Christoph Keller (NSO) and Oskar von der Luehe (Kiepenheuer Institute) used speckle interferometry to obtain diffraction-limited images of magnetic fluxtubes in the solar photosphere. Magnetic elements are the fundamental structures of the solar magnetic field in plages, in active regions, and in the network along the boundaries of supergranular cells. They have a field strength of 1-2 kG in the lower photosphere and diameters ~ 100 km, comparable to the diffraction limit of the largest solar telescopes. Magnetograms at ~ 0.5" resolution have been recorded under excellent conditions, but seeing normally prevents resolution of magnetic elements. The most favored theoretical models of magnetic elements are the so-called fluxtubes. Fluxtubes appear brighter than the average photosphere, since one sees deeper and hotter layers in the partly evacuated fluxtube.
Speckle interferometric techniques can greatly improve the spatial resolution of solar observations. The speckle deconvolution technique developed by Keller and von der Luehe is able to produce diffraction-limited images of the Sun in very narrow spectral bands by combining short-exposure images from a narrow and a broad-band channel. The broad-band images are reconstructed using a modified Knox-Thompson algorithm. By applying this technique to polarimetric observations in the wing of a Zeeman-sensitive spectral line, a few magnetic fluxtubes have been resolved in the past.
Hundreds of fluxtubes in the quiet network are now being studied with observations from the 76 cm Sacramento Peak Vacuum Tower Telescope and the Zurich Imaging Stokes Polarimeter (ZIMPOL) I, using the Universal Birefringent Filter with a bandwidth of 25 pm in the Ca I line at 610.3 nm. 300 simultaneous images in the broad-band and the narrow-band channels with a field of view of 14" X 14" were collected within less than one minute for each area. Bruce Wilton, an NSO summer student supervised by Keller, has analyzed more than 60 regions in the quiet network. Many of these magnetograms have resolution close to 0.2". The figure shows one of the areas where the size, brightness, flux, and dynamics of fluxtubes are determined, which is essential to reach an understanding of important solar phenomena such as chromospheric and coronal heating, irradiance variations, magnetic dynamos, and stellar activity in general. Preliminary results indicate that the larger elements are barely resolved. Most magnetic elements in the network seem to be brighter than the average intensity in white light at disk center, in contrast to the dark structures seen in plages at disk center.