Fr.: paramètre de Rossby
The northward variation of the Coriolis parameter, arising from the sphericity of the Earth.
Fr.: paramètre de taille
A quantity that defines the type of → scattering.
Fr.: paramètre de pente
In a → power-law distribution or → regression, the → exponent that represents the effect of the → independent variable, x, on the → dependent variable, y. X has no association with y if the slope parameter = 0 and x has strong association with y if the slope parameter is large.
Fr.: paramètres de Stokes
Four parameters which are needed to fully describe the
→ polarization state of
→ electromagnetic radiation.
They involve the maximum and minimum intensity, the ellipticity,
and the direction of polarization.
The four Stokes parameters are traditionally defined as follows:
Fr.: paramètre de Tisserand
In celestial mechanics, a combination of orbital elements commonly used to distinguish between comets and asteroids. Objects whose Tisserand's parameter value is smaller than 3 are considered to be dynamically cometary, and those with a value larger than 3 asteroidal. Also called Tisserand's invariant.
Named after François Félix Tisserand (1845-1896), French astronomer, Director of the Paris Observatory (1892).
Fr.: paramètre de Toomre
A quantity that measures the stability of a differentially rotating disk of matter against → gravitational collapse. It is expressed by the relation: Q = csκ / πGΣ, where cs is the → sound speed, κ the → epicyclic frequency, G the → gravitational constant, and Σ the → surface density. The disk is linearly stable for Q > 1 and linearly unstable for Q < 1.
After Alar Toomre (1936-), an American astrophysicist of Estonian origin, professor of mathematics at the Massachusetts Institute of Technology; → parameter.
Fr.: paramètre du viriel
A dimensionless parameter that measures the ratio of thermal plus kinetic energies to gravitational energy of a physical system, such as a molecular cloud. The virial parameter is expressed as: αvir = 5σ2R / GM, where R and M are the radius and mass of the cloud respectively, σ is the one-dimensional → velocity dispersion inside the cloud, and G the → gravitational constant. It indicates whether a cloud could be bound or not. For molecular clouds that are confined by their surface pressure and for which self-gravity is unimportant, αvir is much larger than unity, whereas αvir is ~ 1 when the gravitational energy of a clump becomes comparable to its kinetic energy. See, e.g., Bertoldi & McKee, 1992 (ApJ 395, 140). See also → virial theorem.