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 dA, in a given solid angle dΩ, per unit frequency dν, per unit time. The intensity is therefore,

dE = I_ν(k, x, t)k̂ ∙ n̂ dA dΩ dν 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, p, t) d³x d³p gives the number of photons of spin state α (there are two spin states) at time t within the volume (d³x d³p) of 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/(γmc)

 

~ by jamesdow2013 on March 25, 2013.