The intense solar radiation in this region causes the atoms in the atmospheric gases to breakup into ions. The ionic condition of this area of the atmosphere causes radio waves to bounce back to earth unless there is a disruption from solar storms. The charged particles of solar wind consist mainly of protons and electrons. In a radio, sounds are converted into radio waves which travel through the air at a certain frequency and are then converted back to the original sound. A radio wave is part of the electromagnetic spectrum , a series of waves of both magnetic and electrical energy that vary in frequency.
Radio waves have frequencies lower than visible and infrared light on the electromagnetic spectrum.
When you listen to the radio you can change stations by adjusting the dial. When you do this, you are changing the frequency. AM and Short waves will bounce off the ionosphere if the conditions are right. This makes it possible to send and receive AM and Short wave band radio messages very long distances. Bouncing a radio signal off the ionosphere is called skip.