Physics – Optics
Scientific paper
Oct 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008an....329..773d&link_type=abstract
Astronomische Nachrichten, Vol.329, Issue 8, p.773
Physics
Optics
2
Methods: Data Analysis, Sun: Photosphere, Sunspots, Techniques: High Angular Resolution
Scientific paper
We observed a cluster of extremely bright penumbral grains located at the inner limb-side penumbra of the leading sunspot in active region NOAA 10892. The penumbral grains in the cluster showed a typical peak intensity of 1.58 times the intensity I_0 of the granulation surrounding the sunspot. The brightest specimen even reached values of 1.8--2.0 I_0, thus, exceeding the temperatures of the brightest granules in the immediate surroundings of the sunspot. We find that the observed sample of extremely bright penumbral grains is an intermittent phenomenon, that disappears on time scales of hours. Horizontal flow maps indicating proper motions reveal that the cluster leaves a distinct imprint on the penumbral flow field. We find that the divergence line co-located with the cluster is displaced from the middle penumbra closer towards the umbra and that the radial outflow velocities are significantly increased to speeds in excess of 2 km s^{-1}. The extremely bright penumbral grains, which are located at the inner limb-side penumbra, are also discernible in off-band Hα images down to Hα ± 0.045 nm. We interpret the observations in the context of the moving flux tube model arguing that hotter than normal material is rapidly ascending along the inner footpoint of the embedded flux tube, i.e., the ascending hot material is the cause of the extremely bright penumbral grains. This study is based on speckle-reconstructed broad-band images taken at 600 nm and chromospheric Hα observations obtained with two-dimensional spectroscopy. All data were taken with adaptive optics under very good seeing conditions at the Dunn Solar Telescope, National Solar Observatory/Sacramento Peak, New Mexico on 2006 June 10.
Deng Nimao
Denker Carsten
Tritschler Alexandra
Verdoni Angelo P.
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