When a signal is returned to Earth by the ionosphere, what is this called?

Sky-wave propagation, often referred to as 'skip', occurs when radio signals travel upward into the ionosphere and are refracted (bent) back down to the Earth's surface. This is the primary mechanism that allows for worldwide long-distance communication on the HF (High Frequency) bands. Other options describe entirely different physical paths. Ground-wave propagation relies on waves hugging the curvature of the Earth, tropospheric propagation happens in the lower weather-producing atmosphere (affecting VHF/UHF), and Earth-Moon-Earth involves bouncing signals off the Moon's surface.