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.