  Empirically, the X-ray luminosity LX from M dwarfs has been found to have an
upper limit of about 0.2% of the bolometric flux Lbol. In the limit where
magnetic fields in M dwarfs are generated in equipartition with convective
motions, we use stellar models to calculate the energy flux of Alfven waves FA
as a function of depth in the sub-surface convection zone. Since Alfven waves
have the optimal opportunity for wave modes to reach the corona, we suggest
that FA sets an upper limit on the mechanical flux Fmech which causes coronal
heating. This suggestion accounts quantitatively for the saturated values of
LX/Lbol which have been reported empirically for M dwarfs.
