Scientists observed and captured the first clear evidence of auroras on Neptune using the James Webb Space Telescope (JWST), marking a milestone in the study of the outer reaches of the solar system.
NASA's JWST revealed an aurora shimmering across Neptune's atmosphere, captured in infrared detail for the first time. This provides new insights into the planet's atmosphere and magnetic field. Previously, hints of auroral activity on Neptune were detected during NASA's Voyager 2 flyby in 1989, but confirming and imaging the phenomena had not been achieved until now.
"Auroras on Neptune form when high-energy particles from the Sun are trapped by the planet's magnetic field and collide with its upper atmosphere, producing a glowing light display," explained Henrik Melin, scientist at Northumbria University and principal investigator of the study, according to SciTechDaily. "The level of detail and clarity of the images was surprising."
Neptune's magnetic field is tilted about 47 degrees relative to the planet's rotation axis, which explains why the auroras are positioned far from the poles. Unlike on Earth, where auroras tend to occur near the polar regions, Neptune's auroras are located at mid-latitudes due to the unusual inclination of its magnetic field. They are positioned similar to where South America or Africa are located on Earth.
In images taken by JWST, the bright auroras on Neptune appear as a series of cyan-colored patches. The research team used a powerful infrared measurement instrument equipped on JWST to take spectroscopic images of Neptune and analyze the different light wavelengths emitted by the planet. This was only possible due to JWST's near-infrared sensitivity.
On Neptune, the researchers succeeded in recording a clear emission line that suggests the presence of the trihydrogen cation (H₃⁺), which can form in auroras. "We've long expected this ion to exist on Neptune, but we needed the power of JWST to finally make the detection—this observatory has opened the window onto this last, previously hidden ionosphere of the giant planets," said Professor Leigh Fletcher, according to SciTechDaily.
"This detection will help scientists understand how Neptune's magnetic field interacts with solar particles that stream from the Sun to the distant reaches of our solar system."
"I was astonished—Neptune's upper atmosphere has cooled several hundred degrees. In fact, the temperature in 2023 was a little more than half of what it was in 1989," said Melin. The data collected in June 2023 through the Webb NIRSpec spectrograph made it possible to clearly detect the presence of H₃⁺ on Neptune, indicating auroral activity. In addition to images of Neptune, astronomers obtained a spectrum to characterize the composition and measure the temperature of Neptune's upper atmosphere, the ionosphere.
Equipped with new findings, astronomers now hope to study Neptune with the James Webb Space Telescope over a full solar cycle, an 11-year period of activity driven by the Sun's magnetic field. The combination of the magnetic field tilt and the cold atmospheric temperature explains why Neptune's auroras had not been detected for so long. The results could shed light on Neptune's erratic magnetic field and provide insights into its origin.
"Only with a machine like Webb have we finally gotten that confirmation," said Heidi Hammel of the Association of Universities for Research in Astronomy. The observations were taken as part of Hammel's Guaranteed Time Observation program 1249. The results of the study on Neptune's auroras were published in the journal Nature Astronomy.
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