Magnetic fields and activity of the sun and stars: an overview

The author gives a very personal overviewing outlook on the problem of stellar activity and its relation to magnetic field dynamics in stellar convection zones. Solar physics is said to be suitable for extrapolating to the stellar domains, and consequently, the association between hot plasma in the outer layers of the sun and solar surface magnetic fields is hoped to go beyond the relatively narrow confines of solar physics, giving a kind of generic behaviour of astrophysical objects whose surfaces are turbulent. On various grounds, he concludes that, at least at first order, 'activity'-phenomena on late spectra-type dwarf stars may be thought of as a variant of familiar solar physics. Further, he indicates that observations of solar bolometric luminosity fluctuations suggest that magnetic fields in the outer convection zones of stars modulate the total stellar luminosity, while both solar and stellar data give observational grounds for believing that the interaction of magnetic fields with turbulent fluids is central to the chromospheric and coronal phenomenon on late dwarf stars, and, on the other hand, that observations of stellar surface activities probe magnetic field dynamics in the outer convection zones of stars. In a separate section, some theoretical questions have briefly been discussed concerning nonlinear magnetohydrodynamical equations, flux tube dynamics and plasma heating. (G.J.P.)