ATO Mission Computers Practice Test

What causes the night effect?

The changing height of the ionosphere causing it to reflect skywaves. Variations in transmitter power. Ground conductivity changes during night. Thermal noise in receivers.

Correct Answer:

The changing height of the ionosphere causing it to reflect skywaves.

Explanation:

The night effect comes from the ionosphere changing its properties when solar radiation drops. During the day the D layer is present and absorbs lower HF signals, limiting how far they can skip. At night the D layer fades away, reducing that absorption, and the main reflecting layer (the F layer) also shifts in height. This change in height and density means skywaves are refracted back to Earth from higher altitudes, increasing the distance over which they can travel and be received. In short, the ionosphere’s height and reflective behavior change with the day–night cycle, enabling longer-range skywave propagation.

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