F-层

{{#category_index:F|F-layer}} 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.