DRIVEN ACOUSTIC-OSCILLATIONS WITHIN A VERTICAL MAGNETIC-FIELD

We examine the effects of a vertical magnetic held on p-mode frequenci es, line widths, and eigenfunctions. We employ a simple solar model co nsisting of a neutrally stable polytropic interior matched to an isoth ermal chromosphere. The p-modes are produced by a spatially distribute d driver. The atmosphere is threaded by a constant vertical magnetic f ield. The frequency shifts due to the vertical magnetic held are much smaller than the shifts caused by horizontal fields of similar strengt h. A large vertical held of 2000 G produces shifts on the order of 1 m u Hz while a weak field of 50 G produces very small shifts of several nanohertz. We find that the frequency shifts decrease with increasing frequency and increase with held strength. The shifts are positive, ex cept at high frequency and low field strength, where small negative sh ifts are possible. Coupling of the acoustic fast mode to escaping slow modes is extremely inefficient. Constant vertical magnetic field mode ls are therefore incapable of explaining the high level of absorption observed in sunspots and plage. The damping due to this mode conversio n process produces very narrow line widths. For a 2000 G field the lin e widths are several microhertz and for a 50 G field the line widths a re several nanohertz.