Radiative transfer

Radiative transfer is the physical phenomenon of energy transfer in the form of electromagnetic radiation. The propagation of radiation through a medium is affected by absorption, emission and scattering processes. The equation of radiative transfer describes these interactions mathematically. Equations of radiative transfer have application in wide variety of subjects including optics, astrophysics, atmospheric science, and remote sensing. Analytic solutions to the radiative transfer equation (RTE) exist for simple cases but for more realistic media with complex multiple scattering effects numerical methods are required.

The present article is largely focused on the condition of radiative equilibrium. .

Definitions

The fundamental quantity which describes a field of radiation is nowadays called the spectral radiance, traditionally called the specific intensity. If we think of a very small area element in the radiation field, there will be radiation energy flowing through that area element. The flow can be completely characterized by the amount of energy flowing per unit time per unit solid angle, the direction of the flow, and the wavelength interval being considered (polarization will be ignored for the moment).

In terms of the spectral radiance, I_ν, the energy flowing across an area element of area $da\,$ located at $\mathbf{r}$ in time $dt\,$ in the solid angle dΩ about the direction $\hat{\mathbf{n}}$ in the frequency interval $\nu\,$ to $\nu+d\nu\,$ is

$dE_\nu = I_\nu(\mathbf{r},\hat{\mathbf{n}},t) \cos\theta \ d\nu \, da \, d\Omega \, dt$

where θ is the angle that the unit direction vector $\hat{\mathbf{n}}$ makes with a normal to the area element. The units of the spectral radiance are seen to be energy/time/area/solid angle/frequency. In MKS units this would be W·m^-2·sr^-1·Hz^-1 (watts per square-metre-steradian-hertz).

The equation of radiative transfer

The equation of radiative transfer simply says that as a beam of radiation travels, it loses energy to absorption, gains energy by emission, and redistributes energy by scattering. The differential form of the equation for radiative transfer is:

$\frac{1}{c}\frac{\partial}{\partial t}I_\nu + \hat{\Omega} \cdot \nabla I_\nu + (k_{\nu, s}+k_{\nu, a}) I_\nu = j_\nu + \frac{1}{4\pi c}k_{\nu, s} \int_\Omega I_\nu d\Omega$

where j_ν is the emission coefficient, k_ν,s is the scattering cross section, and k_ν,a is the absorption cross section.

Solutions to the equation of radiative transfer

Solutions to the equation of radiative transfer form an enormous body of work. The differences however, are essentially due to the various forms for the emission and absorption coefficients. If scattering is ignored, then a general solution in terms of the emission and absorption coefficients may be written:

$I_\nu(s)=I_\nu(s_0)e^{-\tau_\nu(s_0,s)}+\int_{s_0}^s j_\nu(s') e^{-\tau_\nu(s',s)}\,ds'$

where τ_ν(s₁,s₂) is the optical depth of the medium between positions s₁ and s₂:

$\tau_\nu(s_1,s_2) \ \stackrel{\mathrm{def}}{=}\ \int_{s_1}^{s_2} \alpha_\nu(s)\,ds$

Local thermodynamic equilibrium

A particularly useful simplification of the equation of radiative transfer occurs under the conditions of local thermodynamic equilibrium (LTE). In this situation, the absorbing/emitting medium consists of massive particles which are in equilibrium with each other, and therefore have a definable temperature. The radiation field is not, however in equilibrium and is being entirely driven by the presence of the massive particles. For a medium in LTE, the emission coefficient and absorption coefficient are functions of temperature and density only, and are related by:

$\frac{j_\nu}{\alpha_\nu}=B_\nu(T)$

where B_ν(T) is the black body spectral radiance at temperature T. The solution to the equation of radiative transfer is then:

$I_\nu(s)=I_\nu(s_0)e^{-\tau_\nu(s_0,s)}+\int_{s_0}^s B_\nu(T(s'))\alpha_\nu(s') e^{-\tau_\nu(s',s)}\,ds'$

Knowing the temperature profile and the density profile of the medium is sufficient to calculate a solution to the equation of radiative transfer.

