The temperature of the Sun’s surface is 5500º C. But the solar corona, which lies about 2,000 km above the surface, rages at a few million degrees celsius. We don’t know why.
Sarah Iqbal is a senior research fellow at the department of biochemistry, Aligarh Muslim University, India.
The Sun’s atmosphere is hotter than its surface, and physicists have been baffled by this for seven decades. Nuclear fusion occurring at the Sun’s core produce all the star’s heat and light. So scientists naturally assumed that the surface closer to the core would be hotter. However, while the temperature of the photosphere – the starstuff that sits around the core – measures 5500º C, the solar corona, which lies about 2,000 km above the surface, rages at few million degrees celsius.
“This is against the rules of physics,” Shin-nosuke Ishikawa, an astrophysicist at the Japanese Aerospace Exploration Agency (JAXA) told The Wire. “When you move away from a bonfire, the temperature drops. This is true for any heat source. But we don’t see this on the Sun.”
The solar corona heating problem is among the oldest fundamental paradoxes in all of physics. It was discovered in the 1940s, and since then many theories have been proposed to make sense of the anomalous temperature readings. But the answer has eluded us to this day.
Tiny explosions in the corona
Hopeful about a method to this madness, Ishikawa and his colleagues at JAXA teamed up with scientists from NASA in the US. In an experiment that used readings from an X-ray instrument onboard a research rocket, they detected multiple low-energy explosions in the corona. And they think these explosions might be able to help explain why the corona is so much hotter than it should be.
“Things work differently on the Sun because it has a strong magnetic field,” Ishikawa explained. Recall high-school physics: if two magnets are placed next to each other, their magnetic field lines won’t intersect. Instead, they will merge and flow as if there is one bigger field. But in the Sun’s atmosphere, there are many magnetic fields and they’re not equally strong. Sometimes, when some of their field lines try to merge, they will do so violently, releasing a large amount of magnetic energy.
Such magnetic reconnections fuel large explosions called solar flares. Using data from their X-ray instrument, Ishikawa & co. say they have reason to believe similar explosions occur in the corona but at a much smaller scale, producing ‘nanoflares’. And these nanoflares could be responsible for heating the corona to millions of degrees.
This idea had in fact first been proposed by Eugene Parker, the noted American solar astrophysicist, in the 1980s. Parker reasoned that turbulence in the Sun’s atmosphere could disturb the magnetic field and the subsequent reconnection would release bursts of energy called nanoflares.
The JAXA/NASA researchers followed up on their observations using complex computer models. These simulations showed that nanoflares could generate small pools of extremely hot plasma in the corona, whose heat becomes redistributed through the corona in time.
However, there is a roadblock: to maintain the corona at a few million degrees, nanoflares should be frequent. Observations from the X-ray instrument as well as other instruments don’t hold this up.
One reason for this could be the scale of the explosions. Nanoflares are minuscule in terms of their strength, about a billion-times weaker than the flares that disrupt satellite communications around Earth. “Such small disturbances are rarely picked up by the current gamut of tools deployed in space,” Dipankar Banerjee, a scientist at the Indian Institute of Astrophysics, Bengaluru, told The Wire. “Detecting such faint traces of heat across a vast background of cooler and brighter emission is not easy.”
The FOXSI-2 sounding rocket
Unfazed by this holdup, the researchers from JAXA and NASA tried a different approach. They reasoned that any change – however small – should have an accompanying collateral effect. For instance, we can’t see the air in Earth’s atmosphere but we can feel it when it blows around.Similarly, the scientists decided that instead of directly focussing on the temperature, they would try to find a consequence of the high temperature.
The hot pools of plasma can energise electrons, which can then emit X-rays. If the scientists could find signs of high-energy electrons or such X-ray emissions, it would give away the presence of a plasma.
This can’t be done in a straightforward way because Earth’s magnetic field and atmosphere filters out such charged particles. If they have to be detected, detectors have to be sent into space – which is exactly what was done in December 2014, when the researchers launched the aforementioned rocket, fit with hard X-ray detectors. During its flight, the detectors had a window of about six minutes during which the rocket, called FOXSI-2, would be able to capture emissions from a region of the Sun showing high magnetic activity. The region was designated 12234.
FOXSI-2 was able to detect multiple high-energy electrons that could be present only if they had come in contact with very hot plasma. At the same time, scientists did not detect any flaring activity emerging from 12234. This confirmed by proxy that the streaks of high-energy electrons could have been produced by nanoflares.
When the FOXSI-2 data was further analysed, Ishikawa & co. detected several signatures of tiny explosions in the corona. “It shows for the first time that nanoflares are more frequent than we thought them to be,” Ishikawa said.
However, does this mean we’ve solved the solar corona problem? “Even though these observations agree with the frequent occurrence of nanoflares and provide an additional clue to solving the heating puzzle, it doesn’t really tell which mechanism is behind these nanoflares,” Patrick Antolin, a postdoctoral fellow at the University of St. Andrews, UK, told The Wire.
Unlike Ishikawa, Antolin believes that magnetic waves, which move from the surface of the Sun out into the corona, heat the star’s atmosphere to exorbitantly high temperatures. Last year, he published a paper where he argued that such waves could play a more fundamental role in generating the small-scale turbulence necessary for the release of energy. Such disturbances could also explain the occurrence of magnetic reconnection. This is still a subject of much debate.
According to Banerjee, both Antolin and Ishikawa could be right, “There are certain regions on the Sun that are extremely active, where the magnetic field is particularly strong, while in some areas the effect is diminished. So it is possible that both the waves and the nanoflares play a role in heating the sun’s corona.”
India’s Aditya set to join the quest
“There are two significant limitations to solving the solar corona problem,” James A. Klimchuk, a scientist at the NASA Goddard Space Flight Centre, Maryland, told The Wire. “First, [the problem] involves important processes occurring on both extremely small and extremely large spatial scales, and it is not possible to accurately simulate all of them in a single computer model” (such modeling has been a prevalent way to study the Sun). “Second,” he continued, “we normally observe the heated gases long after they have cooled. To diagnose the heating process directly requires observations of extremely hot emissions.” Klimchuk was not involved in the JAXA/NASA study.
So, Ishikawa and colleagues have only just scratched the surface of this venerable problem.
According to a review Klimchuk wrote a decade ago, in order to understand coronal heating in its entirety, three pieces of the puzzle had to be figured out needed:
1. A source of energy and a mechanism for converting that energy into heat
2. How the plasma responds to the heating
3. The kind of radiation that is emitted as a result
Only by accomplishing all of these steps can a truly rigorous and meaningful comparison with actual observations be made. And much of this has yet to be worked out with nanoflares.
“We have not solved the paradox yet,” Ishikawa says. “The next step for us is resolving the energies of individual flares and comparing that to the energy and temperature changes in the entire coronal disk.”
To this end, the researchers in JAXA and NASA are planning another joint mission with even more sensitive instruments to detect even fainter flares. The rocket, an updated FOXSI, is expected to be launched next summer.
India’s own solar astrophysicists have been working on a space probe named Aditya, with a launch date set for around 2020. Among the instruments it will carry are a coronagraph, several X-ray detectors and a particle detector. “The coronagraph will help us resolve high energy waves that have never been observed before,” according to Banerjee.
Such missions are key to understanding the space environment. We are shielded from many forms of radiation emitted by the Sun by Earth’s atmosphere and magnetic field. Space probes that go these natural shields are exposed to solar events. “A very important objective of Aditya is to study the coronal mass ejections that can harm satellites,” Banerjee said.
The JAXA/NASA paper was published in the journal Nature Astronomy on October 9, 2017.