Editing Ionosphere (section)

==Layers of ionization==
{{more citations needed section|date=December 2024}}
[[File:Ionosphere Layers en.svg|thumb|upright=1.4|Ionospheric layers.]]

At night the F layer is the only layer of significant ionization present, while the ionization in the E and D layers is extremely low. During the day, the D and E layers become much more heavily ionized, as does the F layer, which develops an additional, weaker region of ionisation known as the F{{sub|1}} layer. The F{{sub|2}} layer persists by day and night and is the main region responsible for the refraction and reflection of radio waves.

[[File:Ionospheric layers from night to day.png|thumb|Ionospheric sub-layers from night to day indicating their approximate altitudes]]
[[File:Lightning sprites.jpg|thumb|[[Sprite (lightning)|Lightning sprites]].]]

===D layer===
The D layer is the innermost layer, {{convert|48|to|90|km|mi|abbr=on}} above the surface of the Earth. Ionization here is due to [[Lyman series]]-alpha hydrogen radiation at a [[wavelength]] of 121.6 [[nanometre]] (nm) ionizing [[nitric oxide]] (NO). In addition, [[solar flares]] can generate hard X-rays (wavelength {{nowrap|< 1 nm}}) that ionize N{{sub|2}} and O{{sub|2}}. Recombination rates are high in the D layer, so there are many more neutral air molecules than ions.

Medium frequency (MF) and lower high frequency (HF) [[radio wave]]s are significantly attenuated within the D layer, as the passing radio waves cause electrons to move, which then collide with the neutral molecules, giving up their energy. Lower frequencies experience greater absorption because they move the electrons farther, leading to greater chance of collisions. This is the main reason for [[Ionospheric absorption|absorption of HF radio waves]], particularly at 10&nbsp;MHz and below, with progressively less absorption at higher frequencies. This effect peaks around noon and is reduced at night due to a decrease in the D layer's thickness; only a small part remains due to [[cosmic rays]]. A common example of the D layer in action is the disappearance of distant AM [[broadcast band]] stations in the daytime.

During [[solar proton event]]s, ionization can reach unusually high levels in the D-region over high and polar latitudes. Such very rare events are known as Polar Cap Absorption (PCA) events, because the increased ionization significantly enhances the absorption of radio signals passing through the region.<ref name="Rose1962">{{cite journal|last1=Rose|first1=D.C.|last2=Ziauddin|first2=Syed|title=The polar cap absorption effect|journal=Space Science Reviews|date=June 1962|volume=1|issue=1|page=115|doi=10.1007/BF00174638|bibcode=1962SSRv....1..115R|s2cid=122220113}}</ref> In fact, absorption levels can increase by many tens of dB during intense events, which is enough to absorb most (if not all) transpolar HF radio signal transmissions. Such events typically last less than 24 to 48 hours.

===E layer===
{{main|Kennelly–Heaviside layer}}

The [[Kennelly–Heaviside layer|E layer]] is the middle layer, {{convert|90|to|150|km|mi|sigfig=2|abbr=on}} above the surface of the Earth. Ionization is due to soft X-ray (1–10&nbsp;nm) and far ultraviolet (UV) solar radiation ionization of molecular [[oxygen]] (O{{sub|2}}). Normally, at oblique incidence, this layer can only reflect radio waves having frequencies lower than about 10&nbsp;MHz and may contribute a bit to absorption on frequencies above. However, during intense [[sporadic E]] events, the E{{sub|s}} layer can reflect frequencies up to 50&nbsp;MHz and higher. The vertical structure of the E layer is primarily determined by the competing effects of ionization and recombination. At night the E layer weakens because the primary source of ionization is no longer present. After sunset an increase in the height of the E layer maximum increases the range to which radio waves can travel by reflection from the layer.

This region is also known as the [[Kennelly–Heaviside layer]] or simply the Heaviside layer. Its existence was predicted in 1902 independently and almost simultaneously by the American electrical engineer [[Arthur Edwin Kennelly]] (1861–1939) and the British physicist [[Oliver Heaviside]] (1850–1925). In 1924 its existence was detected by [[Edward V. Appleton]] and [[Miles Aylmer Fulton Barnett|Miles Barnett]].

===E{{sub|s}} layer===
The E{{sub|s}} layer ([[wikt:sporadic#Adjective|sporadic]] E-layer) is characterized by small, thin clouds of intense ionization, which can support reflection of radio waves, frequently up to 50&nbsp;MHz and rarely up to 450&nbsp;MHz. Sporadic-E events may last for just a few minutes to many hours. [[Sporadic E propagation]] makes VHF-operating by [[Amateur radio high bands|radio amateurs]] very exciting when long-distance propagation paths that are generally unreachable "open up" to two-way communication. There are multiple causes of sporadic-E that are still being pursued by researchers. This propagation occurs every day during June and July in northern hemisphere mid-latitudes when high signal levels are often reached. The skip distances are generally around {{convert|1640|km|mi|abbr=on}}. Distances for one hop propagation can be anywhere from {{convert|900|to|2500|km|mi|abbr=on}}. Multi-hop propagation over {{convert|3500|km|mi|abbr=on}} is also common, sometimes to distances of {{convert|15000|km|mi|abbr=on}} or more.

===F layer===
{{main|F region}}

[[File:Msis atmospheric composition by height.svg|thumb|350px|Main gases of the ionosphere (about {{convert|50|km|mi|abbr=on|disp=semicolon}}and above on this chart) vary considerably by altitude]]

The [[F region|F layer]] or region, also known as the Appleton–Barnett layer, extends from about {{convert|150|km|mi|sigfig=2|abbr=on}} to more than {{convert|500|km|mi|sigfig=2|abbr=on}} above the surface of Earth. It is the layer with the highest electron density, which implies signals penetrating this layer will escape into space. Electron production is dominated by [[extreme ultraviolet]] (UV, 10–100&nbsp;nm) radiation ionizing atomic oxygen. The F layer consists of one layer (F{{sub|2}}) at night, but during the day, a secondary peak (labelled F{{sub|1}}) often forms in the electron density profile. Because the F{{sub|2}} layer remains by day and night, it is responsible for most [[skywave]] propagation of [[radio]] waves and long distance [[high frequency]] (HF, or [[shortwave]]) radio communications.

Above the F layer, the number of [[oxygen]] ions decreases and lighter ions such as hydrogen and helium become dominant. This region above the F layer peak and below the [[plasmasphere]] is called the topside ionosphere.

From 1972 to 1975 [[NASA]] launched the [[AEROS (satellite)|AEROS and AEROS B]] satellites to study the F region.<ref name="Yenne">{{cite book|author=Yenne, Bill|title=''The Encyclopedia of US Spacecraft''|publisher=Exeter Books (A Bison Book), New York|date=1985|isbn=978-0-671-07580-4}} p. 12 '''AEROS'''</ref>