1. One of the layers of ionized air in the ionosphere; it reflects radio waves up to about 50 MHz. In the daytime the F-layer subdivides into two layers, the lower of which (F1) is usually 175 to 250 km high and is found only during daylight. The higher F2 layer at 250 to 400 km is present both night and day and is the principal reflector of HF radio waves at night. See Figure A-22. 2. A transition layer between the Earth’s outer and inner cores; see Figure E-1.