What defines the highest frequency at which waves sent vertically upward are reflected directly back to earth?

The highest frequency at which waves sent vertically upward will be reflected directly back to Earth is known as the critical frequency. This concept is crucial in radio wave propagation as it determines the maximum frequency that can be effectively transmitted via the ionosphere. When a radio wave is transmitted upward, it can be refracted back to Earth, but only if its frequency is below this critical threshold. Frequencies above the critical frequency tend to continue into space rather than returning.

The critical frequency is influenced by the ionosphere's electron density, which varies with time of day, solar activity, and atmospheric conditions. Understanding this frequency allows operators to select appropriate frequencies for effective communication, particularly for long-distance transmissions using ionospheric propagation.

The terms reflection frequency, propagation frequency, and transmission frequency do not specifically define this phenomenon in the context of radio waves reflecting off the ionosphere, which is why they do not serve as suitable answers to the question.