The Eddington approximation

The Eddington approximation is a special case of the two stream approximation. It can be used to obtain the spectral radiance in a "plane-parallel" medium (one in which properties only vary in the perpendicular direction) with isotropic frequency-independent scattering. It assumes that the intensity is a linear function of μ = cos θ. i.e.

I_ν(μ,z) = a(z) + μb(z)

where z is the normal direction to the slab-like medium. Note that expressing angular integrals in terms of μ simplifies things because dμ = − sin θdθ appears in the Jacobian of integrals in spherical coordinates.

Extracting the first few moments of the spectral radiance with respect to μ yields

$J_\nu=\frac{1}{2}\int^1_{-1}I_\nu d\mu = a$

$H_\nu=\frac{1}{2}\int^1_{-1}\mu I_\nu d\mu = \frac{b}{3}$

$K_\nu=\frac{1}{2}\int^1_{-1}\mu^2 I_\nu d\mu = \frac{a}{3}$

Thus the Eddington approximation is equivalent to setting K_ν = 1 / 3J_ν. Higher order versions of the Eddington approximation also exist, and consist of more complicated linear relations of the intensity moments. This extra equation can be used as a closure relation for the truncated system of moments.

Note that the first two moments have simple physical meanings. J_ν is the isotropic intensity at a point, and H_ν is the flux through that point in the z direction.

The radiative transfer through an isotropically scattering medium at local thermodynamic equilibrium is given by

$\mu \frac{dI_\nu}{dz}=- \alpha_\nu (I_\nu-B_\nu) + \sigma_{\nu}(J_\nu -I_\nu)$

Integrating over all angles yields

$\frac{dH_\nu}{dz}=\alpha_\nu (B_\nu-J_\nu)$

Premultiplying by μ, and then integrating over all angles gives

$\frac{dK_\nu}{dz}=-(\alpha_\nu+\sigma_\nu)H_\nu$

Substituting in the closure relation, and differentiating with respect to z allows the two above equations to be combined to form the radiative diffusion equation

$\frac{d^2J_\nu}{dz^2}=3\alpha_\nu(\alpha_\nu+\sigma_\nu)(J_\nu-B_\nu)$

This equation shows how the effective optical depth in scattering-dominated systems may be significantly different from that given by the scattering opacity if the absorptive opacity is small.

See also

• Absorption (electromagnetic radiation)
• Atomic line spectra
• Beer-Lambert law
• Emission
• List of atmospheric radiative transfer codes
• Scattering
• Radiative transfer equation and diffusion theory for photon transport in biological tissue
• Spectral radiance
• Specific intensity

Further reading

• Subrahmanyan Chandrasekhar (1960). Radiative Transfer. Dover Publications Inc.. p. 393. ISBN 0-486-60590-6. 

• Jacqueline Lenoble (1985). Radiative Transfer in Scattering and Absorbing Atmospheres: Standard Computational Procedures. A. Deepak Publishing. p. 583. ISBN 0-12-451451-0. 

• Grant Petty (2006). A First Course in Atmospheric Radiation (2nd Ed.). Sundog Publishing (Madison, Wisconsin). ISBN 0-9729033-1-3. 

• Dimitri Mihalas, Barbara Weibel-Mihalas (1984). Foundations of Radiation Hydrodynamics. Dover Publications, Inc.. ISBN 0-486-40925-2. 

• George B. Rybicki, Alan P. Lightman (1985). Radiative Processes in Astrophysics. Wiley-Interscience. ISBN 0-471-82759-2. 

• G. E. Thomas and K. Stamnes (1999). Radiative Transfer in the Atmosphere and Ocean. Cambridge University Press.. ISBN 0-521-40124-0.