They're invisible to the naked eye and can be seen only through telescopes — but the Sun is covered in millions of tendrils of gas that constantly spurt from its surface.

Key facts: "Spicules" replicated in computer model of the Sun

"Spicules" replicated in computer model of the Sun Movement of magnetic fields could play key part in their formation

Movement of magnetic fields could play key part in their formation Could help us better understand solar wind

Since their discovery in the 19th century, the mechanism that creates these "spicules" has been a mystery.

Now a team of scientists, using a sophisticated simulation of parts of the Sun's atmosphere, believe they've cracked the case.

The secret appears to be a release of energy caused by "knotted" magnetic fields rapidly straightening out in the Sun's atmosphere.

Spiky solar hairdo

"It's like the Sun has hair — it's pretty short hair too. The whole edge of the Sun is full of these spiky features. And if you see movies of them they're continuously on the move," said Bart De Pontieu, an author on today's Science paper and IRIS science lead at Lockheed Martin.

According to Dr De Pontieu, about 20 different models have been proposed to explain how the spicules form.

"But what we've done now is very different. Not only is it a different idea, but we have a realistic simulation."

The scientists' model uses supercomputers to create a "Sun in a box" that calculates the minute-to-minute changes in the features of the Sun.

And they have found that the model closely matches what's actually happening in the Sun's chromosphere — a layer of the Sun's atmosphere sandwiched between the inner and outer layers.

"You can go and compare the synthetic models to the observable, and that's where we found a really good match. That's not something anyone's been able to do before," Dr De Pontieu said.

What is a spicule?

Spicules are geysers of gas spewing from the surface of the Sun and into the atmosphere at speeds of up to 100km/s.

They can reach a height of ten thousand kilometres before they collapse and fall back down. And though they're exceptionally tall, most spicules are only a few hundred kilometres wide.

They form and collapse in a matter of minutes, and it's estimated that there are millions of spicules on the surface of the Sun at any one time. Their tall, thin profile makes the Sun appear fuzzy and hairy when viewed with powerful telescopes, Dr De Pontieu said.

Knotted magnetic fields

The research team's model suggests that the untangling of magnetic fields as they move through the Sun's atmosphere is responsible for the formation of spicules.

Energy moving through the Sun's interior, like bubbles in boiling water, "tangles" magnetic fields that exist in the same space, Dr De Pontieu said.

And when those magnetic fields scatter in the chromosphere, they release their tension like a snapped rubber band.

"When you release these knots in the magnetic field all of a sudden, the magnetic field straightens, and it's kind of like a whiplash effect — you actually accelerate gas that way," Dr De Pontieu said.

Electrical currents generated by the tangled magnetic fields also dissipate, heating the gas at the same time as it speeds up.

Secrets of the solar wind

Scientists are interested in studying spicules because it's believed they may play a part in two enduring puzzles posed by the Sun.

The first is why the Sun's outer atmosphere is so much hotter than its surface — about 100 times hotter, with a temperature that can reach millions of degrees Celsius.

Dr De Pontieu said spicules may be partly responsible for that mysterious heat, depending on how much energy they dump into the Sun's atmosphere.

"Now that we know how spicules work, we're working on a follow-up that suggests they do have an impact on the corona. We're working on quantifying that.

The researchers found observational data (like NASA's IRIS spectrograph data in the top image) closely matched their modelling (the bottom image) ( Supplied: NASA Iris Spectrograph/Bitfrost Code - University of Oslo )

The scientists' model has also found that when spicules form, magnetic waves known as Alfvén waves are a byproduct.

Dr De Pontieu said there's a strong suspicion Alfvén waves are responsible for accelerating the streams of charged particles that make up solar winds.

"We've found now that the spicules are intimately connected to these waves. We're not saying that's the only source of Alfvén waves, but it's an important source."

The next step in the research, he added, was simulating the Sun in full 3D.

Currently, the team's models analyse the chromosphere in 2.5D — meaning the model is simulating a slice of the sun's atmosphere in 2D and includes the effects of magnetic and electric fields in the third dimension.

"[Simulating in 3D] takes time, at least another year. That means there's some questions about whether it will work in exactly the same way in 3D. But we believe the mechanism will still produce spicules," he said.

A 'major step forward'

Australian experts say the research is highly significant in furthering our understanding of spicules.

Professor Iver Cairns from the University of Sydney said the inclusion of "ambipolar diffusion" — the mechanism by which the research suggests magnetic fields diffuse upwards through the chromosphere — was a big change.

"They've included that for what seems like the very first time, and it makes a really big difference," Professor Cairns said.

"[This paper] does have what seems to be a major step forward in the understanding of spicules and their consequences for understanding solar physics."

Dr Alina Donea from Monash University said the modelling done by the research team was "impressive".

She said some scientists were likely to be surprised by the proposed explanation for spicules.

"In the solar physics community … one group has a particular theory, another group has another theory. So there will definitely be some discussion about how this changes what we know about spicules."