When Shah Bahauddin was deciding what to research for his doctorate, he had no intention of getting involved in one of the most embarrassing problems in astrophysics: why is the solar distant atmosphere so hotter than it spins?
His humble theme of choice was a tiny and short loop of sunlight, barely noticeable given the large scheme of the Sun.
But size is not everything. Turns out astronomers have been looking for a tiny eruption like this for more than half a century.
Flashing just below the Sun’s hot corona, the explosion Bahauddin encountered could be the first complete insight into the solar “nanoflames” – from its sudden bright onset to the inevitable sizzling death. And we might as well miss it.
If subtle and transient loops like this are a common thing, it could explain how the Sun’s corona has become hundreds of times hotter than its visible surface – a mystery known as the problem of coronal warming.
“I thought maybe the loops made the surrounding atmosphere a little hot,” Bahauddin admits.
“I never thought it would create enough energy to actually be able to run hot plasma to the corona and heat it.”
A billion times smaller than conventional solar flares, nanowires are incredibly difficult to spot and only existed in theory, so researchers are still reluctant to call the discovery by that official name.
In theory, we have an idea of what nanoflares should look like, but this is based on several assumptions.
“No one really knows, because no one has seen it before,” Bahauddin says. “Let’s say it’s an educated assumption.”
Ever since astrophysicist Eugene Parker first proposed the idea of nanoflares, experts have been trying to figure out what these eruptions might look like in reality.
If they really exist, they are almost impossible to see, and they occur millions of times per second without our instruments ever noticing. Although our technology is getting better.
For example, in 2017, our best insight into nanoflora came from the absence of a larger one. The active region on the Sun, which hosted very few torches of normal size, showed an unusual level of heating. Something unseen obviously had to contribute to the energy of the atmosphere. The case corresponded to a nanoflare.
Technically, in order to be considered a suitable nanoflame, a heat shock must cause the tangled magnetic fields of the Sun, which are created from the bubbles of the swirling plasma below.
When these fields are reconnected, they are thought to cause an explosive process – the equivalent of about 10 billion tons of TNT. This energizes and accelerates the surrounding particles, and if all this activity is strong enough to heat the solar corona, thousands of kilometers above, it is called nanoflora.
Above: Close to one of the studied belt belts. Each insertion frame is further zoomed in (from left to right), showing the alleged nanoflora.
Analyzing some of the finest images of the Sun’s corona, taken from NASA’s Interface Region Imaging Spectrograph or IRIS satellite, the new discovery marks both of those boxes.
This little loop of light was not only millions of degrees hot from its surroundings, but the way it erupted also looked curious.
“You have to examine whether the energy from the nanoflora can dissipate in the corona,” Bahauddin explains.
“If the energy goes somewhere else, it doesn’t solve the coronal heating problem.”
Looking at the data, it turned out that heavy elements, like silicon, became much hotter and more energetic than lighter elements like oxygen, which is just the opposite of what you would expect.
Searching for the type of heat that could affect an oxygen atom differently from a silicon atom in just that way, the researchers found only one match: a magnetic reconnection event.
In these complex chaotic circumstances, heavier ions have the advantage, because they can plow a bunch of lighter ions and steal all the energy, while creating great heat in the process.
But that was just a hypothesis and it seemed like a long blow. The conditions required to achieve this type of heating required just the right ratio of silicon to oxygen. Can it really exist?
“So we looked back at the measurements and saw that the numbers matched exactly,” explains Bahauddin.
To the astonishment of the team, they seem to have come across a real explanation for crown heating. The next step was to see if it actually warmed the crown.
Analyzing data from the region just above the glowing loop, just before it erupted, the team discovered their final clue.
“And there it was, just a 20-second delay,” Bahauddin recalls. “We saw the lightening, and then we suddenly saw the corona overheat to multi-million temperatures.”
The team has already found nine other loops on the surface of the Sun that also show a similar transfer of energy to the corona.
Whether this localized warming is sufficient to explain the higher temperatures found on the solar corona will depend on how many other loops astronomers can find.
If their frequency and locations are frequent and widespread enough, these bursts of energy could at least partially answer the mystery surrounding coronary warming.
Still, astronomers believe there are probably more invisible mechanisms at play. It is probably not just one thing that heats the solar atmosphere to such high temperatures, and many of the ideas we have now are not mutually exclusive.
Other theories include electromagnetic waves that are washed out of the Sun, heating the particles and allowing them to “surf” toward the outside atmosphere.
This little loop is just one small piece of the puzzle.
The study was published in Astronomy of nature.