Atomic excitations occur through collisions with electrons.
The excited atoms decay radiatively and the emitted radiation escapes without any further interaction with the emitting plasma. As a consequence, the emission is optically thin, and the total flux emitted in some spectral band or in a given emission line is proportional to the emission measure EM , defined as the integral of the square of the plasma density n over the emitting volume elements d V through . Thus, observationally, the contributions of density and volume to a given observed value of EM cannot be disentangled. Typically, one observes star to star variations in of up to four orders of magnitude, with the largest X-ray luminosities found among the stars with the largest rotation rates. The emission measure is directly linked to the structure of stellar coronae, if we assume, going along with the solar analogy, that the X-ray emitting plasma of a stellar corona is confined in magnetic loops.