In discussing HF ionospheric propagation, you hear about both "critical" and "maximum usable" frequencies. For example:
Above the critical frequency, the ionosphere is unable to refract the signal back to Earth and it escapes to space. The critical frequency during the day is in the neighborhood of 6m: depending on the space weather, 6m way work for skywave, or it may not.
I've found maps for both F2-Layer Critical Frequency and Maximum Usable Frequency, and they're quite different — the Maximum Usable Frequency map seems much more "optimistic", that is, the frequencies are much higher at any given location.
What's the difference between the two measures?