science has solved the mystery of plasma rain

Although it may seem incredible, it rains in the Sun. But it is not a rain of water like the one we know on Earth. It’s a rain of incandescent plasmaa phenomenon that for decades has baffled scientists by not understanding it. Now, a team from the University of Hawaii has solved the mysteryand the answer is completely changing our way of understanding the atmosphere of our ‘reference’ star.

The discovery. Published in the prestigious magazine The Astrophysical Journal, not only explains why these spectacular plasma condensationsbut also gives us new tools to predict space weather that affects our technology here on Earth.

The mystery. The “solar storm“, or more technically coronal rain, occurs in the corona, the outermost and hottest layer of the Sun. There, masses of denser and relatively “cold” plasma condense and fall back towards the solar surface, creating bright arcs and loops. And although we talk about ‘cold’, the reality is that we are talking about tens of thousands of degreescompared to the millions of degrees in the surrounding plasma. Although for us it would be something unthinkable.

The big enigma was speed. Solar models predicted that this cooling and condensation process should take hours, or even days. However, observations showed that rain formed within minutes during solar flares. Something didn’t add up.

Now the problem has been located in the models that were used. And they assumed that the chemical composition of the corona was static and uniform, a simplification that has undoubtedly resulted in us calculating the phenomena that occur in our star much worse.

The key. The key breakthrough came when the researchers, led by graduate student Luke Fushimi Benavitz, decided to abandon that old assumption. They introduced into their simulations a factor that until now had been overlooked: the abundance of chemical elements varies in space and time without being static. And this is where physics gets very interesting.

The mechanism. The first thing that happens in this case is a solar flare that heats the chromosphere (the layer below the corona). This impulsive heating causes a large amount of plasma in the chromosphere to “evaporate” and rise at high speed towards the coronal loops. This ‘new’ plasma will have a composition similar to that of the photosphere, which is the surface we see of the Sun.

Once the plasma was already in the coronal loop, rich in materials such as iron or silicon, it is pushed and concentrated at the highest point of the arc, creating a ‘peak’ with these elements. One property of these elements is that they can radiate a lot of energy quickly and this causes the plasma to cool. And this sudden concentration at the apex of the loop acts as an ultra-powerful radiator, causing localized and very rapid cooling.

Finally, this sudden cooling causes a pressure drop. As a result, more plasma from the surrounding area is sucked into that area, increasing the density. The most interesting thing is that the higher the density, the cooling becomes even more efficient and a ‘thermal runaway’ occurs. As its name indicates, the temperature will plummet and the plasma will condense, forming rain.

The importance. For the first time, this model has done something that had not been achieved before: simulate the formation of rain on the Sun. And understanding it goes far beyond solving an old riddle, but it affects us completely.

Most importantly for us, it improves our ability to predict space weather. solar flares They can launch enormous amounts of energy and particles into space which, upon reaching Earth, can damage satellites, disrupt communications and overload electrical networks. More precise models of the Sun’s behavior allow us to better anticipate these events that until now gave us very little preparation time.

Rewriting. This discovery forces us to rewrite a fundamental part of solar physics. The idea that the composition of the solar atmosphere is dynamic and not static opens a large field of research ahead to understand exactly how energy moves through the star.

Images | Javier Miranda

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