Eclipses do strange things to radio waves. An army of amateur broadcasters want to know why

On a 7.25-acre (3-hectare) plot of land off an interstate south of Indianapolis, Baker’s array of barbed antennas pointing in different directions allow for transmissions across the United States and far beyond. He has taken his voice to the other side of the world and struck up conversations with fellow radio amateurs in Europe and even in New Zealand, 13,000 kilometers (8,000 miles) away.

One of the antennas in Baker Park is designed at a special angle, he says, so that it transmits a radio signal that initially stays low to the ground. But eventually that signal will head out into space. “When it gets to the ionosphere, it’s going to jump. It’s going to bounce,” Baker says.

This phenomenon, in which radio waves reflect off some of the upper layers of the atmosphere, greatly expands the distance over which radio operators can communicate. It’s called the “sky wave effect” and this is how the first radio transmission was sent across the Atlantic Ocean in 1901.

This means that the curvature of the Earth can be overcome. Radio transmissions can zigzag up and down, bouncing between Earth and the ionosphere, which lies at an altitude of about 80-650 kilometers (50-400 miles). You could say that a person’s voice, which travels in the form of electromagnetic waves, literally touches the sky during long-distance broadcasts that rely on this effect.

“The fact that you can pick up radio signals from the other side of the Earth is really amazing,” says Catherine Mitchell, professor of radio science at the University of Bath.

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The truly amazing thing is that the sky wave effect is not stable, and scientists still do not fully understand it. The ionosphere is strange. It fluctuates, moves, expands and contracts, and is far from homogeneous. Sometimes it’s filled with the same waves, which ripple as the sun rises and sets — almost like throwing a stone into a pond, Mitchell says.

The presence or absence of the sun is one of the reasons for this. During the day, the ionosphere thickens because sunlight hits atmospheric gases, ionizing them to produce electrons. At night, collisions decrease, and the lower layer of the ionosphere disappears. This nighttime thinning allows radio waves to travel much farther, because they reach higher altitudes before being bounced back toward Earth by electrons. That’s why people have long been able to pick up distant radio stations in the wee hours of the night.