Radio Communication and the Ionosphere

The ionosphere controls HF propagation, based on factors like solar activity, time of day, and frequency.

The ionosphere is a region in the upper-atmosphere that meets space (exosphere).

Figure 1: Ionosphere (Thermosphere) is part Earth's Atmosphere

When solar energy ionizes gases 48 to 800 kilometers above the earth's surface, it produces a mixture of free electrons and ions (known as "plasma").

The high densities of free electrons significantly affect radio communication.

Scientists divide the ionosphere into regions, from top to bottom:

1. Skywave communications rely heavily on the uppermost F region of the ionosphere. This region is divided into F1 (lower) and F2 (higher). Solar extreme ultraviolet (EUV) radiation causes the most ionization between 200 and 300 km at noon.
2. The E region affects radio waves differently compared to other regions.
3. The D region is the lowest. The largest part of the HF absorption takes place in this region, in particular frequencies lower than 10 MHz.

Figure 2: Illustration of Ionospheric regions

The ionospheric regions do not have sharp boundaries, and the altitudes at which they occur vary during the course of a day and from season to season.

What makes the ionospheric regions distinct?

Figure 3: Typical Ionospheric electron density profiles

The above graph is based on a review from U.C. Berkeley by Bob Brown, Ph.D, NM7M (SK).↗

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