Volume 552, April 2013
|Number of page(s)||4|
|Published online||03 April 2013|
Formation of a penumbra in a decaying sunspot ⋆
Leibniz-Institut für Astrophysik Potsdam (AIP),
An der Sternwarte 16,
2 Udaipur Solar Observatory, Physical Research Laboratory, Dewali, Badi Road, 313004 Udaipur, Rajasthan, India
3 National Solar Observatory, Sacramento Peak, 3010 Coronal Loop, Sunspot, New Mexico 88349, USA
Received: 18 February 2013
Accepted: 12 March 2013
Context. Penumbrae are an important characteristic of sunspots, whose formation is intricately related to the nature of sub-photospheric magnetic fields.
Aims. We study the formation of a penumbra in a decaying sunspot and compare its properties with those seen during the development of a proto-spot.
Methods. High-resolution spectropolarimetric observations of active region NOAA 11283 were obtained from the spectro-polarimeter on board Hinode. These were complemented with full-disk filtergrams of continuum intensity, line-of-sight magnetograms, and dopplergrams from the Helioseismic and Magnetic Imager at high cadence.
Results. The formation of a penumbra in the decaying sunspot occurs after the coalescence of the sunspot with a magnetic fragment/pore, which initially formed in the quiet Sun close to an emerging flux region. At first, a smaller set of penumbral filaments develop near the location of the merger with very bright penumbral grains with intensities of 1.2 IQS, upflows of 4 km s-1, and a lifetime of 10 h. During the decay of these filaments, a larger segment of a penumbra forms at the location of the coalescence. These new filaments are characterized by nearly supersonic downflows of 6.5 km s-1 that change to a regular Evershed flow nearly 3 h later.
Conclusions. The coalescence of the pore with the decaying sunspot provided sufficient magnetic flux for the penumbra to form in the sunspot. The emerging flux region could have played a decisive role in this process because the formation occurred at the location of the merger and not on the opposite side of the sunspot.
Key words: sunspots / Sun: photosphere / Sun: surface magnetism / techniques: photometric / techniques: polarimetric
An animation of the HMI data is available in electronic form at http://www.aanda.org
© ESO, 2013
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