From AMS Glossary
The highest permanently observable layer of the ionosphere.
It exhibits a distinct maximum of free-electron density occurring at a height that ranges from about 225 km in the polar winter to over 400 km in daytime near the magnetic equator. Like the other ionospheric layers, the F2-layer is formed by absorption of short-wavelength solar radiation, but its behavior and properties are more complex. Unlike the other ionospheric layers, the F2-layer tends to rise during the middle of the day, except at middle to high latitudes in winter. Its maximum electron density occurs during the day, its minimum usually just before sunrise. It is the layer that is most useful for long-range radio transmission.