Excitation temperature

The Excitation Temperature (T_ex) (often called the spin temperature for the 21 cm line) is defined for a population of particles via the Boltzmann factor. It satisfies

Additional recommended knowledge

Don't let static charges disrupt your weighing accuracy

Daily Sensitivity Test

What is the Sensitivity of my Balance?

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 thermodynamical equilibrium. The excitation temperature can even be negative for a system with inverted levels (such as a maser).