Researchers at UCL have studied the behaviour of the Sun's coronal mass ejections, explaining for the first time the details of how these huge eruptions² behave as they fall back onto the Sun's surface. In the process, they have discovered that coronal mass ejections have a surprising twin in the depths of space: the tendrils（卷须） of gas in the Crab³ Nebula⁴, which lie 6500 light-years away and are millions of times larger. On 7 June 2011, the biggest ejection of material ever observed erupted from the surface of the Sun. Over the days that followed, the plasma⁵ belched⁶ out by the Sun made its way out into space. But most of the material propelled up from the Sun's surface quickly fell back towards our star's surface.

 

For the solar physicists⁷ at UCL's Mullard Space Science Laboratory, watching these solar fireworks was a unique opportunity to study how solar plasma behaves.

 

"We've known for a long time that the Sun has a magnetic field, like the Earth does. But in places it's far too weak for us to measure, unless we have something falling through it. The blobs of plasma that rained down from this beautiful explosion were the gift we'd been waiting for," says David Williams, one of the study's authors.

 

Since 2010, the NASA Solar Dynamics⁸ Observatory⁹ (SDO) has been constantly photographing the surface of the Sun. To our eyes, our star seems almost unchanging, with occasional fleeting¹⁰ sunspots the only changes that can be seen without special apparatus¹¹. But the SDO's instruments can cut through the dazzling brightness, magnify the detail and see wavelengths¹² of light which are blocked by the Earth's atmosphere. This combination of high-quality imaging and constant monitoring means that scientists can now see the detail of how the Sun's dynamic surface changes over time.

 

The 7 June 2011 eruption¹ was by some margin¹³ the biggest recorded since this constant monitoring began, meaning the huge cascade¹⁴ of matter that fell back into the Sun following the eruption was a unique opportunity to study, on an unusually large scale, the fluid dynamics of these phenomena¹⁵.

 

"We noticed that the shape of the plume¹⁶ of plasma was quite particular," says Jack¹⁷ Carlyle, lead author of the study. "As it fell into the Sun, it repeatedly split apart like drops of ink falling through water, with fingers of material branching out. It didn't stick together. It's a great example of an effect where light and heavy fluids mix."

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