PLASMA MECHANISMS FOR VARIABILITY IN ACTIVE GALACTIC NUCLEI

We consider several physical processes, dominated by plasma effects, t hat might play roles in producing variability in AGN. One class of pro cesses involves oscillations in the plasma itself. If accretion disks around supermassive black holes have coronae not too different from th ose of the Sun, then magnetoacoustic oscillations of coronal loops can yield fluctuations on timescales from days to years. Such coronal loo ps could also produce flares with substantial emission variations over the short times comparable to those associated with the recently obse rved broad-band microvariability. A second class of processes depends upon electromagnetic fields acting to modulate themselves; these modul ational instabilities can produce filamentation and thence rapid varia bility. We also consider how a shift in emission and scattering mechan ism (e.g., from Compton scattering to Raman scattering) can yield vari ability in the observed emission.