Radial filamentary structure in a sunspot umbra

Abstract

THAT the dark umbrae of sunspots are not uniformly dark has long been recognized¹, but our knowledge of umbral structure is still limited. Here I present two white-light photographs of the solar disk, of different exposure, recently obtained during excep-tional seeing conditions with the McMath Telescope on Kitt Peak (Fig. 1). The shorter exposure reveals the solar granulation, and the longer brings out details in the darker umbrae. When these two images are suitably combined, the umbra of one small sunspot is resolved into an approximately filamentary structure, connecting to the penumbra, along with featureless voids. A 'light bridge9 partially spanning the spot is found to have a broader underlying filamentary foundation that completes the crossing. These photo-graphs suggest that the contemporary view of umbral structure as consisting primarily of granulation or 'umbral dots' is question-able. Given that a sunspot is a place where kilogauss magnetic fields concentrate and diverge outwards, filamentary brightness modulation can perhaps be attributed to partly inclined field lines, and the voids may be places where the field becomes perpendicular to the surface.