The excitation temperature (
) is defined for a population of particles via the
Boltzmann factor. It satisfies
:
where ''n''
u and ''n''
l represent the number of particles in an upper (''e.g.'' excited) and lower (''e.g.'' ground) state, and ''g''
u and ''g''
l their statistical weights respectively.
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 line
The hydrogen line, 21 centimeter line, or H I line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation has a precise frequency of , w ...
of hydrogen, the apparent value of the excitation temperature is often called the "spin temperature".
References
Temperature
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