Electromagnetic radiation — it might sound like something that you’d be better off avoiding, but electromagnetic waves of various kinds underpin our senses and how we interact with the world — from the light emissions through which your eyes perceive these words, to the microwaves that carry the Wi-Fi signal to your laptop or phone on which you’re reading it.
More or less every form of modern communication is carried by electromagnetic waves. They whisk through the antenna on your car, travel through walls whenever you need to make a phone call inside, yet also inexplicably reflect from seemingly nothing in the Earth’s upper atmosphere.
This happens because the atmosphere becomes a plasma at high altitudes — a state of matter where atoms split apart and electrons are no longer bound to their parent nuclei. Plasmas have interesting properties, as they react very strongly to electromagnetic fields. In this case usefully: at low enough frequencies it becomes possible to bounce radio signals around the world, extending their range.