When Shah Bahauddin decided what to research for his PhD, he had no intention of getting involved in one of the most irritating problems in astrophysics: why is the sun’s distant atmosphere so much hotter than its swirling surface?
His modest subject of choice was a small and short loop of sunlight, barely discernible given the sun’s large schedule.
But size isn’t everything. It turns out that astronomers had been looking for a small eruption like this one for more than half a century.
The explosion Bahauddin encountered, flickering just below the sun’s super-hot corona, could very well be the first full glimpse of a solar-powered ‘nanoflare’ – from its sudden bright onset to its inevitable blistering demise. And we might as well have missed it.
If subtle and fleeting loops like these are common, it could help explain how the Sun’s corona got hundreds of times hotter than the visible surface – a mystery known as the coronal heating problem.
“I thought maybe the loops made the surrounding atmosphere a bit hotter,” admits Bahauddin.
“I never thought it would generate so much energy that it would push really hot plasma towards the corona and heat it up.”
Loop clarifications observed. (Bahauddin et al., Nature Astronomy, 2020)
Nanoflares, a billion times smaller than regular solar flares, are incredibly difficult to spot and have only existed in theory, so the researchers are still hesitant to call the discovery by that official name.
In theory, we have an idea of what a nanoflare should look like, but that’s based on several assumptions.
“No one really knows, because no one has seen it before,” said Bahauddin. “It’s an educated guess, say.”
Ever since astrophysicist Eugene Parker first proposed the idea of nanoflares in the 1970s, experts have been trying to figure out what these eruptions might look like in reality.
If they really exist, they are nearly impossible to see and occur millions of times per second without our instruments ever noticing. Although our technology keeps getting better.
For example, in 2017, our best glimpse of a nanoflare came from the absence of a larger one. An active region in the sun, with very few normal-sized flares, showed a curious level of heating. Something invisible clearly had to contribute energy to the atmosphere. A nanoflare suited the case.
Technically, to be considered a true nanoflare, an explosion of heat must be caused by the sun’s tangled magnetic fields, which are produced by bubbles from swirling plasma beneath.
When these fields reconnect, they are believed to cause an explosive process – equivalent to about 10 billion tons of TNT. This activates and accelerates surrounding particles, and when all that activity is strong enough to heat the Sun’s corona thousands of miles up, it’s called a nanoflare.
(NASA / SDO / IRIS / Bahauddin)
Above: A close-up of one of the loop clears studied. Each inset box zooms in further (from left to right) and shows the putative nanoflare.
When analyzing some of the most beautiful images of the Sun’s corona, taken with NASA’s Interface Region Imaging Spectrograph, or IRIS satellite, the new discovery ticks both boxes.
Not only was this small loop of light millions of degrees hotter than its surroundings, the eruption also seemed curious.
“You have to investigate whether the energy of a nanoflare can be dissipated in the corona,” explains Bahauddin.
“If the energy goes elsewhere, it won’t solve the coronal heating problem.”
Looking at the data, it turned out 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.
In search of a kind of heat that could thus affect an oxygen atom differently than a silicon atom, researchers found only one similarity: a magnetic reconnection.
Under these complex chaotic conditions, heavier ions have an advantage, as they can plow through the mass of lighter ions and steal all the energy, generating a lot of heat in the process.
But that was just a hypothesis, and it seemed like a long shot. The conditions required to achieve this type of heating required just the right silicon to oxygen ratio. Could that really exist?
“So we looked back at the measurements and saw that the numbers matched exactly,” Bahauddin explains.
To the surprise of the team, it turned out that they had stumbled upon a real explanation for coronal warming. The next step was to see if it actually heats the corona.
Analyzing data from the region directly above the bright loop just before it flared up, the team uncovered their final clue.
“And there it was, only a 20 second delay,” Bahauddin recalls. “We saw the clarification, and then we suddenly saw that the corona was overheating to temperatures of several million degrees.”
The team has already found nine other loops on the sun’s surface that also show a similar transfer of energy to the corona.
Whether this local warming is enough to explain the higher temperatures in the Sun’s corona depends 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 coronal warming.
But astronomers probably think there are probably several invisible mechanisms at play. It’s probably not one thing that warms the sun’s atmosphere to such sweltering temperatures, and many of the ideas we have now aren’t mutually exclusive.
Other theories include electromagnetic waves that wash out from the sun, heat particles and let them “surf” to the outer atmosphere.
This little loop is just a little piece of the puzzle.
The study is published in Nature astronomy.
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