Physics.
the highest of the radio-reflective ionospheric layers, beginning at an altitude of ab. 80 mi. (130 km) and consisting of two parts, the lower part (F 1 layer) being detectable in the daytime only, the higher (F 2 layer or Appleton layer) being constant and constituting the ionospheric layer most favorable for long-range radio communication.
[ 1925-30 ]