The Kennelly–Heaviside layer, commonly called the E region of the ionosphere, is a partly ionized layer of Earth's upper atmosphere located roughly between 90 km and 150 km above the surface. Solar ultraviolet and X‑ray radiation ionize neutral molecules there, creating free electrons and positive ions; the balance between ionization and recombination determines the electron density. This region is important because it can refract and sometimes reflect radio waves back toward the ground, enabling long‑distance communication by skywave propagation.

Characteristics and behavior

The E region is distinguished by moderate electron densities that vary with solar activity, local time, season and geographic latitude. During daytime photoionization is stronger and the layer is better developed; at night the electron density usually drops because recombination dominates. The layer is also the site of transient phenomena known as "sporadic E," in which localized, dense patches of ionization form and can reflect higher‑frequency signals than the background E region. Sporadic E can enable unexpected long‑range reception of VHF signals such as FM radio and VHF television.

How it affects radio and navigation

  • Medium‑frequency (MF) and lower high‑frequency (HF) radio waves may be refracted or reflected by the E region under suitable conditions, permitting skywave propagation beyond the line of sight.
  • Sporadic E can occasionally reflect much higher frequencies, producing unusual propagation events on VHF bands.
  • The E region contributes to ionospheric delays and signal scattering that can affect radio navigation and satellite communication, although the F region generally dominates long‑range GPS errors.

History and discovery

At the turn of the twentieth century two scientists independently proposed the existence of an ionized atmospheric layer that could explain radio signals being received at distances beyond the horizon. The American engineer Arthur E. Kennelly and the British physicist Oliver Heaviside each advanced ideas that anticipated the later concept of the E region. Subsequent experimental work in radio propagation and dedicated sounding techniques confirmed the presence and properties of the layer. Early long‑distance radio successes—such as Guglielmo Marconi's transatlantic experiments—were made before the ionospheric mechanism was fully understood; later analysis linked those early transmissions to ionospheric reflection and propagation effects (Marconi's transatlantic tests, early 20th‑century studies).

Measurement, research and practical importance

Scientists study the E region using vertical‑incidence ionosondes that produce ionograms, as well as by incoherent scatter radars, sounding rockets and satellite instruments. These measurements reveal the layer's vertical structure, its diurnal and seasonal variability, and transient events such as sporadic E and mid‑latitude summer enhancements. Understanding the E region helps engineers predict radio propagation conditions for broadcasting, aviation communications, amateur radio, and certain military and scientific applications.

Notable distinctions

The Kennelly–Heaviside layer (E region) is one component of the multilayer ionosphere, which also includes the lower D region and the higher F region(s). Each layer has different typical altitudes, electron densities and effects on radio waves. While the E region plays a central role for some frequency ranges and transient propagation phenomena, the F region is usually responsible for long‑distance HF communication, especially at night.

Because the ionosphere is driven by solar radiation and the Earth's magnetic field, its state changes continually; forecasts and real‑time monitoring are therefore important for managing radio services that depend on ionospheric propagation.