Excitation temperature
In statistical mechanics, the excitation temperature is defined for a population of particles via the Boltzmann factor. It satisfies
where
- is the number of particles in an upper state;
- is the statistical weight of those upper-state particles;
- is the number of particles in a lower state;
- is the statistical weight of those lower-state particles;
- is the exponential function;
- is the Boltzmann constant;
- is the difference in energy between the upper and lower states.
In observations of the 21 cm line of hydrogen, the apparent value of the excitation temperature is often called the "spin temperature".