The Mystery of Rain on The Sun Can Finally Be Explained

It rains on the Sun, the gigantic thermonuclear orb that burns with the multi-million-degree 'fires' of fusion.

This rain is made of superheated plasma, and researchers might have discovered its secret: quickly shifting flows of elements such as iron, silicon, and magnesium.

In a poetic, scientific twist, these new findings come courtesy of researchers at the Institute for Astronomy (IfA) at the University of Hawai'i, from the chain of sunny, volcano-forged islands that have their own distinct rainfall patterns.

Related: Stunning Images Reveal The Sun's Surface in Unprecedented Detail

But does it really rain on the Sun? Yes, and no. There are some similarities to rain on Earth, in that coronal rain consists of cool, dense blobs falling from the Sun's corona, the outermost layer of its atmosphere, down to its surface.

However, rain on the Sun is made of plasma, an electrically charged, million-degree gas. As this coronal rain falls, it reveals another generally invisible aspect of the Sun: magnetism. Since plasma is electrically charged, it traces the Sun's magnetic fields and loops, forming giant streaming arcs as it falls.

Such arcs can grow as tall as five Earths stacked atop one another – no word from NASA on how many turtles that is.

We don't know exactly how this solar rain forms. It's often observed in the aftermath of violent solar flares, and downfalls have been linked to the impulsive injection of heat that gives rise to coronal loops. Despite intense study, coronal rain remains mysterious and is difficult to model or predict.

Now, researchers reveal that it could depend on material flows driven by the uneven distribution of elements within the Sun's corona. This discovery counters assumptions baked into previous simulations of the solar atmosphere that these elemental abundances were relatively fixed.

"At present, models assume that the distribution of various elements in the corona is constant throughout space and time, which clearly isn't the case," says Luke Benavitz, an astronomy graduate student at IfA and one of the study's co-authors.

In their simulations, which allowed for variations in the distribution of elements in the Sun's corona, Benavitz and colleagues found that coronal rains started condensing after just 35 minutes, whereas earlier models required hours or days of heating to explain coronal rain.

"It's exciting to see that when we allow elements like iron to change with time, the models finally match what we actually observe on the Sun. It makes the physics come alive in a way that feels real," says Benavitz.

Other mechanisms are likely involved, but the researchers think these shifting elemental abundances influence radiative energy loss, where spikes in radiation cause the temperature to plummet at the peak of coronal loops compared to elsewhere in the Sun's aura. This sucks more material up through the loop and triggers a runaway cooling effect, which results in coronal rain.

Shifting elemental abundances "are critical to understanding the cooling of plasma in the Sun's atmosphere and, as we have shown, can directly cause coronal rain," the team concludes in their paper.

"This discovery matters because it helps us understand how the Sun really works," adds Jeffrey Reep, IfA astronomer and study co-author.

Not only does this study reveal the intricacies of solar rain, an oft-observed but enigmatic phenomenon, but it also suggests that by extension, there may be more to coronal heating than we've appreciated.

"We might need to go back to the drawing board on coronal heating, so there's a lot of new and exciting work to be done," says Reep.

This research is published in The Astrophysical Journal.