ANALYSIS OF A FRAGMENTING SUNSPOT USING<i>HINODE</i>OBSERVATIONS

Abstract

We employ high resolution filtergrams and polarimetric measurements from\nHinode to follow the evolution of a sunspot for eight days starting on June 28,\n2007. The imaging data were corrected for intensity gradients, projection\neffects, and instrumental stray light prior to the analysis. The observations\nshow the formation of a light bridge at one corner of the sunspot by a slow\nintrusion of neighbouring penumbral filaments. This divided the umbra into two\nindividual umbral cores. During the light bridge formation, there was a steep\nincrease in its intensity from 0.28 to 0.7 I_QS in nearly 4 hr, followed by a\ngradual increase to quiet Sun (QS) values in 13 hr. This increase in intensity\nwas accompanied by a large reduction in the field strength from 1800 G to 300\nG. The smaller umbral core gradually broke away from the parent sunspot nearly\n2 days after the formation of the light bridge rendering the parent spot\nwithout a penumbra at the location of fragmentation. The penumbra in the\nfragment disappeared first within 34 hr, followed by the fragment whose area\ndecayed exponentially with a time constant of 22 hr. The depleted penumbra in\nthe parent sunspot regenerated when the inclination of the magnetic field at\nthe penumbra-QS boundary became within 40 deg. from being completely horizontal\nand this occurred near the end of the fragment's lifetime. After the\ndisappearance of the fragment, another light bridge formed in the parent which\nhad similar properties as the fragmenting one, but did not divide the sunspot.\nThe significant weakening in field strength in the light bridge along with the\npresence of granulation is suggestive of strong convection in the sunspot which\nmight have triggered the expulsion and fragmentation of the smaller spot.\nAlthough the presence of QS photospheric conditions in sunspot umbrae could be\na necessary condition for fragmentation, it is not a sufficient one.\n