“All of these things are way more weird than anyone had predicted,” says astrophysicist John Beacom of the Ohio State University who with colleagues, led by astrophysicist Tim Linden, sifted nearly 10 years of observations from the Fermi Gamma-ray Space Telescope, a NASA observatory that scans the sky from its outpost in low-Earth orbit showing that the sun’s gamma rays do a number of weird things. “And that means the magnetic fields must be way more weird than anyone had thought.”
“It’s amazing that we were so spectacularly wrong about something we should understand really well: the sun,” said Brian Fields, a particle astrophysicist at the University of Illinois, Urbana-Champaign.
“Every 11 years, the whole magnetic field of the sun reverses,” said Igor Moskalenko, a senior scientist at Stanford University who is part of the Fermi scientific collaboration. “We have south in the place of north and north in the place of south. This is a dramatic change. The sun is huge, and why we observe this change of polarity and why it is so periodic nobody actually knows.” Cosmic rays, he said, and the pattern of gamma rays they produce “may answer this very important question: Why is the sun changing polarity every 11 years?”
“The almost certain thing that’s going on here is the magnetic fields are much more powerful, much more variable, and much more weirdly shaped than we expect,” said Beacom.
In 2018 scientists discovered that the sun puts out more of this light, called high-energy gamma rays, overall than predicted. But what’s really weird is that the rays with the highest energies appear when the star is supposed to be at its most sluggish, researchers reported in Physical Review Letters. The research is the first to examine these gamma rays over most of the solar cycle, a roughly 11-year period of waxing and waning solar activity.
The decade’s worth of Fermi telescope observations revealed a stunning mystery: gamma rays, the highest frequency waves of light, radiate from our nearest star seven times more abundantly than expected. Stranger still, writes Natalie Wolchover in Quanta, “despite this extreme excess of gamma rays overall, a narrow bandwidth of frequencies is curiously absent. The surplus light, the gap in the spectrum, and other surprises about the solar gamma-ray signal potentially point to unknown features of the sun’s magnetic field, or more exotic physics.”
The unexpected signal that has emerged in the Fermi data reveals the spectrum of gamma rays coming from the sun in ever-greater detail, only proliferated the puzzles.
“We just kept finding surprising things,” said Annika Peter of Ohio State University, a co-author of a recent white paper summarizing several years of findings about the solar gamma-ray signal. “It’s definitely the most surprising thing I’ve ever worked on.”
The sun’s high-energy gamma rays aren’t produced directly by our star. Instead, the light is triggered by cosmic rays — protons that zip through space with some of the highest energies known in nature, reported Science News, “that smack into solar protons and produce high-energy gamma rays in the process.
“All of those gamma rays would get lost inside the sun, if not for magnetic fields. Magnetic fields are known to take charged particles like cosmic rays and spin them around like a house in a tornado. Theorists predicted that cosmic rays whose paths have been scrambled by the tangled mass of magnetic fields at the solar surface should send high-energy gamma rays shooting back out of the sun, where astronomers can see them.”
“There are no good guesses about how the sun’s magnetic field might create the dip in the gamma-ray spectrum at 10 trillion trillion hertz,” writes Wolchover in Quanta. “It’s such an unusual feature that some experts doubt that it’s real. But if the absence of gamma rays around that frequency is a miscalculation or a problem with Fermi’s instruments, no one has figured out the cause.”
“It does not seem to be any instrumental effect,” said Elena Orlando, an astrophysicist at Stanford and a member of the Fermi team.
Not only is the gamma-ray signal far stronger than a decades-old theory predicts, concludes Walchover, “it also extends to much higher frequencies than predicted, and it inexplicably varies across the face of the sun and throughout the 11-year solar cycle. Then there’s the gap, which researchers call a “dip” — a lack of gamma rays with frequencies around 10 trillion trillion hertz.”
“The dip just defies all logic,” said Tim Linden, a particle astrophysicist at Ohio State who helped analyze the signal.