## Radiation Processes

Thermal processes include **Absorption**, **Emission** and **Scattering**. These describe interactions between photons (γ) and material (electrons, ions, and atoms). In scattering the γ maybe re-directed but their energy (approximately) remains unchanged. Electrons may scatter photons by the Thomson or Compton mechanisms.

**Thomson Scattering** occurs in low energy collisions, hν ≪ *kT* and the electrons oscillate, forced by the photon’s electromagnetic field.

**Compton Scattering** is a ‘hard’ collision.

Rayleigh Scattering occurs when the photon excites an atom, which quickly de-excites, emitting a photon of the same energy, but in a different direction.

Thermal absorption processes not only change a photon’s direction but also its energy.

**Photo Ionization (Bound-Free Absorption)** is when the photon energy is sufficient to ionize the originally bound electron.

**Photo Excitation (Bound-Bound Absorption)** the photon excites an electron to a higher energy state.

**Free-Free Absorption** is a three body process where an electron absorbs a photon while in the electromagnetic field of a positive ion. The electron then moves on a different hyperbolic path around the ion.

Each absorption process has its inverse, Photo ionization and **Recombination**, Photo Excitation and** Radiative De-Excitation**, Free-Free Absorption and** Bremsstrahlung.
**

Thermal processes include both line producing (bound-bound) and continuum (bound-free, free-bound, free-free) spectra.

Astrophysical non-thermal processes are from **Masers** and **Synchrotron Radiation**.

Masers are microwave lasers and the non-thermal nature of masers comes from the **stimulated emission** of photons, whereas thermal sources rely on **collisional** or **spontaneous** Radiative processes.

Synchrotron radiation occurs when electrons are moving at near light speed.

Intensity is defined in terms of the energy that passes through a given area *d*A, in a given solid angle *d*Ω, per unit frequency *d*ν, per unit time. The intensity is therefore,

*dE* = I_{ν}(**k**, *x*, *t*)**k̂ ****∙ ****n̂ ***dA **d*Ω *d*ν *dt *(the units are Joules m^{-2} s^{-1} Hz^{-1} steradian^{-1})

Where **k **is the wave vector (= (ω/c)**n **=(ν/2πc)**n**) whose amplitude is the **wave number** and** k ∙ r **is the phase of the wave.

The Planck black body distribution is one of the most fundamental concepts in Astrophysics, and is the mechanism by which thermal sources emit their continuum radiation.

The Distribution Function F_{α} is used in the sense that F_{α} (** x**,

**, t)**

*p**d*

^{3}x*d*gives the number of photons of spin state α (there are two spin states) at time

^{3}p*t*within the volume (

*d*) of

^{3}x d^{3}p**Phase Space**.

**Cyclotron Radiation** is caused by non-relativistic electrons undergoing helical motion when traversing a magnetic field. The cyclotron frequency is ω_{c} = 2πν_{e} = *qB/(mc*)

Synchrotron Radiation arises from relativistic electrons in a magnetic field. The Relativistic Frequency differs from the cyclotron frequency by a logical factor, γ.

ω_{s} = ω_{c}/γ = *qB*/(γ*mc*)