Eclipses do odd things to radio waves. An army of amateur broadcasters wants to find out why
Chris Baraniuk/ BBC:
It’s the huge tower in his back yard that gives Todd Baker’s hobby away. Bristling with antennae, the 30m (100ft) structure is taller than many of the mature trees nearby. Baker, an industrial conveyor belt salesman from Indiana, goes not just by his name, but also his call-sign, the short sequence of letters and numbers that he uses to identify himself over the air: W1TOD. He is a member of the amateur radio, or ham radio, community.
“You name it, I’ve been in it,” he says, referring to different radio systems, including citizens band, or CB radio, that he has dabbled with over the years. “Communications were just plain-o cool to me.”
Now, he dabbles in celestial citizen science, too. On 14 October, he and hundreds of other amateur radio enthusiasts will deliberately fill the airwaves during an annular solar eclipse, as it crosses the Americas. They’ll do it again next April, when a full solar eclipse becomes visible from Newfoundland to Mexico.
Why? Solar eclipses are known to affect radio transmissions, and Baker is planning to take part in a giant experiment designed to monitor how cosmic events affect radio broadcasts.On his 7.25 acre (3 hectare) plot off a highway south of Indianapolis, Baker’s collection of spiky antennae pointing in various directions allows him to make transmissions across the US and much further afield. He has transmitted his voice to the other side of the world and had conversations with fellow amateur radio enthusiasts in Europe and even New Zealand, 13,000km (8,000 miles) away.
One of the antennae in Baker’s garden is specially angled, he says, so that it transmits a radio signal that initially stays low to the ground. But eventually that signal will head off towards space. “As it reaches the ionosphere,” says Baker, “it’ll make a hop. It’ll bounce.”
This phenomenon, in which radio waves are purposefully reflected off some of the atmosphere’s upper layers, vastly extends the distance over which radio operators can communicate. It’s called the “skywave effect” and it’s how the first radio broadcast was sent across the Atlantic Ocean in 1901.
It means that the curvature of the Earth is surmountable. Radio transmissions can zigzag up and down, bouncing between the ground and ionosphere, which sits at an altitude of roughly 80-650km (50-400 miles). You could say that a person’s voice, transmitted 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,” says Cathryn Mitchell, professor of radio science at the University of Bath, “it is really quite astonishing.”The really amazing thing is that the skywave effect is not stable – and scientists still don’t fully understand it. The ionosphere is weird. It fluctuates, it moves around, expands and contracts, and is far from uniform. It is sometimes full of waves itself, says Mitchell, which ripple away during sunrise and sunset – almost like throwing a stone into a pond.
The Sun’s presence or absence is one of the reasons for this. In the daytime, the ionosphere thickens because sunlight strikes atmospheric gases, ionising them to produce electrons. At night, the collisions lessen, and the ionosphere’s lower layer disappears. This night-time thinning allows radio waves to travel much further, because they reach higher altitudes before the electrons bounce them back towards Earth. It is why people have long been able to pick up distant radio stations in the early hours. The ionosphere’s weirdness requires investigation on a large scale. An eclipse provides the perfect opportunity to bring together lots of people in order to test our understanding of what happens to it as it fluctuates, and also at a convenient time – during daylight hours. Plus, eclipses aren’t the same as night-time in terms of how they affect ionisation activity. The shadow cast by the Moon is particular – a point shape that travels quickly across the Earth’s surface. During an eclipse, therefore, unexpected things might happen to the ionosphere, which scientists are keen to observe.
Enter the amateur “ham” radio community, whose members have signed up to a citizen science collective called HamSCI. As the two upcoming American eclipses approach, hundreds of volunteers will begin broadcasting, so they can track their experiences and share them with scientists. Baker is among the volunteers. “They want to be able to hear how signals come and go,” he explains. “No other times do you have the ability to shut the Sun off and turn it right back on.”Leading the experiment is Nathaniel Frissell, a space physicist and electrical engineer at the University of Scranton in Pennsylvania, who founded HamSCI. Via Zoom, he explains why there’s still so much to learn about the ionosphere. He shows me an animation featuring the ionosphere figured as a curved, fuzzy layer neatly sheltering the ground below. “Do you see how smooth this ionosphere is? In real life, the ionosphere is not that smooth,” he says. There are transient lumps and bumps, for want of a better phrase, that scientists are still not good at predicting.
During the upcoming eclipses, Frissell – whose own call-sign is W2NAF – will gather data from, among other sources, individual ham radio operators, volunteers who plan to use highly sensitive transmitting equipment during the events, and online databases that track public radio activity. It is an experiment on a scale that simply would not be possible with standard academic instrumentation alone, he stresses – there’s just not enough of it over a wide enough area. The upcoming experiments follow a similar effort during a total solar eclipse above the US in 2017. Baker took part in that, too – he’s kept the local newspaper cutting with his picture in it.Understanding the ionosphere matters because, during military or disaster response operations, for example, precisely targeted radio communications can make the difference between life and death. It’s crucial for radio operators to know how best to set up their transmission for a successful broadcast. Also, ionospheric variations sometimes affect satellites, says Ruth Bamford of RAL Space, based at the Rutherford Appleton Laboratory, which is part of the UK’s Science and Technology Facilities Council.
Solar flares, for instance, cause the ionosphere to expand, which increases the drag on satellites orbiting the Earth, meaning they might have to get boosted back up to a higher altitude lest they plummet towards the ground.
Bouncing radio waves off the ionosphere is like probing a giant, ever-shifting sea of ethereal matter. “It’s quite a hard thing to do,” says Bamford. “You’ve got a mirror up there that is changing with the Sun.”Bamford praises the radio volunteers.