As the moon’s shadow races across North America on August 21, hundreds of radio enthusiasts will turn on their receivers — rain or shine. These observers aren’t after the sun. They’re interested in a shell of electrons hundreds of kilometers overhead, which is responsible for heavenly light shows, GPS navigation and the continued existence of all earthly beings.
This part of the atmosphere, called the ionosphere, absorbs extreme ultraviolet radiation from the sun, protecting life on the ground from its harmful effects. “The ionosphere is the reason life exists on this planet,” says physicist Joshua Semeter of Boston University.
It’s also the stage for brilliant displays like the aurora borealis, which appears when charged material in interplanetary space skims the atmosphere. And the ionosphere is important for the accuracy of GPS signals and radio communication.