Radiative equilibrium is one of the several requirements for thermodynamic equilibrium, but it can occur in the absence of thermodynamic equilibrium. There are various types of radiative equilibrium, which is itself a kind of dynamic equilibrium.

## Definitions

There are several types of radiative equilibrium.

### Prevost's definitions

An important early contribution was made by Pierre Prevost in 1791.[1] Prevost considered that what is nowadays called the photon gas or electromagnetic radiation was a fluid that he called "free heat". Prevost proposed that free radiant heat is a very rare fluid, rays of which, like light rays, pass through each other without detectable disturbance of their passage. Prevost's theory of exchanges stated that each body radiates to, and receives radiation from, other bodies. The radiation from each body is emitted regardless of the presence or absence of other bodies.[2][3]

Prevost in 1791 offered the following definitions (translated):

Absolute equilibrium of free heat is the state of this fluid in a portion of space which receives as much of it as it lets escape.

Relative equilibrium of free heat is the state of this fluid in two portions of space which receive from each other equal quantities of heat, and which moreover are in absolute equilibrium, or experience precisely equal changes.

Prevost went on to comment that "The heat of several portions of space at the same temperature, and next to one another, is at the same time in the two species of equilibrium."

Following Planck (1914),[4] a radiative field is often described in terms of specific radiative intensity, which is a function of each geometrical point in a space region, at an instant of time.[5][6] This is slightly different from Prevost’s mode of definition, which was for regions of space. It is also slightly conceptually different from Prevost’s definition: Prevost thought in terms of bound and free heat while today we think in terms of heat in kinetic and other dynamic energy of molecules, that is to say heat in matter, and the thermal photon gas. A detailed definition is given by Goody and Yung (1989).[6] They think of the interconversion between thermal radiation and heat in matter. From the specific radiative intensity they derive ${\displaystyle \mathbf {F} _{\nu }}$, the monochromatic vector flux density of radiation at each point in a region of space, which is equal to the time averaged monochromatic Poynting vector at that point (Mihalas 1978[7] on pages 9–11). They define the monochromatic volume-specific rate of gain of heat by matter from radiation as the negative of the divergence of the monochromatic flux density vector; it is a scalar function of the position of the point:

${\displaystyle h_{\nu }=-\nabla \cdot \mathbf {F} _{\nu }}$.

They define (pointwise) monochromatic radiative equilibrium by

${\displaystyle \nabla \cdot \mathbf {F} _{\nu }=0}$ at every point of the region that is in radiative equilibrium.

They define (pointwise) radiative equilibrium by

${\displaystyle h=\int _{0}^{\infty }h_{\nu }d\nu =0}$ at every point of the region that is in radiative equilibrium.

This means that, at every point of the region of space that is in (pointwise) radiative equilibrium, the total, for all frequencies of radiation, interconversion of energy between thermal radiation and energy content in matter is nil. Pointwise radiative equilibrium is closely related to Prevost's absolute radiative equilibrium.

Mihalas and Weibel-Mihalas (1984)[5] emphasise that this definition applies to a static medium, in which the matter is not moving. They also consider moving media.

Karl Schwarzschild in 1906[8] considered a system in which convection and radiation both operated but radiation was so much more efficient than convection that convection could be, as an approximation, neglected, and radiation could be considered predominant. This applies when the temperature is very high, as for example in a star, but not in a planet's atmosphere.

Subrahmanyan Chandrasekhar (1950, page 290)[9] writes of a model of a stellar atmosphere in which "there are no mechanisms, other than radiation, for transporting heat within the atmosphere ... [and] there are no sources of heat in the surrounding" This is hardly different from Schwarzschild's 1906 approximate concept, but is more precisely stated.

Planck (1914, page 40)[4] refers to a condition of thermodynamic equilibrium, in which "any two bodies or elements of bodies selected at random exchange by radiation equal amounts of heat with each other."

The term radiative exchange equilibrium can also be used to refer to two specified regions of space that exchange equal amounts of radiation by emission and absorption (even when the steady state is not one of thermodynamic equilibrium, but is one in which some sub-processes include net transport of matter or energy including radiation). Radiative exchange equilibrium is very nearly the same as Prevost's relative radiative equilibrium.

