Luc H. M. Rouppe van der Voort, Mats G. Löfdahl, Dan Kiselman, 
& Göran B. Scharmer
Institute for Solar Physics of the Royal Swedish Academy of Sciences

Penumbral structure at 0"1 resolution I. General appearance and power
spectra

Submitted to Astronomy & Astrophyics

Abstract:

We analyse sunspot filtergrams of unprecedented quality obtained by
Scharmer (2002) with the Swedish 1-m Solar Telescope on La Palma.
The observations comprise images in three different wavelength bands:
488, 436, and 430 nm (G-band).  We find that there are still
unresolved penumbral filaments which must have widths smaller than
80 km.  The fine structuring along the filaments is limited.
Penumbral grains have internal structure and look like they are split
or crossed by narrow dark structures.  We calculate intensity power
spectra of the penumbra from images that are corrected for seeing
using the Phase Diversity technique.  The effects of high order
aberrations that are not corrected for are estimated to be too low to
be consistent with a flat power spectrum.  The penumbral power spectra
do not show any signs of bumps or peaks that could correspond to a
preferred scale at 0"35 for the width of penumbral filaments.  We
argue that the power spectrum is not a very reliable source of
information concerning preferred scales.
  
Keywords Sun: photosphere -- sunspots