When Shah Bahauddin decided what to research for his doctorate, he had no intention of getting bogged down in one of the most troublesome problems in astrophysics: why is the Sun’s distant atmosphere so hotter than the roaming surface?
His modest theme of choice was a small, short loop of sunlight, barely detectable given the large scheme of the Sun.
But size is not everything. As it turns out, astronomers have been looking for a tiny eruption like this for more than half a century.
Blinking right under the Sun’s super-hot corona, the explosion Bahauddin tripped over may well be the first complete sight of a solar “nanoflare” – from its abruptly bright beginning to its inevitable heartbreaking disappearance. And just as easily we could have missed him.
If subtle, transient curls like this are a common problem, it could help explain how the Sun’s corona became hundreds of times hotter than its visible surface – a mystery known as the coronary warming problem.
“I thought maybe the curls made the surrounding atmosphere a little hotter,” Bahauddin admits.
“I never thought it would produce so much energy that it could propel hot plasma to the crown and heat it.”
Observed loop lights. (Bahauddin et al., Nature Astronomy, 2020)
One billion times smaller than ordinary solar flares, nanoflars are incredibly difficult to observe and existed only theoretically, so researchers are still reluctant to call the discovery by this official name.
In theory, we have an idea of what a nanoflare should look like, but this is based on several hypotheses.
“No one really knows, because no one has seen it before,” says Bahauddin. “It’s a polite assumption, let’s say.”
Since astrophysicist Eugene Parker first proposed the idea of nanoflars in the 1970s, experts have been trying to figure out what these eruptions might actually look like.
If they really exist, they are almost impossible to see, they appear millions of times a second without our instruments ever noticing. Although our technology is getting better.
In 2017, for example, the best image of a nanoflare came from the absence of a larger one. An active region in the Sun, which hosted very few normal-sized missiles, showed a curious level of warming. Something unseen must have contributed to the energy in the atmosphere. A nanoflare suited the case.
From a technical point of view, in order to be considered a suitable nanoflare, an explosion of heat must be triggered by the Sun’s tangled magnetic fields, which are produced by the plasma bubbles that stir from below.
When these fields reconnect, they are thought to cause an explosive process – equivalent to about 10 billion tons of TNT. It energizes and accelerates the surrounding particles, and if all that activity is strong enough to heat the Sun’s corona, thousands of miles above, it’s called nanoflare.
(NASA / SDO / IRIS / Bahauddin)
Above: A close-up of one of the studied loop lights. Each inserted frame moves closer (from left to right), showing the subdued nanoflare.
Analyzing some of the most beautiful images of the Sun’s corona, taken from the Interface Region Imaging Spectrograph or NASA’s IRIS satellite, the new discovery ticks both boxes.
Not only was this little loop of light millions of degrees hotter than its surroundings, the way it erupted seemed curious.
“You have to examine whether the energy of a nanoflare can be dissipated in the corona,” explains Bahauddin.
“If the energy goes elsewhere, that doesn’t solve the problem of coronary warming.”
Looking at the data, it appeared that heavy elements, such as silicon, became much hotter and more energetic than lighter elements such as oxygen, which is the exact opposite of what you would expect.
Looking for a type of heat that could have a different impact on an oxygen atom than a silicon atom in this way, the researchers found only one match: a magnetic reconnection event.
In these complex chaotic circumstances, heavier ions have an advantage because they can plow through the multitude of lighter ions and steal all the energy, accumulating high heat in the process.
But that was just a hypothesis and it seemed like a long shot. The conditions necessary for this type of heating required only the correct proportion of silicon to oxygen. Could it really exist?
“So we looked back at the measurements and saw that the numbers matched exactly.” Bahauddin explains.
To the amazement of the team, it seemed that they had come across a real explanation for coronary warming. The next step was to see if it actually warms the crown.
Analyzing the data from the region just above the light loop, just before it lit up, the team discovered the last clue.
“And there it was, just a 20-second delay,” Bahauddin recalls. “I saw the glow and then I suddenly saw that the crown overheated to temperatures of millions of degrees.”
Already, the team has found nine other loops on the surface of the Sun that also show a similar transfer of energy to the corona.
Whether this localized heating is sufficient to explain the higher temperatures found in the Sun’s corona will depend on how many other loops astronomers can find.
If their frequency and locations are often and sufficiently widespread, these bursts of energy could at least partially answer the mystery surrounding coronary warming.
However, in all likelihood, astronomers believe that there are probably more invisible mechanisms in play. It’s probably not just something that warms the Sun’s atmosphere to such boiling temperatures, and many of the ideas we have now aren’t mutually exclusive.
Other theories include electromagnetic waves that wash out from the Sun, heating particles and allowing them to “navigate” in the outer atmosphere.
This little loop is just a small piece of the puzzle.
The study was published in Nature Astronomy.
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