The ionosphere is the ionized portion of Earth's atmosphere produced primarily by solar extreme ultraviolet (EUV) and X‑ray radiation, extending from about 80 km to more than 600 km and forming the interface where the neutral upper atmosphere meets near‑Earth space. Its plasma affects radio propagation, satellite communication, and navigation systems through refraction, reflection, absorption, and scintillation of trans‑ionospheric signals, and varies with local time, latitude, season, and the solar cycle (NOAA Space Weather Prediction Center; Britannica; NASA Science). (swpc.noaa.gov)

Structure and layers

• –D region (about 50–90 km): Dominated by high electron‑neutral collision frequencies and molecular/cluster ions, this region is chiefly responsible for daytime absorption of high‑frequency (HF) radio waves; absorption intensifies during solar X‑ray flares and with solar proton precipitation (NOAA SWPC D‑region absorption documentation). (swpc.noaa.gov)
• –E region (about 90–150 km): Formed mainly by photoionization of N₂ and O₂, including transient, thin “sporadic E” layers composed largely of long‑lived metallic ions of meteoric origin concentrated by wind shear (Annales Geophysicae; Atmospheric Chemistry and Physics; Atmospheric Chemistry and Physics). (angeo.copernicus.org)
• –F region (≈150–600+ km): Typically splits into F1 and F2 in daylight; the F2 layer at ≳220 km holds the peak electron density and is the principal refractor/reflector for long‑range HF skywave communication. Above ~200 km, O⁺ is typically the dominant ion in daylight, with composition and density modulated by dynamics and solar conditions (Space Weather, IRI‑2016; NASA NTRS). (merit.url.edu)

Composition and photochemistry Photoionization by the Sun produces ion pairs; in the lower ionosphere, rapid ion‑molecule reactions create molecular and hydrated cluster ions (e.g., NO⁺·(H₂O)_n, O₂⁺·(H₂O)_n), whereas in the F2 region atomic oxygen ions (O⁺) prevail. Recombination (dissociative and radiative) and transport establish the electron density profile, which follows approximate Chapman‑type production during the day and decays after sunset, thinning the ionosphere on the nightside. Studies of cluster‑ion chemistry and hydrated complexes help explain D‑region absorption behavior (Physical Chemistry Chemical Physics; NASA ICON overview; NOAA SWPC D‑region absorption documentation; [The Earth's Ionosphere](book://Michael C. Kelley|The Earth's Ionosphere: Plasma Physics and Electrodynamics|Academic Press|2009); [Ionospheres](book://R. W. Schunk & A. F. Nagy|Ionospheres: Physics, Plasma Physics, and Chemistry|Cambridge University Press|2009)). (pubs.rsc.org)

Dynamics and variability Solar EUV/X‑ray flux varies by nearly an order of magnitude over the ≈11‑year solar cycle, imprinting strong climatological changes on electron density and layer heights. Space weather drivers (flares, coronal mass ejections, and geomagnetic storms) alter the ionosphere through enhanced ionization, electric fields, and thermospheric winds, frequently increasing total electron content (TEC) and disturbing radio systems. Coupling from the lower atmosphere via tides and gravity waves also modulates ionospheric structure (NOAA SWPC; NOAA Space Weather and GPS Systems; NASA “Revolutions in Understanding the Ionosphere”). (swpc.noaa.gov)

Equatorial Ionization Anomaly and equatorial processes Near the magnetic equator, the E‑region dynamo and pre‑reversal enhancement produce an upward E×B drift that lifts plasma, which then diffuses along field lines to form the Equatorial Ionization Anomaly (EIA) with crests on either side of the equator. Satellite imaging has revealed rapid changes and unusual X‑ and C‑shaped patterns in the EIA, underscoring the ionosphere’s day‑to‑day “weather” (NASA GOLD; NASA Science; NOAA repository, JGR Space Physics, 2024). (nasa.gov)

Aurora and airglow Energetic particle precipitation along geomagnetic field lines deposits energy into the ionosphere, producing Aurora with characteristic altitude‑dependent emissions (e.g., O‑green at ~100–200 km, O‑red above ~200 km). Separate from aurora, photochemical reactions generate persistent airglow bands that enable global remote sensing of ionospheric density and winds (NASA Science: Auroras; NASA ICON). (science.nasa.gov)

Radio propagation and navigation impacts The ionosphere refracts, reflects, or absorbs radio waves depending on frequency and density structure. Long‑range HF “skywave” links exploit refraction from the E/F layers, with performance quantified by parameters such as critical frequency (fo) and maximum usable frequency (MUF) standardized by the International Telecommunication Union (ITU‑R P.533). At VHF/UHF and microwave bands used by the Global Positioning System, integrated electron density along the signal path (TEC) introduces group delay and phase advance; disturbed conditions can cause scintillation that degrades or breaks receiver lock (NOAA SWPC: Total Electron Content; NOAA SWPC: Ionospheric Scintillation; NOAA SWPC: Space Weather and GPS Systems). (itu.int)

Total Electron Content (TEC) TEC is the line‑of‑sight integral of electron density; by convention, 1 TEC unit (TECU) equals 10^16 electrons/m². Vertical TEC commonly ranges from a few to several hundred TECU, with strong enhancements during geomagnetic storms. NOAA’s operational GloTEC and U.S./North America TEC products provide near‑real‑time situational awareness for GNSS users (NOAA SWPC GloTEC; NOAA SWPC NA‑TEC; NCEI TEC overview). (swpc.noaa.gov)

Observation and modeling The ionosphere is monitored with ionosondes (vertical HF sounding), incoherent scatter radars, GNSS dual‑frequency receivers for TEC, radio occultation from low‑Earth‑orbiting satellites (e.g., COSMIC‑2/FORMOSAT‑7), all‑sky imagers, and spaceborne ultraviolet instruments. The International Reference Ionosphere (IRI‑2016) is the widely used empirical standard, maintained under COSPAR/URSI, and is routinely evaluated and refined with new datasets; operational data‑assimilation systems blend observations with IRI‑based backgrounds to yield global electron density analyses (NASA CCMC: IRI‑2016; Space Weather, IRI‑2016; Annales Geophysicae, IRI evaluation; NOAA NESDIS: COSMIC‑2; NOAA NESDIS news). (ccmc.gsfc.nasa.gov)

Missions and recent advances NASA’s ICON (launched 2019) and GOLD (launched 2018) provided complementary low‑Earth‑orbit and geostationary views that revealed rapid regional changes, new morphology in the EIA, and detailed airglow dynamics; ICON concluded operations after completing its prime mission, while GOLD continues to monitor the upper atmosphere with ~30‑minute cadence (NASA ICON mission page; NASA mission update; NASA GOLD overview). (science.nasa.gov)

History The existence of an ionized, radio‑reflecting region was theorized by A. E. Kennelly and O. Heaviside (the Kennelly–Heaviside layer). In 1924–1926, Edward V. Appleton and colleagues provided direct experimental verification and discovered a higher reflecting region, later termed the Appleton (F) layer. Appleton received the 1947 Nobel Prize in Physics for investigations of the upper atmosphere and the ionosphere (Britannica; Nobel Prize facts; Nobel biographical note). (britannica.com)

Applications and standards Practical uses include HF beyond‑line‑of‑sight communication, over‑the‑horizon radar, frequency management via MUF/foF2 forecasting, and GNSS error mitigation through dual‑frequency techniques and TEC‑driven corrections. International recommendations (e.g., ITU‑R P.533) and national space‑weather services provide methods and products for predicting ionospheric impacts on systems (ITU‑R P.533; NOAA SWPC TEC; NOAA SWPC scintillation). (itu.int)