To a first approximation, an example of radiative exchange equilibrium is in the exchange of non-window wavelength thermal radiation between the land-and-sea surface and the lowest atmosphere, when there is a clear sky. As a first approximation (Swinbank 1963,[10] Paltridge and Platt 1976, pages 139–140[11]), in the non-window wavenumbers, there is zero net exchange between the surface and the atmosphere, while, in the window wavenumbers, there is simply direct radiation from the land-sea surface to space. A like situation occurs between adjacent layers in the turbulently mixed boundary layer of the lower troposphere, expressed in the so-called "cooling to space approximation", first noted by Rodgers and Walshaw (1966).[12][13][14][15]

### Definition for an entire passive celestial system such as a planet that does not supply its own energy

Liou (2002, page 459)[16] and other authors use the term global radiative equilibrium to refer to putative or theoretically conceived radiative exchange equilibrium globally between the earth and extraterrestrial space; such authors intend to mean that, in a putative or theoretically conceived steady state, incoming solar radiation absorbed by the earth and its atmosphere would be equal to outgoing longwave radiation from the earth and its atmosphere. Prevost[1] would say then that the earth and its atmosphere regarded as a whole were in absolute radiative equilibrium. Some good texts, for example Satoh (2004),[17] are rather cavalier here and simply refer to "radiative equilibrium", apparently not distinguishing between pointwise and global exchange radiative equilibrium. These usages assume that the supply of energy from chemical and nuclear reactions within the planet are negligibly small, the very opposite assumption to the one that motivates a contrary definition given hereunder.

The various global temperatures that may be theoretically conceived for any planet in general in putative or theoretically conceived global radiative equilibrium are of considerable interest. Such temperatures include the equivalent blackbody temperature of a planet,[18] also called the effective radiation emission temperature of the planet.[19] Related putative or theoretical concepts include the global-mean surface air temperature,[20] which specifically considers the presence of an atmosphere.

### Definition for an entire active celestial system such as a star that supplies its own energy

#### A contrary definition

Cox and Giuli (1968/1984)[21] use a definition quite contrary to all the above usages. They define 'radiative equilibrium' for a star, taken as a whole and not confining attention only to its atmosphere, when the rate of transfer as heat of energy from nuclear reactions plus viscosity to the microscopic motions of the material particles of the star is just balanced by the transfer of energy by electromagnetic radiation from the star to space. This is a way of using the words 'radiative equilibrium' that is contrary to the above-mentioned usages. They note that a star that is radiating energy to space cannot be in a steady state of temperature distribution unless there is a supply of energy, in particular they have in mind a steady supply of energy from nuclear reactions within the star, to support the radiation to space. Likewise the condition that is used for the above definition of pointwise radiative equilibrium, namely, ${\displaystyle h=\int _{0}^{\infty }h_{\nu }d\nu =0}$ at every point of the region that is in radiative equilibrium, cannot hold throughout a star that is in a steady state of temperature distribution, without an energy supply. Cox and Giuli's definition allows them to say at the same time that a star is in a steady state of temperature distribution and is in 'radiative equilibrium'; they are assuming that all the radiative energy to space comes from within the star, the very opposite assumption to the one made by the above-mentioned users of the concept 'global radiative equilibrium'.[21]

When there is enough matter in a region to allow molecular collisions to occur very much more often than creation or annihilation of photons, for radiation one speaks of local thermodynamic equilibrium. In this case, Kirchhoff's law of equality of radiative absorptivity and emissivity holds.[22]

Two bodies in radiative exchange equilibrium, each in its own local thermodynamic equilibrium, have the same temperature and their radiative exchange complies with the Stokes-Helmholtz reciprocity principle.

