Within Earth’s atmosphere, temperatures drop with increasing altitude. On the Sun however, the reverse occurs: its outermost layer, the corona, is nearly 500 times hotter than the underlying layer, the photosphere. To explain this cosmic conundrum, astronomers have long theorized that some unknown mechanism must be vigorously heating the corona.
One possible mechanism is nanoflares: tiny explosions on the solar surface that randomly occur and rapidly dissipate. But while researchers have proposed the existence of nanoflares for some time, it’s been difficult to detect them with current technology.
But now, in a study published June 1 in The Astrophysical Journal Letters, researchers from the National Centre for Radio Astrophysics (NCRA) in India have discovered fresh evidence for nanoflares. They spotted the weakest radio emissions detected yet from our star on a day when the Sun was not flaring — hinting that random, ubiquitous nanoflares indeed exist, and they could explain why the corona is so hot.
Observing the quiet Sun
The team made the detection by studying the Sun with a radio telescope called the Murchison Widefield Array (MWA). Once they were sure that there were no powerful solar flares occurring on either the near or far side of the Sun, they took 70 minutes of data at four distinct frequencies.
During this time, the array captured weak emissions on the quiet Sun — the first observational radio evidence of nanoflares.
Looking at the inactive Sun is important because the corona’s constantly high temperature suggests that the extreme heating is not due to larger intermittent flares, which are the type usually studied by astronomers.
“Powerful flares, although [they] dump a significant amount of energy,” occur less frequently and are associated with localized regions on the Sun, where its magnetic field is strong, Surajit Mondal, the lead author of the study, tells Astronomy. “However, since the corona is always hot, a mechanism is necessary which shall continuously dump energy into the corona and is present everywhere.”
And nanoflares perfectly fit the bill.
In 2017, scientists using the Focusing Optics X-ray Solar Imager (FOXSI) published a paper in Nature Astronomy presenting evidence that nanoflares might be the cause of the Sun’s high coronal temperature. In their study, the FOXI team had observed an extremely hot region of the corona that wasn’t associated with any normal, full-sized flares. They posited that instead, nanoflares had heated the region they observed to more than 10 million kelvins.
Mondal and his team saw emissions similar to those seen in the 2017 study, but at a time when the entire Sun was quiet, increasing the likelihood that nanoflares are the best explanation for the hot corona.
“These are exciting results that add to the growing evidence that nanoflares play an important role in heating the solar corona to its multi-million degree temperatures,” James Klimchuk, an astrophysicist at the NASA Goddard Space Flight Center who is not involved in the NCRA research, tells Astronomy.
Strength in numbers
Compared to the powerful solar flares you’re familiar with, nanoflares are weak. They occur suddenly and fade quickly. So, how could they possibly be responsible for heating — much less maintaining — the Sun’s coronal temperature of several million kelvins?
The answer, as Klimchuk explained in a 2015 press release, lies in the large number of nanoflares that occur on the solar surface. “Although puny by solar standards, each one is the equivalent of a 50-megaton hydrogen bomb, the largest ever detonated on Earth. Millions of nanoflares occur every second across the Sun, and together they pack a real wallop,” he said.
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So, although each nanoflare by itself is weak, their ubiquitous presence on the solar surface could explain the high coronal temperature.
Mondal and his team observed this: In 70 minutes of data, they detected more than 81,000 events. But while their research serves as clear evidence for the abundance of nanoflares even in areas with low magnetic field strength, exactly how nanoflares generate the high coronal temperature is still unknown.
The road ahead
“The novelty of our work lies in the fact that we, for the first time, show that impulsive emissions play a significant role in heating the quiet solar corona, and also characterize the nature of these emissions,” Mondal says. But since the work is based only on a single observation, Mondal notes that multiple observations carried out at different times and during varying levels of solar activity are necessary to determine whether nanoflares are indeed the mechanism behind coronal heating. Additionally, “There are definitely weaker emissions which we have not detected,” Mondal says.
Engulfed in the Sun’s brilliance, the corona reflects little visible sunlight and is barely visible to telescopes near Earth, a whopping 94 million miles (150 million kilometers) away. While observing the dim nanoflares is hard enough, the task is made harder with limited telescope sensitivity. Missions dedicated to studying the Sun’s corona up close, such as NASA’s ongoing Parker Solar Probe and the Indian Space Research Organisation’s upcoming Aditya-1 will expand current knowledge of exactly how the corona is heated.
And this knowledge is important for more than simply understanding how the Sun works.
“If nanoflares are responsible for heating the corona, as seems to be the case, then they are [also] the source of ultraviolet and X-ray radiation [from the Sun] that is absorbed by the upper atmosphere of the Earth,” Klimchuk says. That radiation affects the properties of the upper atmosphere, which in turn influences the amount of drag experienced by satellites and space debris. So, the rate of nanoflares occurring on the solar surface could ultimately affect the efforts made by satellite controllers to avoid collisions in space.