What term refers to the maximum frequency returned by the ionosphere when aimed vertically?

The term that describes the maximum frequency returned by the ionosphere when a signal is aimed vertically is known as the critical frequency. This frequency is specific to the ionosphere's characteristics at a given time, based on the ionization levels and the atmospheric conditions. When a radio wave is transmitted vertically, the critical frequency is the highest frequency at which the wave can be successfully reflected back to Earth by the ionosphere.

Above this frequency, the wave will not be reflected but instead will penetrate through the ionosphere into space. This concept is crucial for radio communication, as it defines a limit for effective communication using ionospheric propagation during different times of the day and in varying atmospheric conditions. Understanding critical frequency is essential for operators to optimize the transmission frequencies for effective radio communication.