ON THE ORIGIN OF PENUMBRAL LINE ASYMMETRIES

The common interpretation of observed shifts and asymmetries of spectr al lines emerging from sunspot penumbrae (Evershed effect) is a homoge neous material flow such that the flow velocity increases with depth. This interpretation is found to be inconvincing in view of theoretical arguments which predict a decreasing flow speed. As an alternative it is suggested that the fraction of the penumbral volume which contains the streaming gas increases with depth. The spectroscopic signature o f such a penumbral structure is investigated by calculating synthetic line profiles. Provided that the postulated spatial inhomogeneity cann ot be resolved with present day telescopes, it is shown that the calcu lated line shifts and asymmetries are consistent with observations eve n if the absolute value of the velocity decreases with depth.