Kinetic temperature ($T_k$)

For an isotropic nonrelativistic Maxwellian particle population, kinetic temperature describes random translational motion, with mean kinetic energy $3k_B{T}_k/2$ per particle after subtracting bulk motion. Different species can have different kinetic temperatures outside thermal equilibrium. A rotational excitation temperature equals the kinetic temperature only when the excitation is sufficiently thermalized; a radiation brightness temperature is another distinct quantity.