Ionospheres

The ionosphere is a region of Earth’s upper atmosphere where solar radiation ionizes atmospheric gases, creating a plasma of free electrons and positive ions. This ionized layer extends roughly from about 60 to 1,000 kilometers above the surface and overlaps with the region where the atmosphere gradually fades into space.

The ionosphere is commonly described in layers: the D layer (~60–90 km), the E layer (~90–150 km), and the F layer, which during the day splits into F1 and F2 and rises to higher altitudes. The peak electron density typically occurs in the F2 layer at about 250–400 kilometers, with daytime and seasonal variations as well as changes tied to solar activity.

Ionization is driven mainly by solar ultraviolet and X-ray radiation, with additional contributions from energetic particle precipitation during geomagnetic events. The ionosphere’s density and composition vary with local time, season, and the solar cycle; it is generally lower at night when ionizing radiation is reduced.

The ionosphere plays a crucial role in radio communication by refracting and reflecting certain radio waves back toward Earth, enabling long-range transmission in the high-frequency band. It also influences navigation systems such as GPS and is a key component of space weather. Measurements are made using ionosondes, incoherent scatter radars, and GPS-based total electron content data, among other methods.

Ionospheres are not unique to Earth; many planets and some moons with atmospheres possess ionospheres formed by the interaction of solar radiation and the solar wind with atmospheric constituents. Their properties reflect environmental conditions and are of interest in both planetary science and telecommunications.