Magnetic activity in lower main sequence stars.

The IUE satellite studied transition region plasma in main sequence F-M stars. The fractional flux (fTR/fbol) is proportional to Ro to the power -2.5, where Ro is a parameter resembling the Rossby-number. Other chromospheric-coronal indicators (from CaK to X-rays) show similar correlations, but with different slopes. An interpretation is given where the transition region flux is proportional to the area coverage filling factor Ar, and the clear saturation observed (fTR/fbol constant when Ro 0.3) is due to the total filling of the surface with coronal loops (Ar = 1). A simple dynamo model gives the behavior observed quantitatively. Binary effects, as a cause for the saturation, are small. Evidence against any clear decrease in the dynamo ability as the star becomes fully convective is produced. The magnetic braking in cataclysmic variables is discussed.