What can the eclipse tell us about the corona’s magnetic field?

The star of any solar eclipse is, of course, the sun. And total eclipses give the sun’s wispy, tenuous atmosphere the spotlight. This region, called the corona, is normally too dim to observe directly. But with the moon blocking the sun’s bright disk, the corona comes into view.

And the view is dazzling. The corona’s hot plasma is a radiant, ever-changing tiara, full of bright loops and whorls that bend, sway and snap. Sometimes, one of those loops breaks off, sending high-energy material rippling through space in what’s called a coronal mass ejection, or CME. When aimed at Earth, these bursts can trigger auroras — or damage satellites and knock out power grids.
It’s the motion of charged particles, like those in the corona’s plasma, that create the magnetic field that, in turn, choreographs this disordered dance. Understanding the sun’s magnetic inner life is key to understanding and predicting the corona’s dramatic tendencies. But surprisingly, although the magnetic field at the sun’s surface is well known, the corona’s field is so weak, we barely know it at all.

“To get a full understanding of the corona, you need to understand the magnetic field,” says Jenna Samra, an applied physics graduate student at Harvard University. “It’s the heart of everything.”
As a first step, during the eclipse on August 21, Samra and others will observe the corona in wavelengths of infrared light between 1 and 4 micrometers. These are the wavelengths emitted when a heavy element like magnesium, iron, sulfur or silicon loses an electron to the hot plasma around it.

Magnetic fields of different strengths make those electrons spiral in particular ways, and that spiraling changes the orientation of the light as it travels toward Earth. Ultimately, to probe the magnetic field, scientists will have to measure this orientation, or polarization.

During this eclipse, though, scientists will be satisfied with picking out which wavelengths are the best diagnostic tools. Among them will be Paul Bryans and Philip Judge of the High Altitude Observatory at the National Center for Atmospheric Research (NCAR) in Boulder, Colo. Along with their colleagues, the solar physicists will haul a spectrometer — a device that splits sunlight into its component wavelengths — to the top of Casper Mountain in Wyoming in the back of a truck.

“The whole infrared spectrum has never been acquired before, and that’s one of the things we want to do,” Judge says.

Samra will have another infrared spectrometer at an altitude of 15 kilometers. Her team, including her adviser Edward DeLuca, and their instrument will take off from near Chattanooga, Tenn., on a Gulfstream V jet owned by the National Science Foundation and operated by NCAR. The extra height will get them above most of the water in the Earth’s atmosphere, which absorbs some of the infrared wavelengths they’re interested in. Flying east in the shadow of the moon will also extend the eclipse a bit — giving them more time to collect data. The team will get four minutes of the moon entirely blocking the sun, compared with 2½ minutes on the ground.

The spectrum these teams measure during the eclipse won’t translate immediately to a measurement of the strength or shape of the magnetic field. But it will help identify which wavelengths are easiest to observe. That in turn will set the stage for the work of future telescopes, like the Daniel K. Inouye Solar Telescope under construction in Maui, Hawaii, which will come online in 2019.

“Half of its mission will be to observe the infrared and the corona, but right now we don’t know what it should look for,” Judge says. Measurements made during this year’s eclipse will help point the way.