Radio waves in these frequency ranges can be used for very long distance (transcontinental) communication because they can reflect off layers of charged particles in the ionosphere and return to Earth beyond the horizon, a mechanism called skywave or “skip” propagation.
Radio propagation and possible communication distances vary depending on the time of day, the season and the level of solar activity.
These bands are used by powerful long range AM radio stations, many operated by governments, which broadcast to multiple countries.
It is particularly useful in providing emergency communication where standard telecommunications infrastructure is compromised or nonexistent, such as a disaster area or remote region of the globe.
Similar rules exist in Europe, where it has become necessary for European amateurs to police the bands due to overcrowding.
Above 10 MHz there are numerous frequencies set aside for radio astronomy, space research (FCC terminology) and standard- frequency-and-time services.
RF diathermy equipment uses 27.12 MHz to heat bulk materials or adhesives for the purpose of drying or improving curing.