Why is the F2 region mainly responsible for the longest distance radio-wave propagation?

The main reason the F2 region is responsible for the longest distance radio-wave propagation is that it is the highest layer of the ionosphere. Situated at altitudes typically between 200 and 400 kilometers above the Earth's surface, the F2 layer has the capacity to reflect radio waves effectively due to its high electron density, which increases during the solar daytime.

During the daytime, the ionization levels are higher due to solar radiation, enabling this region to reflect higher frequency radio waves back towards the Earth, facilitating long-distance communication. The ability to support a range of frequencies, along with its altitude, allows radio waves transmitted from one point to travel farther before dissipating, making it crucial for long-distance radio communication.

Other choices do not accurately explain the primary reason for long-distance propagation. For example, while the thickness of a layer or its specific power capabilities may contribute to its characteristics, these factors do not directly correlate with the extent of radio-wave propagation as effectively as its altitude does. The F2 region’s position at the top of the ionosphere allows it to maintain strong ionization levels that are optimal for reflecting radio waves over vast distances.