The ionosphere is a region of the Earth's upper atmosphere with high concentrations of electrically charged particles, or ions. These ions are produced by the intense ionizing radiation from the sun, which strikes molecules of oxygen and nitrogen present in the upper atmosphere, yielding positively charged ions and negatively charged free electrons. The ionosphere forms an electrically conducting layer capable of reflecting radio waves transmitted from the Earth's surface. Because of this unique property, the ionosphere enables radio and television broadcasts to be received at far distances over the Earth's horizon as the radio waves "bounce" between the ionosphere and the surface. The ionosphere is found at an altitude of about 50 miles (80 km).

Within the ionosphere are several layers, identified by letters, which are distinguished by how they interact with radio waves. Layer D is between 50 and 60 miles (80 and 100 km). With the loss of solar radiation at night...