Spatial and temporal fluctuations in sunspots derived from MDI data

The penumbra radiates an energy flux that is 0.75-0.8 times the photospheric value. One mechanism proposed to bring this flux to the surface is interchange convection according to which hot flux tubes rise to the surface, lie horizontally there while they cool and finally sink down again. We search for possible signatures of such a process using time series of magnetograms and continuum images recorded by the Michelson Doppler Imager (MDI) in its high resolution mode (0.6" pixels). The data reveal that at the spatial scales accessible to MDI, magnetic structures are on average smaller in the azimuthal direction than brightness features. The small-scale magnetic pattern resolvable by MDI lives for well over two hours, i.e. longer than the brightness pattern. As shown in a parallel paper (Schlichenmaier & Solanki 2003) this result, taken together with theoretical predictions, suggests that interchange convection is unable to account for the observed penumbral radiative flux. The need for higher resolution data obtained under stable conditions is pointed out.