The sun’s wispy upper atmosphere, called the corona, is an ever-changing jungle of sizzling plasma. But mapping the strength of the magnetic fields that largely control that behavior has proved elusive. The fields are weak and the brightness of the sun outshines its corona.
Now though, observations taken using a specialized instrument called a coronagraph to block out the sun’s bright disk have allowed solar physicists to measure the speed and intensity of waves rippling through coronal plasma (SN: 3/19/09). “This is the first time we’ve mapped the coronal magnetic field on a large scale,” says Steven Tomczyk, a solar physicist at the High Altitude Observatory in Boulder, Colo., who designed the coronagraph.
In 2017, Tomczyk had been part of a team that took advantage of a total solar eclipse crisscrossing North America to take measurements of the corona’s magnetic field (SN: 8/16/17). He trekked to a mountaintop in Wyoming with a special camera to snap polarized pictures of the corona just as the moon blocked the sun. (I was there with them, reporting on the team’s efforts to help explain why the corona is so much hotter than the sun’s surface (SN: 8/21/17).) The team observed a tiny slice of the corona to test whether a particular wavelength of light could carry signatures of the corona’s magnetic field. It can (SN: 8/21/18).
But it’s the observations from the coronagraph, made in 2016, that allowed researchers to look at the whole corona all at once. Theorists had shown decades ago that coronal waves’ velocities can be used to infer the strength of the magnetic field. Such waves might also help carry heat from the sun’s surface into the corona (SN: 11/14/19). But no one had measured them across the whole corona before.
The corona’s magnetic field strength is mostly between 1 and 4 gauss, a few times the strength of the Earth’s magnetic field at the planet’s surface, the researchers report in the Aug. 7 Science.
Making a map is a big step, the team says. But what solar physicists would really like to do is track the corona’s magnetic field continuously, at least once a day.
“The solar magnetic field is evolving all the time,” says solar physicist Zihao Yang of Peking University in Beijing. Sometimes the sun releases magnetic energy explosively, sending bursts of plasma can shooting out into space (SN: 3/7/19). Those ejections can wreak havoc on satellites or power grids when they strike Earth. Continuously monitoring coronal magnetism can help predict those outbursts. “Our work demonstrated that we can use this technique to map the global distribution of coronal magnetic field, but we only showed one map from a single dataset,” Yang says.
Measuring the strength of the corona’s magnetic field is “a really big deal,” says solar physicist Jenna Samra of the Smithsonian Astrophysical Observatory in Cambridge, Mass. “Making global maps of the coronal magnetic field strength … is what’s going to allow us to eventually get better predictions of space weather events,” she says. “This is a really nice step in that direction.”
Tomczyk and colleagues are working on an upgraded version of the coronagraph, called COSMO, for Coronal Solar Magnetism Observatory, that would use the same technique repeatedly with the ultimate goal of predicting the sun’s behavior.
“It’s a milestone to do it,” Tomczyk says. “The goal is to do it regularly, do it all the time.”