http://arxiv.org/abs/1509.00312
Umbral dots (UDs) are small isolated brightening observed in sunspot umbrae. They are convective phenomena existing inside umbrae. UDs are usually divided into central UDs (CUDs) and peripheral UDs (PUDs) with respect to their positions inside an umbra. Our purpose is to investigate UD properties and analyze their relationships, and further to find whether or not the properties depend on the umbral magnetic field variation. For the purpose, we selected the high-resolution TiO images of four active regions (ARs) obtained under the best seeing conditions with the \emph{New Vacuum Solar Telescope} (NVST) in Fuxian Solar Observatory of Yunnan Astronomical Observatory, China. The four ARs (NOAA 11598, 11801, 12158, and 12178) include six sunspots. A total of 1220 CUDs were extracted from six sunspots, and 603 PUDs from three sunspots. Meanwhile, the radial component of the magnetic field of the sunspots obtained with the \emph{Helioseismic and Magnetic Imager} onboard the \emph{Solar Dynamics Observatory} was used to analyze the influence to UD properties. To CUDs, their diameters and lifetimes exhibit an increasing trend with brightness, whereas their horizontal velocities exhibit an inverse trend. Moreover, the properties: diameter, intensity and velocity depend on magnetic field variation. To a CUD, its diameter becomes larger and brighter, and its motion shows slower in a weak magnetic field than in a strong field. To PUDs, the similar trends are also found. Moreover, we also find that the lifetimes of UDs located in different sunspots are not obviously different, implying that they are unrelated to the magnetic flux density in which they lived.
J. Kaifan, J. Xia, F. Song, et. al.
Wed, 2 Sep 15
16/87
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