spin temperature

Spin temperature is a way of measuring the temperature of neutral hydrogen (HI) in space in terms of the relative populations of its two possible spin states – parallel (electron and proton spins aligned) and antiparallel (spins opposed). The spin temperature of an HI region is the temperature at which its observed ratio of parallel to antiparallel spins would occur if the gas were in thermal equilibrium. It differs from the kinetic temperature at low densities, where the spin temperature is raised by the tendency of resonant scattering of Lyman-alpha photons to equalize the occupation probabilities in the two spin states.