properties of the turbulence are measured in the solar wind. Then researchers review that data to determine if, and how, the properties are linked. The plasma waves, called "slow waves" and "fast waves" have been studied before, says Brown, and have been observed by satellites in space turbulence.

“We want to see if they exist in our turbulence, too,” he says. “This would help us verify that our high-velocity plasma is very similar to that in the solar wind. In solar wind turbulence, the magnetic field and density are anti-correlated -- when the magnetic field goes up, the density drops,” says Brown. “This points to a particular kind of plasma wave. We’re interested in whether this phenomenon might be a universal aspect of plasma turbulence.”

The funding also will allow them to buy some diagnostics and do experiments both here at Swarthmore and at Bryn Mawr with Assistant Professor of Physics David Schaffner. In addition, Brown says, they have the simulation support of Jason TenBarge now at Princeton. Swarthmore physics majors are involved in all aspects of SSX experiments. 

“We have seniors working on senior theses for honors, and preparing presentations for national conferences,” he says. “This year we have three seniors coming to the American Physical Society conference in Milwaukee in October.” First year students help in the lab, too. 

“As it turns out, NASA’s Parker Solar Probe will be heading into the Sun, so for the first time, we will observe solar wind turbulence that's denser and hotter,” says Brown. “We should be able to predict what the Parker Solar Probe will see.”