Formation of a penumbra in a decaying sunspot

• Published in: Astronomy and astrophysics (Berlin) Vol. 552; p. np
• Main Authors: Louis, R. E., Mathew, S. K., Puschmann, K. G., Beck, C., Balthasar, H.
• Format: Journal Article
• Language: English
• Published: EDP Sciences 01.04.2013
• Subjects: Coalescence; Coalescing; Decay; Filaments; Flux; Formations; Penumbras; Sun: photosphere; Sun: surface magnetism; Sunspots; techniques: photometric; techniques: polarimetric
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/201321314

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.