Where is the strongest field located in sunspots ? - A statistical analysis using Hinode/Spectro-Polarimeter -

Sunspots are concentrations of magnetic fields on the solar surface. The strongest field in a sunspot is generally located in its umbra, but sometimes a stronger field is found outside the umbra, such as in a penumbra or in a light bridge. How is such a strong field created ? This has puzzled us for a long time, but recently we proposed a mechanism that a stronger field can be generated as a result of compression of one umbra pushed by horizontal flows from the other umbra, by analyzing datasets including an extremely-strong magnetic field located in a light-bridge-like feature between the opposite-polarity umbrae in AR 11967. Now to confirm our hypothesis, we performed a statistical analysis of strongest fields in sunspots, using numerous high-quality datasets taken with the Hinode/Spectro-Polarimeter over 10 years. In this study, first we complied a ranking list of active regions (ARs) by their largest field strengths and then investigated physical quantities and conditions in and around strong magnetic fields. We found that three-quarters of ARs had their strongest fields in umbrae, while the other quarter of ARs had their strongest field outside umbrae and often obtained high rankings. 84% of them have similar features to those in AR 11967. Hence, we conclude that our scenario is statistically correct.