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Instatistical mechanics, the excitation temperature (Tex) is defined for a population of particles via the Boltzmann factor. It satisfies
where
Thus the excitation temperature is the temperature at which we would expect to find a system with this ratio of level populations. However it has no actual physical meaning except when in local thermodynamic equilibrium. The excitation temperature can even be negative for a system with inverted levels (such as a maser).
In observations of the 21 cm lineofhydrogen, the apparent value of the excitation temperature is often called the "spin temperature".[1]
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