How the 2019 eclipse will differ from 2017’s — and what that means for science

Two years ago, scientists towed telescopes and other equipment into fields and up mountains across the United States for a celestial spectacle: the 2017 Great American Eclipse.

Now, they’re at it again. On July 2, the next total solar eclipse will be visible shortly before sunset from the Pacific Ocean and parts of Chile and Argentina.

Eclipse watchers hope to study some of the same solar mysteries as last time, including the nature of our star’s magnetic field and how heat moves through the Sun’s wispy outer atmosphere, known as the corona (SN Online: 8/11/17). But every eclipse is different, and this year’s event offers its own unique opportunities and challenges.

“There are all sorts of outside things you have to be lucky about” in watching an eclipse, says astronomer Jay Pasachoff of Williams College in Williamstown, Mass., who will be viewing his 35th total solar eclipse from the Cerro Tololo Inter-American Observatory in northern Chile. Here are some of the challenges, and potential rewards, facing astronomers.

1. The sun is in a period of low solar activity.

A total solar eclipse occurs on August 21, 2017, at Mary’s River Covered Bridge, in Chester, IL, USA. (Photo by Patrick Gorski/NurPhoto via Getty Images)

One of the main reasons, scientifically speaking, to observe a total solar eclipse is to catch a glimpse of the corona, whose wisps and tendrils of plasma are visible only when the sun’s bright disk is blocked. This region could hold the key to predicting the sun’s volatile outbursts, including giant burps of plasma called coronal mass ejections that can wreak havoc with satellites and power grids if they hit Earth (SN Online: 4/9/12). But the corona is one of the least well-understood parts of our nearest star.

This animation shows the path of the 2019 total solar eclipse over the Pacific Ocean and South America. The gray shadow indicates areas that will see a partial eclipse, and the black dot is the shadow of the moon, where viewers will see a total eclipse.A.T. SINCLAIR/NASAStill, “some of the science we can do is contingent on what’s happening on the sun at that particular time,” says solar physicist Paul Bryans of the National Center for Atmospheric Research in Boulder, Colo.

In every eclipse, the corona looks different, thanks partly to the sun’s 11-year magnetic activity cycle. Magnetic fields in the corona guide the motions of its wisps and whorls, and determine when the sun gives off more powerful bursts like flares or the mass ejections. Scientists have never measured the corona’s magnetic fields directly (SN Online: 8/16/17), but they keep trying.

In 2017, the sun was in a period of low magnetic activity (SN Online: 8/19/17), showing a pitiful number of sunspots, flares and coronal mass ejections. This year, it’s even quieter. “We are in the minimum of the sunspot cycle,” Pasachoff says. “If you look at an image of the sun today, it’s entirely blank.”

That’s both lucky and unlucky. Low solar activity means certain features in the corona, such as plasma plumes that stream away from the sun’s poles, will be more visible now than when the sun is more active. Catching a glimpse of those magnetically controlled plumes could give relatively rare insights into how the corona behaves.

But low activity also means that scientists are less likely to catch a fleeing coronal mass ejection, like those detected by Adalbert Ding of the Institute for Optics and Atomic Physics in Berlin and colleagues in 2015 and 2017 (SN Online: 5/29/18).

Although such an eruption is unlikely, it’s not impossible. The sun “does produce explosive events during this quiet phase,” Bryans says. “Part of the reason we’re doing these experiments is to predict when events like these happen.”

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