When scientists need to learn about something, recreating it in the lab is often one of the best ways – and now that even applies to the Sun itself. Physicists from the University of Wisconsin-Madison have built a mini-Sun in the lab, and used it to probe the secrets of the real thing.
This isn't the first time scientists have tried to replicate the Sun on Earth. An artificial Sun in Germany focuses on producing Sun-like light, for experiments into producing solar fuels like hydrogen. Meanwhile, another so-called artificial Sun in China is being used to study nuclear fusion, in hopes of one day generating energy from it.
But the UW-Madison artificial Sun replicates different aspects again. Known as the Big Red Ball, the device is made up of a 3-m-wide (10-ft) hollow sphere, with a strong magnet in the middle. To fire it up, the team pumps in helium gas and ionizes it, creating a plasma. Then an electric current is applied that stirs the plasma up. The end result is a swirling, spinning ball of plasma interacting with electromagnetic fields, producing a pretty good approximation of the Sun itself.
The team set out to use the Big Red Ball to study the solar wind. This constant stream of charged particles is created as plasma is thrown out from the Sun's surface and escapes the magnetic field.
"The solar wind is highly variable, but there are essentially two types: fast and slow," says Ethan Peterson, lead author of the study. "Satellite missions have documented pretty well where the fast wind comes from, so we were trying to study specifically how the slow solar wind is generated and how it evolves as it travels toward Earth."
The researchers started with the Parker Spiral, the twisted structure of the magnetic field that emanates from the Sun and permeates the solar system. It's long been thought that close to the Sun, the magnetic field heads straight out. But at a certain distance – called the Alfvén surface – the magnetic field is dragged into a spiral shape. The Big Red Ball showed this was the case.
"Satellite measurements are pretty consistent with the Parker Spiral model, but only at one point at a time, so you'd never be able to make a simultaneous, large-scale map of it like we can in the lab." says Peterson. "Our experimental measurements confirm Parker's theory of how it is created by these plasma flows."
The Big Red Ball was also able to recreate the plasma burps that the Sun has been known to give off, allowing the team to identify what was causing them. In certain regions where the magnetic field was weaker, the fast-moving plasma was able to escape it, breaking off from the main ball and creating an ejection.
"These ejections are observed by satellites, but no one knows what drives them," says Peterson. "We ended up seeing very similar burps in our experiment, and identified how they develop."
The research was published in the journal Nature Physics.
Source: University of Wisconsin-Madison