Ham Amateur Radio Technician Practice Exam

Which ionospheric region is closest to the surface of Earth?

1. The E region

2. The D region

3. The F region

4. The A region

The correct answer is: The D region

The D region is the ionospheric layer that is closest to the Earth's surface, typically located between about 30 to 90 miles (48 to 144 kilometers) above sea level. It plays a critical role in radio wave propagation, particularly at lower frequencies, where it tends to absorb electromagnetic radiation, especially during the daytime. This absorption occurs due to the presence of free electrons generated by solar radiation interacting with atmospheric gases. The characteristics of the D region significantly affect radio communication. For instance, it is more pronounced during the daytime when solar ionization is strongest, leading to a situation where higher frequencies may not be effectively transmitted as they could be during nighttime when the D region's density decreases. In contrast, the E region generally lies above the D region and is involved in some reflection phenomena, while the F region is even higher and reflects radio waves at various frequencies, facilitating long-distance communication. The A region, however, is not formally classified as part of the main ionospheric layers and is often a confusion point. Understanding the structure and function of these layers helps amateur radio operators predict radio wave propagation under various conditions.