From AMS Glossary
The layer of the ionosphere that exists as an appendage on the lower part of the [[F2-layer]] during the day.
It exhibits a distinct maximum of free-electron density, except at high latitudes in winter, when it is not detectable. Its virtual height ranges from 200 to 300 km, being lowest at around noon. The F1-layer is formed by absorption of solar radiation in the extreme ultraviolet (EUV) range of wavelengths and is often roughly similar in shape to an idealized Chapman layer.