4. Evershed flow in interchange flux tubes
In the model of Schlichenmaier et al. (1998) the gas flows along horizontal flux tubes which owe their inclination and position to the interchange instability. The flow is not driven by the magnetic gradient between the footpoints of these arched flux tubes and consequently does not constrain the field strength of the downflowing footpoint. Rather, the process appears to be more akin to convection, with the field lines acting as a guiding funnel.
In the upflowing footpoint of their model, the flux tube has a lower field strength than the surroundings, but is hotter. Further out in the penumbra, the horizontal tube cools to the surrounding temperature and its field strength and velocity shift increase.
A major discrepancy between this model and our observations arises from the fact that it does not predict that cooler gas has lower magnetic field strength. The model can be reconciled with the observations if the low-field strength component seen in Ti is only restricted to near the outer penumbral edge, where the field of the ambient hotter medium has become weaker. As pointed out in Sect. 3, due to the seeing our observations are indeed compatible with a confinement of the cool horizontal field component to the outer part of the penumbra. The fact that the outflowing gas near the upflow footpoint is hot in this model does not necessarily contradict the observations, since, firstly, lines formed at higher temperatures than the infrared Ti lines also show the Evershed effect, although less strongly. Secondly, the larger opacity of the hot gas at the upflow footpoint raises the height of formation of continuum and line radiation, so that the spectral lines sample less of the hot, flowing gas.
© European Southern Observatory (ESO) 1999
Online publication: July 26, 1999