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Z.E. Musielak (Uni. Texas at Arlington)
It is now well-established that the Sun and all other late-type stars show chromospheric activity, which is typically identified with the presence of emission in the cores of Ca II and Mg II spectral lines. This activity varies significantly for a given spectral type and a star is more active when a larger portion of its surface is covered by magnetic fields. For stars of very low activity, a minimum core emission flux ("basal flux") has been observed. To explain the required heating and the observed range of activity, two classes of heating mechanisms have been considered: (1) dissipation of acoustic and magnetic waves generated in stellar convection zones; and (2) dissipation of currents generated by photospheric motions of surface magnetic fields. I will review both classes of heating mechanisms and discuss recent results which demonstrate that theoretical models of stellar chromospheres based on the wave heating (1) can explain the "basal flux" and the observed Ca II emission in most stars but cannot account for the observed Mg II emission in active stars. I will also present theoretical arguments which imply that the base of stellar chromospheres is heated by acoustic waves, the heating of the middle and upper chromospheric layers is dominated by magnetic waves associated with magnetic flux tubes, and that other non-wave (2) heating mechanisms are required to explain the structure of the highest layers of stellar chromospheres in active stars.
This work was supported by NSF under grant ATM-0087184, NATO under grant CRG-910058 and The Alexander von Humboldt Foundation.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.