In the interstellar medium the concentration of atoms is small and the optical layer are small, it means that the temperature of the radiation is the radiation temperature of stars (~ 5000 K) and does not correspond to the temperature of the environment. In this electronic and ion temperature plasma may be very different from each other as the exchange of energy in collision occurs very rarely. Thus there is no single temperature, even in a local sense.
Distribution of the number of atoms and ions on human levels determined by the balance of ionization and recombination processes. Ltr requires that these processes were in equilibrium in order to meet the condition of detailed balance, however, in the interstellar medium direct and reverse elementary processes have different nature and, therefore, detailed balance can not be established.
Finally, a small optical depth for radiation hard and fast charged particles leads to the fact that the energy released in any region of space, loss of long-distance. And cool is the whole amount at once, but not in the local area, expanding at the speed of sound in the environment. Similarly, and are heated. The thermal conductivity is not able to transfer heat from a remote source and to join processes, heating a large amount at once.
However, despite the lack of ltr, even in very rarefied space plasma sets Maxwell distribution of electron velocities, the temperature of the environment, so the distribution of particle energies can be used Boltzmann equation and to talk about tempearature. It happens so because of the long-range Coulomb forces in a fairly short period of time (for a pure hydrogen plasma this time around 105 s), much less time to collision between particles.