Magnetic structure in cool stars. XI. Relations between radiative fluxes mesuring stellar activity, and evidence for two components in stellar chromospheres.

Tight power-law relations between chromospheric, transition-region, and coronal surface flux densities of late-type stars are obtained, provided that a lower-limit flux φ is subtracted from radiative flux densities originating in the lower chromosphere. The author determines lower-limit, or basal, flux densities φ_i that optimize correlation coefficients for power-law relations between the soft X-ray flux density and the chromospheric excess flux density (F_i-φ_i) in the Ca II H and K, Mg II h and k, and Si II resonance lines.