In the mythology of ancient Egypt, Apep was the serpent god — the rival of Re, the sun god. He chased Re, and sometimes briefly caught him — creating a solar eclipse.

NASA plans to launch an experiment named for the serpent god to study the eclipse on Monday. Instruments will fly on three different rockets. They won’t look at the Sun, though. Instead, they’ll study the effect of the eclipse on the ionosphere — a high-altitude layer of Earth’s atmosphere.

The ionosphere carries an electric charge. It “bends” radio waves, allowing them to travel far around the planet. During the day, sunlight intensifies the charge. At night, the charge lessens.

Scientists want to understand how the ionosphere reacts to the brief loss of sunlight during an eclipse. That should tell them more about the ionosphere overall, and how it changes during the day.

Among other ways, they’ll do so by launching Apep. Three small rockets will take flight from a NASA center in Virginia. The rockets will climb about 200 miles high. They’ll deploy several instruments, which will make observations for seven or eight minutes. Then they’ll parachute back to Earth.

The same instruments flew during last year’s annular eclipse, when the Moon didn’t fully cover the Sun. This year’s flights will allow scientists to compare the results — learning more about how Earth’s atmosphere reacts when the Sun grows darker.

More about the eclipse tomorrow.