Chromospheric-coronal emission and rotation in cool MS stars : discordant correlations explained.

Data from ground and space observations (IUE and Einstein Observatories) of Ca2, C4, UV and X-ray emission from late main sequence stars F7-M2 are discussed. The relation between rotation and emission in the chromospheres, transition regions, and coronae is examined and compared to relations derived from previous measurements. It is shown that the Lk, L4 and Lx emission luminosities of main stars decrease exponentially when the rotational period increases, regardless of the spectral type of the stars. Coefficients in the correlation are found to be in good agreement with observed power-law relations between C4 and X-ray emission and chromospheric emission. The effect of rotation period on dynamo-generated magnetic field is briefly discussed.