Imagine for a minute that you have been transported to the middle layers of the sun’s atmosphere. The sun’s surface, the “visible disc” scientists call the photosphere, boils below you, red-hot plasma heated to 10,000 degrees Fahrenheit. Above you, the vast corona, an atmospheric aura of gas superheated to several million degrees Fahrenheit, flings heat, light, magnetism, and radioactive particles into space with explosive force. The corona has long been an enigma to scientists: It is much hotter than the layers below it. Traveling outwards toward it from the sun’s surface would be like walking away from a campfire and feeling even more heat than when you were sitting next to the flames.
You’re floating in the chromosphere, the slice of the sun’s atmosphere sandwiched between these two much-studied layers, which is named (“sphere of color”) for its pops of pink that are visible from the Earth during total solar eclipses. Up close, those pink flashes are seas of boiling hydrogen plasma that go on to the sun’s massive horizon. But another, more dominant force is unleashed in the chromosphere: the sun’s magnetic fields. These fields are created far below the sun’s surface by the dynamo effect—heat and rotation on the largest scale in the solar system. The sun’s magnetic fields are massive, but within its inner layers, their forces are channeled and controlled by the pressure of the superheated plasma, convecting its heat outward like a boiling pot of tomato soup.
Don your ultraviolet light glasses, though, and you’ll see something interesting. Rising within the chromosphere, the relative force of the superheated plasma lessens quickly, but the magnetic fields stay relatively strong. The higher you look, the more the forces of magnetism dominate. In the photosphere, magnetic fields push the plasma aside, exploding outward in massive loops, rooted at their bases to the black regions we call sunspots. (In the photosphere, each one is the size of the Earth.) These magnetic loops twist and shear as they interact with the plasma and each other, creating a dynamic, chaotic environment—a superheated brouhaha so powerful that the effects are felt on our own planet 93 million miles away.
What you’d witness within the sun’s atmosphere is hypothetical, of course—not just because the chromosphere would instantly vaporize you, but because for decades scientists have had to guess exactly what’s happening inside it. Unlike the photosphere and the corona, it is very difficult to see and therefore to map. “It’s a really confusing place,” says David McKenzie, the principal investigator of NASA’s Chromospheric Layer Spectropolarimeter 2 mission, or Clasp2, a sounding rocket that briefly shot above the Earth’s atmosphere to observe the sun, then parachuted its payload of instruments and data home. “That’s what makes it exciting. It’s a frontier right in the middle of the sun’s atmosphere.”
McKenzie is a coauthor of a new paper that appeared in February in Scientific Advances, the result of data collected by Clasp2 in 2019, which represents the first successful mapping of the chromosphere’s magnetic field at four layers, using novel ultraviolet imaging techniques of a solar magnetic field. Written by a team from Japan, Europe, and the US, its findings appear to confirm theories about how the corona becomes superheated. Using these new mapping techniques, the scientists believe they will be able to better understand in real time the coronal mass ejections (CMEs) and “space weather” thrown off by the sun—huge magnetic, radioactive fields that cause chaos when they hit the Earth or technology in space.
The new mapping data was collected in only 150 seconds as the rocket orbited 170 miles above the Earth, tracking the sun. To collect it, the team built a special telescope armed with a spectropolarimeter, which read the magnetic polarization of ultraviolet light along a very thin, short stretch of the sun’s chromosphere. The scientists paired this data with measurements of the photosphere in the same stretch of the sun taken by the co-observing Hinode satellite.
The scientists took advantage of the Zeeman effect, which produces circular polarization and shifts in the wavelength of light from certain ions sensitive to magnetic fields. By measuring the polarization of wavelengths associated with iron, magnesium, and manganese, they could infer the strength of the magnetic fields all the way from the sun’s surface, through the chromosphere, and into the lower corona—painting a cohesive picture of how the magnetic fields behaved as they arced and looped, coupling the photosphere, chromosphere, and the base of the corona. That data maps only a single track along the sun’s surface—a thread of information. Given more “passes” using their spectropolarimetric tools, the scientists believe they could knit together entire sections of the sun’s atmosphere in a three dimensional map.
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“It’s clear evidence that in those gradients of the upper chromosphere, where the temperature is large, the magnetic field is also high,” says Javier Trujillo Bueno, one of the principal investigators for the Clasp2 mission, its lead theoretical physicist, and a coauthor of the paper, who hails from the Instituto de Astrofísica de Canarias, Spain. “That correlation is a clear indication of the idea that the physical mechanism that produces the heating of the outer layers of the solar atmosphere has a magnetic origin.”
Those findings were consistent with current theories about how magnetic fields unleash the explosive powers of the sun as CMEs and space weather. “We believe you can think of the sun’s magnetic field loops as rubber bands,” McKenzie says. “If you twist and stretch a rubber band, it can store enough energy to fly a model airplane.” Magnetic fields are the same. When they are twisted and stretched while interacting with plasma and other magnetic fields, they store energy that can be released in the form of extreme heat, light, and eruptions in the corona.
But each magnetic loop is different. “One magnetic loop might be boring as a cauliflower, but another is a loaded gun,” McKenzie says. Advanced spectropolarimeter imaging like that accomplished onboard Clasp2 might help scientists jump from understanding the theory (that a magnetic loop could unleash some energy) to predicting what’s about to happen—when that magnetic loop will “go off,” and in which direction.
To build those prediction models, scientists will need more than 150 seconds of data collection. “Imagine what we could do if we had an instrument like the one in Clasp2, but in a space telescope that orbits around the Earth for years at a time,” says Trujillo Bueno. “This must be the near-future goal for solar physics.”
Any real-time modeling of the chromosphere’s magnetic fields and their direct effect on the solar corona is information NASA, the National Oceanic and Atmospheric Administration, and other organizations could use to improve their predictions of space weather. That’s important business, because the radiated magnetism unleashed in large solar flares and CMEs can knock out communications, damage satellites, and literally melt the enormous transformers connecting America’s power grid.
That’s what makes Clasp2’s findings so exciting. The theory was sound; solar physicists appear to have nailed the theory and stuck the data landing from 93 million miles away. “In the grand scheme of space missions, this sounding rocket was a very small mission,” says Laurel Rachmeler, a NOAA solar scientist who worked as the Clasp2 project scientist at NASA. “But with that small mission, we’ve collected data that humans have never seen before, and learned something about the sun that we could only hypothesize about before. We’re really at the forefront of human knowledge in this specific case.”
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