From AMS Glossary
(Also called Heaviside layer, Kennelly–Heaviside layer.) The region of the ionosphere usually found at an altitude between 100 and 120 km.
It exhibits one or more distinct maxima and sharp gradients of free electron density. It is most pronounced in the daytime but does not entirely disappear at night. Ionosonde recordings show that the E-region is often subdivided into two or more "E-layers," while localized and intermittent regions of fairly high ionization, known as sporadic E-layers, are also frequently observed. The E- region is produced by absorption of solar radiation at a variety of extreme ultraviolet (EUV) and x-ray wavelengths.