Observatories probe the ionosphere at vertical incidence. What term describes the highest frequency that a region can reflect at the time?

Critical frequency is defined as the highest frequency that will be returned to Earth when a radio wave is transmitted straight up (at vertical incidence) into the ionosphere. If a signal is transmitted vertically at a frequency higher than the critical frequency, it will penetrate the ionosphere and escape into space. This differs from the Maximum Usable Frequency (MUF), which refers to the highest frequency that can be used for communication between two specific locations at a shallower angle. The key phrase to watch for is 'vertical incidence,' which strictly points to the critical frequency.