Magnetic structure in cool stars. XVII. Minimum radiative losses from theouter atmosphere.

The emissions in several chromospheric and transition region lines and in coronal soft X-rays are analyzed for a sample of cool stars. The nature of the lower-limit flux densities is explored, and evidence is given for the possibility of a basal, nonmagnetic heating mechanism being responsible for these emission fluxes up to, and perhaps including, the upper transition region. It is argued that the excess flux density, derived by subtraction of the basal flux density from the observed stellar flux, is the proper measure of magnetic activity. The level of the basal flux density as a function of color is determined to be 2 x 10 exp 6 erg/sq cm/s for F-type stars and 2 x 10 exp 5 erg/sq cm/s for K-type stars.