ANALYSIS OF A FRAGMENTING SUNSPOT USINGHINODEOBSERVATIONS

Abstract

We employ high resolution filtergrams and polarimetric measurements from Hinode to follow the evolution of a sunspot for eight days starting on June 28, 2007. The imaging data were corrected for intensity gradients, projection effects, and instrumental stray light prior to the analysis. The observations show the formation of a light bridge at one corner of the sunspot by a slow intrusion of neighbouring penumbral filaments. This divided the umbra into two individual umbral cores. During the light bridge formation, there was a steep increase in its intensity from 0.28 to 0.7 I QS in nearly 4 hr, followed by a gradual increase to quiet Sun (QS) values in 13 hr. This increase in intensity was accompanied by a large reduction in the field strength from 1800 G to 300 G. The smaller umbral core gradually broke away from the parent sunspot nearly 2 days after the formation of the light bridge rendering the parent spot without a penumbra at the location of fragmentation. The penumbra in the fragment disappeared first within 34 hr, followed by the fragment whose area decayed exponentially with a time constant of 22 hr. In comparison, the parent sunspot area followed a linear decay rate of 0.94 Mm 2 hr −1 . The depleted penumbra in the parent sunspot regenerated when the inclination of the magnetic field at the penumbra-QS boundary became within 40 • from being completely horizontal and this occurred near the end of the fragment's lifetime. After the disappearance of the fragment, another light bridge formed in the parent which had similar properties as the fragmenting one, but did not divide the sunspot. The significant weakening in field strength in the light bridge along with the presence of granulation is suggestive of strong convection in the sunspot which might have triggered the expulsion and fragmentation of the smaller spot. Although the presence of QS photospheric conditions in sunspot umbrae could be a necessary condition for fragmentation, it is not a sufficient one. ABSTRACTWe employ high resolution filtergrams and polarimetric measurements from Hinode to follow the evolution of a sunspot for eight days starting on June 28, 2007. The imaging data were corrected for intensity gradients, projection effects, and instrumental stray light prior to the analysis. The observations show the formation of a light bridge at one corner of the sunspot by a slow intrusion of neighbouring penumbral filaments. This divided the umbra into two individual umbral cores. During the light bridge formation, there was a steep increase in its intensity from 0.28 to 0.7 I QS in nearly 4 hr, followed by a gradual increase to quiet Sun (QS) values in 13 hr. This increase in intensity was accompanied by a large reduction in the field strength from 1800 G to 300 G. The smaller umbral core gradually broke away from the parent sunspot nearly 2 days after the formation of the light bridge rendering the parent spot without a penumbra at the location of fragmentation. The penumbra in the fragment disappeared first within 34 hr, followed by the f...

Peripheral downflows in sunspot penumbrae

“…These downflows reach maximum velocities of up to 8 km s −1 in 1.6-6 arcsec 2 sized patches in the inner penumbra of a sunspot and have also been reported by Louis et al (2012). Franz & Schlichenmaier (2009) determined downflow velocities up to 9 km s −1 in the leading spot of NOAA AR 10933, the same as one of the sunspots, which we investigate in the present paper.…”

Small-scale magnetic flux emergence in a sunspot light bridge

“…This is significantly different from quiet-Sun flux emergence, where the field becomes more vertical as the loop rises in the atmosphere (Martínez González & Bellot Rubio 2009). Light bridges are known to be structures where the umbra is subject to convective instabilities (Jurčák et al 2006;Louis et al 2012). The granular morphology of the LB thus offers a natural location for the emergence of small-scale magnetic fields, aided by the upflows of convective cells.…”

“…Light bridges (LBs) are bright structures in the dark umbra of sunspots (Muller 1979), which bear an umbral, penumbral, or quiet Sun morphology depending on their lifetimes (Sobotka et al 1994;Lites et al 2004;Rimmele 2008;Louis et al 2012). They are often seen during the formation and decay of sunspots along fissures where individual fragment spots coalesce or split (Garcia de La Rosa 1987).…”

Evolution of Active Regions

“…The exact role of light bridges in the decay of sunspots is still an open question. Indeed, Louis et al (2012) find that although light bridges may be necessary for the break-up of spots, they are not sufficient to cause the fragmentation. 4.4 Asymmetry in decay of bipoles and polarity imbalance Karachik et al (2010) followed the evolution of four isolated bipolar ARs over two or three rotations as they decayed and measured several parameters of their magnetic fields including total flux, imbalance, and compactness.…”