Astronomy and Astrophysics – Astronomy
Scientific paper
May 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aas...210.1704f&link_type=abstract
American Astronomical Society Meeting 210, #17.04; Bulletin of the American Astronomical Society, Vol. 39, p.117
Astronomy and Astrophysics
Astronomy
Scientific paper
The stretching of the vorticity present in faculae (Falco 2006, Solar Phys. 234), by the downflow in pores and sunspots before they reach maturity, is proposed as the mechanism for the formation of penumbral fine structure. The dark cored filaments which are bordered by bright filaments, taken together, are seen as stretched vortex tubes with strong magnetic fields and cool cores. The classical Evershed flow away from the umbra in the photosphere, its unsteadiness, and the consequent mass flow balance at the outer penumbral boundary are explained in the context of the pressure differences on the inflow and outflow portions of the stretched vortex, and on subsequent vortex breakdown. The two-component ‘interleaved’ magnetic field structure (horizontal dark filaments and more vertical bright filaments) is a consequence of the model. It also explains the very sharp bending at the inner footprint of flux tubes that appear to emerge from the umbra (and inner penumbra). Furthermore, the movement of the inner footprints (also called bright penumbral grains) towards and sometimes into the umbra, and of the ‘Evershed clouds’ away from the umbra, are both consequences. The theory suggests that penumbral asymmetries (an example is the limb-side/center-side dark cored filament differences) are in part due to differences in the reorientation that vortex/flux tubes undergo at different azimuthal positions of a sunspot as the tubes are stretched into the penumbra, and not solely due to viewing angle.
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