Delores Larson, age 15, of Columbus, Ohio, for her question:

HOW THICK IS THE IONOSPHERE?

The ionosphere is a part of the earth's atmosphere that has many ions, or electrically charged atoms and groups of atoms, as well as free electrons. Cosmic rays and radiation from the sun produce these ions.

Several ionized regions make up the ionosphere. The lowest one, called the D region, starts at an altitude of 30 to 55 miles.  This region has an altitude of 55 to 90 miles. The F region starts at 90 miles and extends up to about 190 miles.

The height and ionization of these regions vary from day to night and changes in solar radiation. D region, for example, almost disappears at night.

The ionosphere makes long distance radio communication possible because it reflects certain radio waves thousands of miles back to the earth. At night, when the regions of the ionosphere rise, radio waves can be received even farther from the transmitter than during the day.