Generation and propagation of longitudinal tube waves in late-type stellar atmospheres and the rotation-emission activity relation
Generated longitudinal tube wave (LTW) energy fluxes as a result of the interaction of magnetic flux tubes with the turbulent medium of the convection zones of late-type stars were computed. Solar magnetic flux tubes of different filling factors were heated with adiabatic and radiatively damped LTW. It was found that the tube geometry plays an important role for heating the outer layers. The response of the chromospheres of the Sun and two late-type stars (of spectral types F5V & M0V) to the excitation by different wave modes was studied. It was found that the magnetic atmosphere shows resonance oscillations with longer periods in comparison with the non-magnetic plane parallel atmosphere. Magnetic chromosphere models were constructed for late-type stars based on our computed LTW fluxes. The mean temperatures of all spectral types for a given magnetic filling factor show a monotonic outward (classical) temperature rise. A new picture of the heating of stellar chromospheres is presented. The chromospheric emission can be explained by an ordered sequence of different heating processes which systematically vary as a function of height in the star and with its rotation rate. The low chromosphere is heated by acoustic shock waves. The middle and upper chromosphere by longitudinal and transverse MHD waves via shock dissipation. For the highest chromosphere, wave heating is insufficient and additional non-wave heating (e.g. magnetic reconnection) is necessary. With increasing rotation, the magnetic heating mechanisms become more important (rotation - emission activity relation). Based on our computed Mg II fluxes we estimate the rotational periods of G- and K- type-stars.
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