However, the ionosphere has a very low density. As a matter of fact, the return from the ionosphere is the equivalent of the return you would get from illuminating a penny in the atmosphere, at a height of 300 km. This is equivalent to the distance from Boston to New York. This is the time in Greenwich, England, and is also, in meteorological circles, referred to as Z, or Zulu time. Similar to a weather radar that you may see on the local news, the colors indicate the intensity of the radar return.
Red areas correspond to high electron densities and blue-green areas correspond to low densities.
Note that, during the day electron densities are higher and the red area is closer to the Earth's surface. At night, this area is much higher. In addition, the higher electron densities near the surface during the day can interfere with radio waves. If you want your students to follow along with an experiment as it runs, you may check the Millstone Radar schedule . The implications of knowing the electron density in the ionosphere goes well beyond simply determining how far away you can be from the WEEI radio tower and still be able to listen to Pedro Martinez chalking up another win.