  The flare-prolific active region NOAA 10930 offered both a developing
delta-spot and a decaying satellite sunspot of opposite polarity. The objective
of this study is to characterize the photometric decay of the satellite sunspot
and the evolution of photospheric and chromospheric horizontal proper motions
in its surroundings. We apply the local correlation tracking technique to a
16-hour time-series of Hinode G-band and CaIIH 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. 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 152Mm2/day over
an 11-hour period. Typical lifetimes of intensity features derived from an
autocorrelation analysis are 3-5min for granulation, 25-35min for G-band bright
points, and up to 200-235min 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 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.
Notwithstanding the prominent role of delta-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 Halpha filament.
