Today, on Aug. 21, 2017, the Moon will slide in front of the Sun and for a brief moment, day will melt into a dusky night. Moving across the country, the Moon’s shadow will block the Sun’s light, and weather permitting, those within the path of totality will be treated to a view of the Sun’s outer atmosphere, called the corona.

But the total solar eclipse will also have imperceptible effects, such as the sudden loss of extreme ultraviolet radiation from the Sun, which generates the ionized layer of Earth’s atmosphere, called the ionosphere. This ever-changing region grows and shrinks based on solar conditions, and is the focus of several NASA-funded science teams that will use the eclipse as a ready-made experiment, courtesy of nature.

NASA is taking advantage of the Aug. 21 eclipse by funding 11 ground-based science investigations across the United States. Three of these will look to the ionosphere in order to improve our understanding of the Sun’s relationship to this region, where satellites orbit and radio signals are reflected back toward the Earth.

“The eclipse turns off the ionosphere’s source of high-energy radiation,” said Bob Marshall, a space scientist at University of Colorado Boulder and principal investigator for one of the studies. “Without ionizing radiation, the ionosphere will relax, going from daytime conditions to nighttime conditions and then back again after the eclipse.”

During the total solar eclipse, the Moon will turn off the ionosphere’s source of extreme ultraviolet radiation: The ionosphere will go from daytime conditions to nighttime conditions.

Credits: NASA’s Goddard Space Flight Center/Katy Mersmann

Stretching from roughly 50 to 400 miles above Earth’s surface, the tenuous ionosphere is an electrified layer of the atmosphere that reacts to changes from both Earth below and space above. Such changes in the lower atmosphere or space weather can manifest as disruptions in the ionosphere that can interfere with communication and navigation signals.

“In our lifetime, this is the best eclipse to see,” said Greg Earle, an electrical and computer engineer at Virginia Tech in Blacksburg, Virginia, who is leading another of the studies. “But we’ve also got a denser network of satellites, GPS and radio traffic than ever before. It’s the first time we’ll have such a wealth of information to study the effects of this eclipse; we’ll be drowning in data.”

Pinning down ionospheric dynamics can be tricky. “Compared to visible light, the Sun’s extreme ultraviolet output is highly variable,” said Phil Erickson, a principal investigator of a third study and space scientist at Massachusetts Institute of Technology’s Haystack Observatory in Westford, Massachusetts. “That creates variability in ionospheric weather. Because our planet has a strong magnetic field, charged particles are also affected along magnetic field lines all over the planet — all of this means the ionosphere is complicated.”

But when totality hits on Aug. 21, scientists will know exactly how much solar radiation is blocked, the area of land it’s blocked over and for how long. Combined with measurements of the ionosphere during the eclipse, they’ll have information on both the solar input and corresponding ionosphere response, enabling them to study the mechanisms underlying ionospheric changes better than ever before.