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thermal emission(thermal radiation; thermal bremsstrahlung) Electromagnetic radiation resulting from interactions between electrons and atoms or molecules in a hot dense medium. The term usually refers to thermal emission from ionized gas (but see also thermal black-body radiation). Free electrons in the ionized gas radiate when they are deflected in passing near protons. The electrons are unbound to a nucleus before and after each encounter and the radiation is therefore also known as free-free emission. The radiation has a continuous spectrum, which at low frequencies has the spectral index, α, of a thermal black-body radiator, i.e.
S ∝ ν–α; α = –2where S is the flux density and ν the frequency. At higher frequencies the spectrum flattens to a spectral index of α = 0. The emission measure of an ionized region of length l containing a density n e of free electrons is given by
EM = l 0 n e 2 dswhere ds is an element of the path through the region. Compare nonthermal emission.