Sun’s solar wind recreated in lab with aid of Big Red Ball

The solar wind is made mostly of pure awesome. It is an always-changing, poorly predictable flow of charged particles from the Sun: a giant exhalation right into our faces. It’s responsible for the auroras, which it produces in partnership with the Earth’s magnetic fields. The solar wind has also given rise to possibly the coolest job description on Earth: space meteorologist.

But data on the solar wind is not so easy to come by. Yes, we can always observe the charged particles that hit our world’s magnetic field, but for a more global view, we need to use satellite data—and satellites don’t come cheap. It would be nice if we could recreate the solar wind in the laboratory. And that is exactly what a group of physicists have done, using a machine called the “Big Red Ball.”

A closer look at the solar wind

You’d think we understand the solar wind pretty well given that its existence was predicted before it was observed. But it’s a complex system, and predicting its existence has not made it any easier to predict its behavior. Why is it so complicated?

The wind exists because the Sun has a powerful magnetic field, which forms giant nested loops that begin and end at the solar poles. The further away from the Sun these loops get, the weaker they become, which makes them easier to break. And the Sun itself helps break them. It more or less continuously ejects a soup of charged particles, or plasma. That plasma builds up and eventually gets strong enough to prevent the magnetic field lines of the loops from closing. These broken lines no longer begin or end at the solar poles—instead they extend out through the Solar System, never ending. 

Then you have to add solar rotation: the Sun is spinning like a top that drags the magnetic field with it. For the unbroken lines, which still run pole-to-pole, this doesn’t matter much. However, the broken lines twist up to form a spiral that trails out into the Solar System. The shape of these field lines give the solar wind its characteristic properties.

Now back to the plasma that’s being ejected by the Sun. Remember that the plasma puts pressure on the magnetic field lines that eventually breaks them. This is a violent event that happens at unpredictable moments. That all makes for a very unpredictable system, which in turn makes space weather interesting. It’s also why a laboratory model of this complex process would be so useful: it would let scientists validate their computational models against a denser dataset than can be obtained by observations of the Sun.

My, what a big red ball you have

This is where the Big Red Ball comes in. It is essentially a vacuum chamber surrounded by magnets that allow the researchers to confine a big ball of plasma. However, the fields generated by the BRB’s magnets are designed to be quite weak in the central volume (where experiments are done) so that the plasma is essentially free to generate its own behavior. 

To model the solar wind, the researchers placed a bar magnet at the center of the ball and started it rotating. On that level, the experiment is really simple. But the BRB is a sophisticated machine for characterizing plasmas and magnetic fields, so it needs to be able to observe complicated behavior.

And that’s what it did. Researchers showed that the miniature solar wind produced by their model has all the main characteristics of the real thing. There is a boundary where the field lines aren’t closed anymore. The plasma within the central region “puffs” out by breaking field lines and does so at unpredictable moments in time. The field lines spiral outward just like they do in the Solar System.

But the model is not perfect. A plasma consists of both electrons and positively charged ions. The electrons experience nearly the same conditions as they would in space, but that’s not true for the ions. In space, the ions will almost never collide with each other, while in the Big Red Ball they are constantly colliding. This has two effects: first, it demagnetizes the ion part of the plasma. Each ion is a tiny magnet and given time will align (or anti-align) itself to the Sun’s magnetic field. In the Big Red Ball, the constant collisions disrupt that alignment. That means the plasma will behave a bit differently in the model than it does out in space.

We also suspect the collisions will homogenize the plasma. In space, with no collisions, hot ions stay hot and don’t transfer their energy to other ions. However, in the Big Red Ball, that energy will be spread around fairly quickly. Even with that limitation, however, it seems likely that the model will be a highly useful tool for helping space meteorologists get a better handle on predicting space weather.

Nature Physics, 2019, DOI: 10.1038/s41567-019-0592-7 (About DOIs)

https://arstechnica.com/?p=1556131