Formation of a penumbra in a decaying sunspot ^⋆

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: rlouis@aip.de
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

Abstract

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 I_QS, 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.