A hot mystery about the sun may be close to being solved.
Scientists have been trying for decades to understand why the sun’s outer atmosphere is so much hotter than the surface, even though it is further away from the core. The surface, or photosphere, is millions of degrees Fahrenheit, while the outside air is only about 10,000 F (5,500 C).
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New data from the National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) in Hawaii, the largest ground-based solar telescope in history, is helping scientists learn how solar energy is transported through the atmosphere.
mystery of the sun
Richard Morton, a solar physicist and professor at Northumbria University in the UK who led the study, told Live Science via email that researchers had previously focused on the extreme temperatures of the sun’s corona and the supercharged stream of heated gases called the solar wind that blows out of the sun at more than 1 million miles per hour (1.6 million km per hour).
Both processes require energy, and scientists hypothesized that rotating convection on the sun’s surface would produce the necessary fuel. However, a complication arose during the initial research several decades ago.
“It is unclear how this happens, but [energy] is transferred to the atmosphere and solar wind, and how that energy is converted into heat and momentum,” Morton said.
In 1942, Swedish plasma physicist (and eventual Nobel Prize winner) Hannes Alfvén suggested that magnetic waves could be the culprit. But these waves, now known as Alfvén waves, had never been found in the corona before.
“This is because previous instruments were not sensitive enough to resolve the behavior of Alfvén waves,” Morton said. “Nevertheless, many numerical experiments and space weather prediction tools assume the existence of Alfvén waves in the corona. However, the properties of the waves used within the models are educated guesses.”
“Unprecedented” observations
Morton said DKIST has a 4-meter (13-foot) mirror and has “unprecedented” resolution of the Sun, giving it much “cleaner measurements” (less noise) than previous solar observatories. In the new study, scientists used the telescope’s Cryogenic Near-Infrared Spectropolarimeter (Cryo-NIRSP) to search for coronal Alfvén waves.
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In addition to being able to chart the movement of the corona through images, Morton said, cryo-NIRSP can also study changes in the sun’s plasma (superheated gases) through a phenomenon known as Doppler shift. Doppler shift is the difference in the perceived frequency of a wave when the observer and the wave source move toward or away from each other. (A common example in practice is the change in the sound of an ambulance siren passing a pedestrian on the street.)
“Cryo-NIRSP has provided us with data that allows us to observe the distinct signature of Alfvén waves, the back-and-forth twisting of the magnetic field in plasmas like the corona,” Morton said. “This manifests itself as a pattern of alternating red and blue Doppler shifts on opposite sides of the magnetic field. We found that these waves were continuously present during our observations. And given that there was nothing particularly special about the region we observed, this means that they are likely to be common in other parts of the atmosphere all the time.”
“Perhaps most importantly, our analysis shows that waves are likely to carry significant amounts of energy,” he continued.
This is an important discovery, he noted, because astronomers have debated between solar waves and magnetic reconnection (when the sun’s magnetic fields twist and break around each other, releasing energy) as mechanisms behind the intense heating of the corona.
Evidence that magnetic reconnection is responsible for coronal heating has been found by various spacecraft, but DKIST’s new findings show that the picture is more complex. Solar observatories such as NASA’s Parker Solar Probe and the European Space Agency’s Solar Orbiter, along with the latest data from DKIST, show that “both waves and reconnections are occurring frequently in the Sun’s atmosphere,” Morton said.
“Our study confirms that Alfvén waves exist, carry significant amounts of energy, and may account for at least half of the energy needed to heat the corona,” he added. “However, estimating the exact energy associated with waves remains difficult.”
The ratio of magnetic reconnection to Alfvén waves affects not only the heating of the Sun, but also the light (or more precisely the radiant output) from the Sun and the light from stars outside our solar system. Scientists hope to use this research to understand how planetary systems have evolved over the long term and make better short-term predictions for solar and wind power generation. “We hope that further studies like the one we have published will reveal the properties of Alfvén waves, informing models and improving predictions,” he said.
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