An Etymological Dictionary of Astronomy and Astrophysics
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The interstellar matter outside condensed molecular clouds. Diffuse interstellar medium consists of a hot intercloud medium, a warm intercloud medium, and a cold neutral medium with hydrogen atom densities n_H ~ 0.003, ~ 0.25, and ~ 40 cm^-3, and mean gas → kinetic temperatures T_k ~ 5 x 10⁵, ~ 10⁴, and 80 K, respectively.

→ diffuse; → interstellar; → medium.

A type of → molecular cloud in which the → interstellar radiation field is sufficiently attenuated, so that the local fraction of → molecular hydrogen (H₂) becomes substantial (> 0.1). However, enough interstellar radiation is still present to → photoionize any atomic carbon, or to → photodissociate → carbon monoxide (CO) such that carbon is predominantly still in the form of C⁺ (> 0.5). In steady state, diffuse molecular clouds must necessarily be surrounded by diffuse atomic gas, in order to provide the → shielding of radiation. This means that most sightlines that cross a diffuse molecular cloud will also cross → diffuse atomic gas (Snow & McCall, 2006, ARA&A 44, 367).

→ diffuse; → molecular; → cloud.

An irregularly shaped and low density interstellar cloud visible in the optical wavelengths.

→ diffuse; → nebula.

Reflection of light from a rough or granular surface, which takes place in all directions due to the microscopic irregularities of the interface; opposed to → specular reflection.

→ diffuse; → reflection.

Transmission accompanied by diffusion or scatter to the extent that there is no regular or direct transmission.

→ diffuse; → transmission.

A device used to scatter or disperse light emitted from a source.

From → diffuse + -er.

From paxš, present stem of paxšidan, → diffuse, + -andé or -gar (→ detector).

1) Movement of a gas or liquid as a result of the random thermal motion of its atoms or molecules.
2) The random spreading out of a beam of radiation on reflection (→ diffuse reflection) or transmission through a translucent medium (such as amber, milk, or frosted glass).
3) In chemical thermodynamics, the process of attaining an equilibrium distribution of the → concentration within the phases. A result of diffusion at constant equilibrium is the equalization of the → chemical potentials.
Related terms: → diffraction; → dispersion; → distribution; → scatter; → scattering.

L. diffusionem, from stem of diffundere "scatter, pour out," from dif- "apart, in every direction," → dis-, + fundere "to melt, cast, pour out," from PIE *gheud-, from root *gheu- "to pour."

Paxš, verbal noun and stem of paxšidan→ diffuse.

A factor of proportionality involved in the → diffusion equation. It may be defined as the amount of the quantity diffusing across a unit area through a unit concentration gradient in unit time. → magnetic diffusivity.

→ diffusion; → coefficient.

An equation that expresses the time rate of change of a quantity in terms of the product of the diffusion coefficient and the → Laplacian operating on the quantity. For example the diffusion equation for temperature is: ∂T/∂t = D∇²T.

→ diffusion; → equation.

A narrow boundary layer above the solar → photosphere, between two magnetic field lines, where the plasma becomes demagnetized or unfrozen. The presence of a localized magnetic region is necessary for → magnetic reconnection.

→ local; → -ize; → diffusion; → region.

Tending to diffuse; characterized by → diffusion.

→ diffuse + → -ive.

1) The ability to permit or undergo diffusion.
2) → magnetic diffusivity.

→ diffusive; → -ity.

An act or instance of disqualifying; the state of being disqualified.

→ disqualify; → -tion.

(of a person) declared ineligible for an office, activity, or competition because of an offence or infringement (OxfordDictionaries.com).

→ dis-; → qualified.

To deprive of qualification or fitness; render unfit; incapacitate (Dictionary.com).

→ dis-; → qualify.

Effect of the relative motion of a wave source (light, sound) and the observer. If the source is moving away, the wavelength is stretched (shifted toward lower frequencies). If the source is approaching, the wavelength is compressed (shifted toward higher frequencies). These effects, known as Doppler shifts, are in the case of light waves called redshift and blueshift, respectively.

→ Doppler effect; → shift.

An instability involving two layers of fluid with opposite gradients of properties. Same as → fingering instability. See also → salt finger. Double-diffusive instabilities commonly occur in any astrophysical fluid that is stable according to the → Ledoux criterion, as long as the entropy and chemical stratifications have opposing contributions to the dynamical stability of the system. They drive weak forms of convection, and can cause substantial heat and compositional → mixing. Two cases can be distinguished. In fingering convection, entropy is stably stratified (∇ - ∇ad < 0), but chemical composition is unstably stratified (∇μ < 0); it is often referred to as → thermohaline convection by analogy with the oceanographic context in which the instability was first discovered. In oscillatory double-diffusive convection, entropy is unstably stratified (∇ - ∇ad > 0), but chemical composition is stably stratified (∇μ > 0); it is related to semiconvection, but can occur even when the → opacity is independent of composition (P. Garaud, 2014, arXiv:1401.0928).

→ double;→ diffusive; → system.

1a) General: A driving movement or force; impulse; impetus; pressure.
1b) Physics: A slight change of a quantity with time, for example the sensitivity of an electronic detector continuously operated during a long period as an effect of continued use.
1c) A slow change in frequency of a radio transmitter.
1d) Aerospace: The gradual deviation of a rocket or guided missile from its intended trajectory.
2) (v.intr.) To be carried along by currents of water or air, or by the force of circumstances.

From M.E. drift, from O.E. drifan "to drive," or from O.N. or M.Du. drift, from P.Gmc. *driftiz, related to *dribanan "to drive."

Delek from Lori, Laki, Hamadâni, Malâyeri "push, shove, drive;" variants Gilaki duko, Tâleši dako, Baluchi dhakkk(a) "push, shove, blow," Choresmian dh- "to hit," Kurd. dân/di- "to beat, hit," Proto-Iranian *daH- "to beat, hit, strike" (Cheung 2007); PIE base *dhen- "to hit, push;" delekidan, verb from delek.

In radio astronomy, the output response as a function of position for a given filter as the source passes through the beam.

→ drift; → curve.

The amount of drift, in any of its several senses, per unit time.

→ drift; → rate.