Big Red Ball gives clues to the sun’s wind
US researchers gain insight into the largest magnetic field in the solar system. Barry Keily reports.
A three-metre diameter artificial sun has been used to model the massive and spiralling magnetic field that emanates from the sun, taking researchers one step closer to understanding the behaviour of solar wind.
Solar wind comprises a stream of ionised plasma and particles that spread out from the sun’s surface at speeds of up to 800 kilometres a second.
As the sun turns on its axis, plasma at its core spins, generating a magnetic field that suffuses the entire solar system. At its genesis, the field radiates in straight lines. However, after travelling a certain distance it reaches a region known as the Alfvén surface – the area in which rotation starts to deform the lines of force, and which is considered the definitive boundary between the sun and interplanetary space.
The result of the deformation is a complex spiral shaped magnetic field, known as the Parker spiral, which suffuses the entire solar system.
This Alfvén surface is assumed to be highly dynamic but has proven thus far difficult to study in detail.
To at least partially ameliorate the situation, physicists led by Ethan Peterson from the University of Wisconsin-Madison in the US opted to create a scale (and simplified) model of the sun using a piece of kit owned by the university.
It is known formally as a rapidly rotating plasma magnetosphere, but more commonly as the Big Red Ball.
The machine is essentially a giant sphere, three metres in diameter, with a powerful magnet at its centre. Peterson and colleagues filled it with helium, which they then ionised to create plasma, ran an electric current through it and rotated it rapidly to create a magnetic field.
With the aid of various probes inside the apparatus, the researchers were able to observe motions within the plasma that were in close agreement to measurements taken by satellites, confirming that they had created an accurate, although simplified, model of the sun.
The model allowed observations of the interaction between the magnetic field and plasma flow in three-dimensions – a contrast to existing satellite data which essentially measures a single location at a single point in time.
Importantly, the lab model revealed points at which the plasma was moving fast enough, and the magnetic field was weak enough, that it could break off and eject – corresponding to the observed phenomenon known as “solar burps”, which drive slow-moving solar winds.
“These ejections are observed by satellites, but no one knows what drives them,” Peterson says. “We ended up seeing very similar burps in our experiment and identified how they develop.”
The researchers say that their findings, published in the journal Nature Physics, will be useful for refining the interpretation of data sent back by NASA’s Parker Solar Probe, which was built specifically to measure the solar magnetic field. It was launched in August 2018.
- DOI: 10.1038/s41567-019-0592-7