## References

1. ^ a b Prevost, P. (1791). Mémoire sur l'equilibre du feu. Journal de Physique (Paris), vol 38 pp. 314–322.
2. ^ Maxwell, J.C. (1871). Theory of Heat, Longmans, Green and Co, London, pages 221–222.
3. ^ Partington, J.R. (1949). An Advanced Treatise on Physical Chemistry, volume 1, Fundamental Principles. The Properties of Gases, Longmans, Green and Co, London, page 467.
4. ^ a b Planck, M. (1914). The Theory of Heat Radiation, second edition translated by M. Masius, P. Blakiston's Son and Co., Philadelphia, 1914.
5. ^ a b Mihalas, D., Weibel-Mihalas, B. (1984). Foundations of Radiation Hydrodynamics, Oxford University Press, New York ISBN 0-19-503437-6.
6. ^ a b Goody, R.M., Yung, Y.L. (1989). Atmospheric Radiation. Theoretical Basis, second edition, Oxford University Press, New York, 1989, ISBN 0-19-505134-3.
7. ^ Mihalas, D. (1978). Stellar Atmospheres, 2nd edition, Freeman, San Francisco, ISBN 0-7167-0359-9.
8. ^ Schwarzschild, K. (1906). Ueber das Gleichgewicht der Sonnenatmosphaere. Nachrichten von der Koeniglichen Gessellschaft der Wissenschaften zu Goettingen. Math.-phys. Klasse 195: 41–53. Translation in Selected Papers on the Transfer of Radiation, ed. D.H. Menzel, Dover, New York, 1966.
9. ^ Chandrasekhar, S. (1950). Radiative Transfer, Oxford University Press, Oxford, 1950.
10. ^ Swinbank, W.C. (963). Long-wave radiation from clear skies, Quarterly Journal of the Royal Meteorological Society, 89: 339–348.
11. ^ Paltridge, G.W., Platt, C.M.R., (1976). Radiative Processes in Meteorology and Climatology, Elsevier, Amsterdam, ISBN 0-444-41444-4.
12. ^ Rodgers, C.D., Walshaw, C.D. (1966). The computation of infrared cooling rate in planetary atmospheres, Quarterly Journal of the Royal Meteorological Society, 92: 67–92.
13. ^ Paltridge, G.W., Platt, C.M.R., (1976). Radiative Processes in Meteorology and Climatology, Elsevier, Amsterdam, ISBN 0-444-41444-4, page 172.
14. ^ Goody, R.M., Yung, Y.L. (1989). Atmospheric Radiation: Theoretical Basis, 2nd edition, Oxford University Press, Oxford, New York, 1989, ISBN 0-19-505134-3, page 250.
15. ^ Wallace, J.M., Hobbs, P.V. (2006). Atmospheric Science: An Introductory Survey, 2nd edition, Elsevier, Amsterdam, ISBN 978-0-12-732951-2, page 138.
16. ^ Liou, K.N. (2002). An Introduction to Atmospheric Radiation, second edition, Academic Press, Amsterdam, 2002, ISBN 978-0-12-451451-5.
17. ^ Satoh, M. (2004). Atmospheric Circulation Dynamics and General Circulation Models, Springer-Praxis, Chichester UK, ISBN 3-540-42638-8, page 370.
18. ^ Wallace, J.M., Hobbs, P.V. (2006). Atmospheric Science. An Introductory Survey, second edition, Elsevier, Amsterdam, ISBN 978-0-12-732951-2. Section 4.3.3, pp. 119–120.
19. ^ Stull, R. (2000). Meteorology For Scientists and Engineers. A technical companion book with Ahrens' Meteorology Today, Brooks/Cole, Belmont CA, ISBN 978-0-534-37214-9., p. 400.
20. ^ Wallace, J.M., Hobbs, P.V. (2006). Atmospheric Science. An Introductory Survey, second edition, Elsevier, Amsterdam, ISBN 978-0-12-732951-2., p.444.
21. ^ a b Cox, J.P. with Giuli, R.T. (1968, reprint 1984). Principles of Stellar Structure, Gordon and Breach, New York, ISBN 0-677-01950-5, page 134.
22. ^ Milne, E.A. (1928). The effect of collisions on monochromatic radiative equilibrium, Monthly Notices of the Royal Astronomical Society 88: 493–502