Horizontal flow fields observed in Hinode G-band images

III. The decay of a satellite sunspot and the role of magnetic flux removal in flaring

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

Received: 28 May 2012
Accepted: 24 July 2012

Abstract

Context. Emergence of magnetic flux plays an important role in the initiation of flares. However, the role of submerging magnetic flux in prompting flares is more ambiguous, not the least because of the scarcity of observations.

Aims. The flare-prolific active region NOAA 10930 offered both a developing δ-spot and a decaying satellite sunspot of opposite polarity. The objective of this study is to characterize the photometric decay of the satellite sunspot as well as the evolution of photospheric and chromospheric horizontal proper motions in its surroundings.

Methods. We apply the local correlation tracking technique to a 16-h time-series of Hinode G-band and Ca ii H images and study the horizontal proper motions in the vicinity of the satellite sunspot on 2006 December 7. Decorrelation times were computed to measure the lifetime of solar features in intensity and flow maps.

Results. We observed shear flows in the dominant umbral cores of the satellite sunspot. These flows vanished once the penumbra had disappeared. This slow penumbral decay had an average rate of 152 Mm² day^-1 over an 11-h period. Typical lifetimes of intensity features derived from an autocorrelation analysis are 3–5 min for granulation, 25–35 min for G-band bright points, and up to 200−235 min for penumbrae, umbrae, and pores. Long-lived intensity features (i.e., the dominant umbral cores) are not related to long-lived flow features in the northern part of the sunspot, where flux removal, slowly decaying penumbrae, and persistent horizontal flows of up to 1 km s^-1 contribute to the erosion of the sunspot. Finally, the restructuring of magnetic field topology was responsible for a homologous M2.0 flare, which shared many characteristics with an X6.5 flare on the previous day.

Conclusions. Notwithstanding the prominent role of δ-spots in flaring, we conclude based on the decomposition of the satellite sunspot, the evolution of the surrounding flow fields, and the timing of the M2.0 flare that the vanishing magnetic flux in the decaying satellite sunspot played an instrumental role in triggering the homologous M2.0 flare and the eruption of a small Hα filament. The strong magnetic field gradients of the neighboring δ-spot merely provided the vehicle for the strongest flare emission about 10 min after the onset of the flare.