‘Campfires’ may drive heating of solar atmosphere

doi:10.1126/science.372.6542.557

GSTDTAP > 气候变化

DOI	10.1126/science.372.6542.557
	‘Campfires’ may drive heating of solar atmosphere
	Daniel Clery
	2021-05-07
发表期刊	Science
出版年	2021
英文摘要	Researchers are getting closer to solving an enduring mystery: why the Sun's wispy atmosphere is nearly 200 times hotter than its surface. Results from new instruments that see the Sun in sharper spatial and temporal resolution, presented last week at the European Geosciences Union (EGU) meeting, suggest tiny, flickering flares, dubbed “campfires,” could be sufficient to heat the atmosphere. “It's still one of the major unsolved problems in solar physics,” says Sarah Matthews a solar physicist at University College London. “Campfires are a tantalizing prospect.” Temperatures ought to decline as one moves out from the Sun's core, where its fusion fires burn. However, in the 1930s scientists discovered that the solar atmosphere, or corona, seethes at more than 1 million degrees Celsius, far hotter than the 5500°C of the surface. Flares have been a prime suspect for providing the missing heat. Researchers think these eruptions of energy and particles occur when magnetic field lines, looping up into the corona, become twisted and suddenly snap into a configuration of lower stress, a process known as magnetic reconnection that releases huge amounts of energy. Solar telescopes on Earth and in space reveal large magnetic loops 10 times the diameter of Earth flaring several times a day when the Sun is magnetically active. At those rates, the flares cannot provide enough heat to maintain the corona's temperature. So researchers in the 1960s proposed that the Sun is constantly erupting in “nanoflares” too small to be seen. Enter Solar Orbiter, launched by the European Space Agency just over 1 year ago. In May 2020, while still maneuvering toward its operational orbit, the spacecraft observed the corona from a vantage halfway between the Sun and Earth using its Extreme Ultraviolet Imager (EUI). At extreme ultraviolet wavelengths, hot gases deep in the corona shine brightly, revealing a welter of brief brightenings the team called “campfires”: almost 1500 of them during a 4-minute observation, ranging in size from 400 to 4000 kilometers long. “We didn't expect them to stand out so clearly,” says EUI Principal Investigator David Berghmans, a solar physicist at the Royal Observatory of Belgium. The team was especially surprised to find these nanoflares going off on the Sun's so-called “quiet” regions, where magnetic activity is low and big flares rare. Seeing them there, Berghmans says, is “like a palm tree at the North Pole.” The question solar physicists now want answered is: “Are [campfires] enough to power the quiet Sun on average?” says Solar Orbiter project scientist Daniel Müller at the European Space Agency's research center in the Netherlands. The EUI alone can't settle the question, but, Müller says, “Indications are that they could.” Part of that confidence comes from computer models, such as one presented at EGU by a team led by Yajie Chen of Peking University and Hardi Peter of the Max Planck Institute for Solar System Research. Their 3D model extends from below the Sun's turbulent surface up into the corona and across an expanse of the quiet Sun similar to that observed by EUI. The “brightenings” it produced were remarkably similar to the larger campfires in size and duration, the team reported. “Everything we checked matched,” Peter says. The model also suggested the nanoflares could sustain the million-degree temperatures of the quiet parts of the corona. Radio observations of the Sun support that picture. At EGU, researchers from India's Tata Institute of Fundamental Research reported data from the Murchison Widefield Array, a radio telescope in Western Australia made up of 4100 spiderlike wire antennas spread across several kilometers. During a 70-minute observation of quiet regions on the Sun, the team recorded 20,000 bright radio flashes, each lasting 1 second or less, with estimated energies matching theoretical predictions for nanoflares. “It jumped out of the data,” says Tata's Surajit Mondal. Mondal adds that it's difficult to estimate the energy of a flare from its radio signal alone. But, he says, “If there is this number of flares, it can maintain coronal heating.” The mystery of the corona's heating isn't solved yet—other processes besides the small flares could contribute. The main rival is waves, carrying energy from the churning plasma below the Sun's surface up into the corona. But researchers are struggling to explain how the waves would break and deposit their energy into the diffuse coronal plasma—and whether they arise independently of the flares. “Flare processes also drive wave modes, they are intertwined,” Matthews says. Solar physicists will have their best chance yet to crack this problem in the next few years, as Solar Orbiter teams up with NASA's Parker Solar Probe, launched in 2018, and the Daniel K. Inouye Solar Telescope, currently being commissioned on a mountaintop in Hawaii. Solar Orbiter will be fully in place in 2022, swooping down every 6 months to just one-quarter of the Sun-Earth distance. Never before has a telescope stared directly at the Sun from such a close vantage. Peter says: “It will be an exciting year.”
领域	气候变化 ; 资源环境
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文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/325915
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Daniel Clery. ‘Campfires’ may drive heating of solar atmosphere[J]. Science,2021.
APA	Daniel Clery.(2021).‘Campfires’ may drive heating of solar atmosphere.Science.
MLA	Daniel Clery."‘Campfires’ may drive heating of solar atmosphere".Science (2021).