What we learn in this chapter

A hot plasma of ionized atoms emits thermal bremsstrahlung radiation through the Coulomb collisions of the electrons and ions. The electrons experience large accelerations in the collisions and thus efficiently radiate photons, which escape the plasma if it is optically thin. The energy Q radiated in a single collision is obtained from Larmor's formula. The characteristic frequency of the emitted radiation is estimated from the duration of the collision, which, in turn, depends on the electron speed and its impact parameter (projected distance of closest approach to the ion). Multiplication of Q by the electron flux and ion density and integration over the range of speeds in the Maxwell–Boltzmann distribution yield the volume emissivityjν(ν) (W m−3 Hz−1), the power emitted from unit volume into unit frequency interval at frequency ν as a function of frequency. It is proportional to the product of the electron and ion densities and is approximately exponential with frequency. A slowly varying Gaunt factor modifies the spectral shape somewhat. Most of the power is emitted at frequencies near that specified by hν ≈ kT.

Integration of the volume emissivity over all frequencies and over the volume of a plasma cloud results in the luminosity of the cloud. […]