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The Sun, imaged in ultraviolet light by Europe’s Solar Orbiter, revealing the hottest parts of its surface and atmosphere

Solar Orbiter/EUI Team/ESA & NASA; CSL; IAS; MPS; PMOD/WRC; ROB; UCL/MSSL

Solar ‘campfires’ may heat the Sun’s atmosphere to scorching temperatures

A new space mission is helping researchers solve one of the most enduring mysteries of our Sun: why its wispy atmosphere is nearly 200 times hotter than the surface below. An analysis of observations made last year by the European-led Solar Orbiter mission, presented today at the European Geosciences Union (EGU) meeting, suggests tiny flares just above the Sun’s surface, dubbed “campfires,” could be enough to heat the atmosphere to its prodigious temperature. Other research at the meeting supports the long-held suspicion that small flares, not large ones, may do most of the heating.  

The Sun’s heat comes from nuclear fusion in its core, and temperatures should decline moving outward. However, in the 1930s scientists discovered that the Sun’s atmosphere, or corona, had a temperature of 1 million degrees Celsius, far hotter than the 5500°C temperature of the surface. They’ve puzzled over the mystery ever since. “It’s still one of the major unsolved problems in solar physics,” says Sarah Matthews of University College London.

A prime suspect is the Sun’s turbulent magnetic field. The corona consists of plasma, a gas so hot that atoms fall apart, producing a roiling soup of ions and electrons buffeted by magnetic fields. Solar telescopes can see active regions in the corona where magnetic fields loop up from the surface, channeling superhot plasma up into the atmosphere and back down in arcs visible from Earth. Occasionally, active regions produce flares, eruptions of radiation and high-energy particles that sometimes reach Earth. Researchers think these flares likely erupt when twisted, stress-filled magnetic loops snap and link up with other loops in a lower stress configuration, a process known as magnetic reconnection that releases huge amounts of energy.

But researchers realized in the 1960s that the loops and flares they could see did not provide enough heat to keep the corona cooking. So they proposed that the corona is peppered with loops and flares with a wide range of sizes, most too small to see, that provide the missing heat through a wealth of smaller contributions. Over the years, researchers have developed some “very slick ways of studying the very smallest flares” via their collective effects, says James Klimchuk of NASA’s Goddard Space Flight Center. But until now, solar telescopes only had sharp enough vision to see the largest ones.

Solar Orbiter’s Extreme Ultraviolet Imager found “campfires” (next to the white arrow), thought to be flares up to 4000 kilometers long.

Solar Orbiter/EUI Team/ESA & NASA; CSL; IAS; MPS; PMOD/WRC; ROB; UCL/MSSL

Enter Solar Orbiter, which launched just over 1 year ago with six telescopic instruments and four sensors of its local environment. In May 2020, while still in its instrument-testing phase, the spacecraft swung halfway between the Sun and Earth. From a distance of 77 million kilometers, the team working with spacecraft’s Extreme Ultraviolet Imager (EUI) expected to see some small flares, says David Berghmans, a solar physicist with the Royal Observatory of Belgium and EUI principal investigator. But, “We didn’t expect them to stand out so clearly,” he says. In quiet areas of the surface, he says, “it’s like a palm tree at the North Pole.”

Researchers got a second close look at the campfires when Solar Orbiter approached the Sun again in February, and the team is still analyzing those data. In December, testing will be over, and researchers will unlock the true power of Solar Orbiter by studying the Sun with multiple instruments at the same time. The real job is to work out the statistics, says Daniel Müller of the European Space Agency, principal investigator of the mission. “Are [campfires] enough to power the quiet Sun on average?” Müller wonders. “Indications are that they could.”

Some of those indications are coming from theory. A team led by Yajie Chen of Peking University and Hardi Peter of the Max Planck Institute for Solar System Research used an existing model of the Sun’s turbulent surface to calculate how the corona above it would behave in conditions like those observed by Solar Orbiter in May 2020. The model produced “brightenings” that were remarkably similar to the larger campfires in size and duration, they report today at the EGU meeting. “Everything we checked matched,” Peter says. The model also suggests the brightenings are the result of magnetic reconnection, produce million-degree temperatures, and contribute significantly to coronal heating.

Other instruments are helping tackle the problem. Researchers at India’s Tata Institute of Fundamental Research have used the Murchison Widefield Array, an array of many thousands of wire antennas spread across 3 kilometers that is exquisitely sensitive to long-wavelength radio waves, to look for evidence of small flares in the quiet parts of the Sun. During a 70-minute observation of the Sun, the team saw a multitude of bright flashes, each lasting a second or less, they told the EGU meeting. “It jumped out of the data,” says Tata’s Surajit Mondal. The team logged about 20,000 flashes. Mondal acknowledges the difficulty of estimating the energy of a flare from its radio signal alone, but, “If there is this number of flares, it can maintain coronal heating,” he says.

The mystery of the corona’s heating isn’t solved—there could be other processes adding to the heating in addition to small flares—but over the next few years solar physicists have their best chance yet to crack this problem, with Solar Orbiter starting work as well as NASA’s Parker Solar Probe, launched in 2018, and the Daniel K. Inouye Solar Telescope in Hawaii, now being commissioned. “Campfires are a tantalizing prospect,” Matthews says. But we need more data, she says, to pin down how many there are and how much heat they produce. Peter is looking forward to 2022, when Solar Orbiter will begin to observe with all its instruments working in concert and carrying out joint observations with telescopes on or near Earth. “It will be an exciting year.”