I watched the eclipse with scientists hunting the sun’s secrets

Totality was sudden. The edge of the sun just barely peeked past the moon, the shadow rushed across the landscape towards us at almost 3000 kilometres per hour, and then, abruptly, it was dark. A few stars twinkled in the midday sky, and the temperature dropped by what felt like 20 degrees. The obscured sun revealed its corona, a shimmering ring of hot plasma stretching away from its surface like a spiked collar around the new moon.

As I watched the moment of totality during yesterday’s eclipse, there was an audible gasp and a flurry of activity at Guernsey State Park in Wyoming. I was standing in a giant custom tent designed to house the scientific instruments necessary for observing the sun. A group of researchers and engineers led by Nathalia Alzate from the University of Hawaii’s Institute for Astronomy whisked the solar filters off their telescopes to begin taking images of the corona.

This was one of five groups of Solar Wind Sherpas spread across the western US for the country’s first total solar eclipse in 38 years, headed by Shadia Habbal at the University of Hawaii. Habbal has studied 14 eclipses around the world over 22 years, and her mission, along with the Solar Wind Sherpas, is to create a temperature map of the sun’s corona in order to figure out why it is so hot.

You might think we already know – it is a great big flaming ball of gas. But while the surface of the sun roils at temperatures of thousands of degrees, the corona reaches millions of degrees and nobody is quite sure why.

Past eclipses have revealed that the corona’s temperature distribution is patchy: rather than a smooth transition from relatively cool to sizzling hot, the corona has areas of higher and cooler temperatures that don’t seem to depend on their proximity to the sun’s surface. That irregular heating adds to the corona’s mystery.

“If it was a smooth distribution like we expected, that’d be a much simpler problem to solve,” says Alzate. “Instead, we have to look at these individual events and how they affect the structure of the corona as a whole.”

And the only time to look at that entire structure is during a complete solar eclipse, when the glare from the sun’s surface stops obscuring the dimmer whorls of the corona.

Mapping the corona

There are two sets of instruments that study the sun on a day-to-day basis: imagers that look at the disk of the sun, and those that look at the corona by blocking out the disk. But because the coronagraphs have to block out an area slightly larger than the sun itself in order to prevent too much light from leaking in, there is a ring of space around the sun that neither set of instruments looks at. We can only get data on that ring during a total eclipse.

“We’re missing precious information about these solar events, about their trajectory and development once they leave the surface,” says Alzate. “A solar eclipse fills that gap, and it gives us a chance to make a full map of the corona.”

Alzate and the rest of the Solar Wind Sherpas create their corona temperature maps using sets of telescopes, each with a filter that allows it to view one particular ion in the plasma. Each ion is created from plasma at different temperatures, so by combining data about several of them, researchers can patch together an idea of the corona’s temperature distribution. At Guernsey, we were looking at two types of iron, each of which glows at a temperature of more than a million degrees, as well as helium, which is cooler.

“We’re missing precious information about these solar events… A solar eclipse fills that gap.”

Over the course of the eclipse, the group’s five telescopes took a total of about 450 images of the sun. Across Habbal’s five observing sites, all of which were lucky enough to have clear skies, researchers took images of seven different ion distributions in the corona. “That’s a lot of data,” says Alzate. “We’re going to be really busy.”

Just as suddenly as totality began, it was over. After slightly more than two minutes of darkness, the sun began to emerge, the rest of the stars disappeared, and the temperature surged. Within half an hour, it was a sweltering summer afternoon again. And as the Solar Wind Sherpas began to look over the images they had taken, the tent rang with a resounding cheer.