The role of magnetic fields in stellar chromospheres and transition regions

In this review based largely on observations with the IUE and Einstein satellites, the author summarizes the different roles that magnetic fields play in controlling the structure and energy balance in the chromospheres and transition regions of late-type stars. The author proposes that the existence of a boundary between solar-like stars and those with little or no hot plasma, as well as the different types of G-K giants and supergiants, can be understood in terms of the fractional surface coverage by closed magnetic structures. Transition region downflows, the chromospheric heating mechanism, and the relative heating rates at different layers can be simply explained by the control of the energy balance by magnetic fields. Finally, models computed for active and quiet regions on the Sun are compared with similar models computed for active and quiet stars, that is stars with intrinsically bright or weak emission lines. (Auth.)