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Sunspot Bright Points

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Abstract

We used the flux-calibrated images from the Broad-band Filter Imager and Stokes Polarimeter data obtained with the Solar Optical Telescope onboard the Hinode spacecraft to study the properties of bright points in and around sunspots. The selected bright points are smaller in diameter than 150 km with contrasts exceeding about 3 % in the ratio of sunspot images obtained with the G-band (430.5 nm) and Ca ii H (396.85 nm) filters. The bright points are classified as umbral dot, peripheral umbral dot, penumbral grains, and G-band bright point depending on their location. The bright points are preferentially located around the penumbral boundary and in the fast decaying parts of the umbra. The color temperature of the bright points is in the range of 4600 K to 6600 K with cooler ones located in the central part of the umbra. The temperature increases as a function of distance from the center outward. The G-band, CN-band (388.35 nm), and Ca ii H fluxes of the bright points as a function of their blue-band (450.55 nm) brightness increase continuously in a nonlinear fashion unlike their red (668.4 nm) and green (555.05 nm) counterparts. This is consistent with a model in which the localized heating of the flux tube depletes the molecular concentration, resulting in the reduced opacity that leads to the exposition of deeper and hotter layers. The light curve of the bright points shows that the enhanced brightness at these locations lasts for about 15 to 60 min with the least contrast for the points outside the sunspot. The umbral dots near the penumbral boundary are associated with elongated filamentary structures. The spectropolarimeter observations show that the filling factor decreases as the G-band brightness increases. We discuss the results using the model in which the G-band bright points are produced in the cluster of flux tubes that a sunspot consists of.

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Acknowledgements

One of the authors (DPC) is grateful to the Institute of Space and Astronautical Sciences (ISAS/JAXA) for the hospitality during this work. We would like to thank Eugene Parker for stimulating discussions. A part of the support comes from NSF under grant ATM 05-48260 and NASA grant NNX 08AQ32G. The data were obtained using Hinode Solar Optical Telescope. Hinode is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA and STFC (UK) as international partners. It is operated by these agencies in co-operation with ESA and NSC (Norway).

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Correspondence to Debi Prasad Choudhary.

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Choudhary, D.P., Shimizu, T. Sunspot Bright Points. Sol Phys 288, 171–190 (2013). https://doi.org/10.1007/s11207-013-0306-8

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Keywords

  • Sunspots: magnetic field
  • Sunspots: temperature
  • Sunspots: umbral dots
  • Sunspots: velocity field