Magnetic structure in cool stars. XIV. Deficiency in chromospheric fluxes from M-type dwarfs.

Chromospheric, transition-region and coronal emissions of M-type dwarfs and of dwarfs of earlier spectral type are compared, and related to the rotation period. M-type dwarfs depart from the relation between chromopsheric Ca II H+K excess flux density ΔF_{Ca II} and the coronal soft X-ray flux density F_X as defined by stars of earlier spectral type. The M-type dwarfs also show low chromospheric emissions in comparison to stars of earlier spectral type and the same rotation period P. It is shown that these deviations are caused by a deficiency in the chromopsheric emission, rather than by an increase in the coronal emission.