Seth Putterman started from studying the behavior of plasma for reasons of national security. Extremely fast hypersonic missiles heat and ionize the surrounding air and form a cloud of charged particles called plasma, which absorbs radio waves and makes it difficult for operators on the ground to communicate with the missiles, a problem Putterman was trying to solve. Then it occurred to him that the same plasma physics applies to our sun.
A UCLA scientist and colleagues created what Patterman calls “our sun in a jar,” a 1.2-inch plasma-filled glass sphere that they used to simulate processes similar to those that cause solar flares. . These are explosive releases of energy, sometimes accompanied by the release of a high-speed plasma clot that can damage satellites in orbit and power grids on the ground. “The steps we are taking will impact modeling so that we can anticipate and identify space weather cues,” says Putterman, senior author of the study at Physical Review Letters describing their experiments.
The sun is essentially a swirling hell of plasma, made up of rotating electrically charged particles of gas—mostly electrons and hydrogen atoms stripped of their electrons. (Stellar plasma is slightly different from the low-density plasma used in tokamak fusion reactors.) Researchers have long tried to better understand solar flares, especially when a particularly large chunk of plasma is ejected toward Earth.
The team’s experiments began by placing some partially ionized sulfur dioxide inside a glass bulb and then bombarding it with low-frequency microwaves, similar to those used in a microwave oven, to excite the gas, heating it to about 5,000 degrees Fahrenheit. They found that pulsing microwaves at 30 kHz creates a sound wave that exerts pressure that causes the hot gas to compress. This sound wave pressure creates a kind of “acoustic gravity” and causes the fluid to move as if it were in the Sun’s spherical gravitational field. (The gravitational field of the experiment is about 1,000 times stronger than that of Earth.) This creates plasma convection, a process in which a warm liquid rises and a colder, denser liquid sinks towards the core of the glass ball. In doing so, the team became the first person on Earth to create something resembling the spherical convection normally seen inside a star.
Their project was first funded by DARPA, the Pentagon’s advanced research arm, because of its applications for hypersonic vehicles. He then received support from the Air Force Research Laboratory, as space weather can interfere with aircraft and spacecraft. But astronomers think it can also tell us something fundamental about the behavior of the sun. “I think it’s really important to start modeling solar convection in the lab and therefore gain insight into the Sun’s mysterious solar cycle,” says Tom Berger, executive director of the university’s Center for Space Weather Technology, Research and Education. Colorado in Boulder, which was not included in the study.
Berger is referring to a roughly 11-year cycle in which the Sun’s inner convection zone somehow becomes more active, causing the outer layer, or corona, to generate more frequent and intense flares and plasma ejections called coronal mass ejections. Exploring the interior of the sun is difficult, Berger says, although NASA is trying to do so with a spacecraft called the Solar Dynamics Observatory, which uses sound waves to map the sun’s surface and infers about the plasma below.
Other experts in the field also praise the study by Putterman and his colleagues, but note that it has limitations. “This is an exciting and innovative development. It’s cleverly done. It has always been challenging to model the internal dynamics of a star in the lab,” says Mark Misch, a researcher at the NOAA Space Weather Prediction Center and the University of Colorado.