An Etymological Dictionary of Astronomy and Astrophysics
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Pertaining to or existing in the atmosphere of an astronomical object such as a planet, moon, or star.

→ atmosphere; → -ic.

The absorption of → electromagnetic radiation in the → atmosphere mainly by → water vapor, → carbon dioxide, and oxygen. The atmosphere introduces two more limiting factors in → remote sensing: → atmospheric scattering and → atmospheric turbulence.

→ atmospheric; → absorption.

The large-scale movements of air around areas of high and low pressure whereby heat is distributed on the surface of the Earth. Atmospheric motion is driven by uneven heating of the planet. The atmosphere (and ocean) → transfers the excess heat from → tropics to → poles. The flow is determined by balance between → pressure gradients and the → Coriolis effect.

→ atmospheric; → circulation.

The splitting of starlight into a spectrum in the atmosphere because the atmosphere acts as a refracting prism. This phenomenon brings about a practical problem for spectroscopic observations using a slit. → differential refraction; → atmospheric refraction.

→ atmospheric; → dispersion.

The emission of electromagnetic radiation from the atmosphere due to thermal and → non-thermal processes. → Thermal emission comes mainly from → water vapor. Non-thermal processes result in emission lines oxygen (optical) and OH (near-IR). Atmospheric emission is a very significant source of noise in astronomical observations. See also → airglow, → aurora.

→ atmospheric; → emission.

A process by which a planet loses its atmospheric gases to space. There are three main types: 1) → thermal escape, 2) → suprathermal escape (or → nonthermal escape), and 3) → impact erosion. According to models, the two mechanisms that can most efficiently cause substantial atmospheric loss are hydrodynamic escape and impact erosion (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

→ atmospheric; → escape.

The decrease in the intensity of light from a celestial body due to absorption and scattering by Earth's atmosphere. It increases from the zenith to the horizon and affects short wavelengths more than long wavelengths, so that objects near the horizon appear redder than they do at the zenith.

→ atmospheric; → extinction.

A → subatomic particle produced when → primary cosmic rays, impinge on the Earth's atmosphere producing a particle cascade, in which secondary particles decay into → muons. In the energy range up to 100 → GeV atmospheric muons come mostly from the decay of secondary → pions: π^±→ μ^± + anti-ν_μ. At higher energies, the → kaon contribution to the muon flux become significant, reaching the asymptotic value of 27% at about 10 TeV: K^±→ μ^± + anti-ν_μ.

→ atmospheric; → muon.

A neutrino produced in the collision of → cosmic rays (typically → protons) with nuclei in the → upper atmosphere. This creates a shower of → hadrons, mostly → pions. The pions decay to a → muon and a muon neutrino. The muons decay to an → electron, another muon neutrino, and an electron neutrino.

→ atmospheric; → neutrino.

Noise in radio wavelengths caused by natural atmospheric processes, mainly lightening discharges in thunderstorms. They can affect radio observations.

→ atmospheric; → noise.

The shift in apparent direction of a celestial object caused by the bending of light while passing through the Earth's atmosphere. Since the density of the atmosphere decreases with altitude, the starlight will bend more as it continues down through the atmosphere. As a result, a star will appear higher in the sky than its true direction.

→ atmospheric; → refraction.

The → scattering of → electromagnetic radiation by various particles in the Earth's → atmosphere. The phenomenon is caused by collisions between photons and several scattering agents such as atoms, molecules, → aerosols, and water droplets in clouds. → Rayleigh scattering.

→ atmospheric; → scattering.

Random fluctuations of the atmosphere caused by the constant injection of energy into the atmosphere from solar and local sources, changing the temperature and pressure of the air where it is absorbed and leading to fluid instabilities. The development over time of the instabilities gives rise to fluctuations in the density of air, and therefore the → refractive index of the atmosphere. → turbulence; → seeing.

→ atmospheric; → turbulence.

Gaps in → atmospheric absorption, allowing a range of electromagnetic wavelengths to pass through the atmosphere and reach the Earth.

→ atmospheric; → window.

The method used to detect very brief flashes of → Cherenkov radiation generated by the → cascade shower of → relativistic charged particles produced when a very high-energy → gamma ray (in the range 50 GeV to 50 TeV) strikes the atmosphere at a typical altitude of 10 km. Owing to this technique, it possible to discriminate cosmic gamma rays from the cosmic ray background and to determine their energy and source direction. More specifically, the incoming gamma-ray photon undergoes → pair production in the vicinity of the nucleus of an atmospheric molecule. The electron-positron pairs produced are of extremely high energy and immediately radiate in a → bremsstrahlung process. This radiation is itself extremely energetic, with many of the photons undergoing further pair production. A cascade of charged particles ensues which, due to its extreme energy, produces a flash of Cherenkov radiation lasting between 5 and 20 nano-seconds. The total area on the ground illuminated by this flash corresponds to many hundreds of square meters, which is why the effective area of IACT telescopes should be large.

→ imaging; → atmospheric; → Cherenkov; → technique.