Investigation of Umbral Dots with the New Vacuum Solar Telescope

Abstract

Umbral dots (UDs) are small isolated brightenings observed in sunspot umbrae. They are convective phenomena existing inside umbrae. UDs are usually divided into central UDs (CUDs) and peripheral UDs (PUDs) according 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 umbral magnetic field strengths. Thus, we selected high-resolution TiO images of four active regions (ARs) taken under the best seeing conditions with the \textit{New Vacuum Solar Telescope} in the Fuxian Solar Observatory of the Yunnan Astronomical Observatory, China. The four ARs (NOAA 11598, 11801, 12158, and 12178) include six sunspots. A total of 1220 CUDs and 603 PUDs were identified. Meanwhile, the radial component of the vector magnetic field of the sunspots taken with the \textit{Helioseismic and Magnetic Imager} on-board the \textit{Solar Dynamics Observatory} was used to analyze relationships between UD properties and umbral magnetic field strengths. We find that diameters and lifetimes of UDs exhibit an increasing trend with the brightness, but velocities do not. Moreover, diameters, intensities, lifetimes and velocities depend on the surrounding magnetic field. A CUD diameter was found larger, the CUD brighter, its lifetime longer, and its motion slower in a weak umbral magnetic field environment than in a strong one.Comment: 16 Pages, 7 Figures, and 5 Tables, accepted for publication in Solar Physic

Comparison of different populations of granular features in the solar photosphere

“…The scatter plot derived from PLGs shows a distribution similar to that found by Berrilli et al (2002) and Yu et al (2011) in their studies on quiet-Sun granulation. Indeed, the UDs distribution revealed from our study is not in agreement with the results found by Kilcik et al (2012) and Ji et al (2016). The LBGs distribution is similar to the UDs distribution, although for LBGs the range of intensity values is shifted into higher intensities.…”

“…Other authors studied the mean UDs intensity referring to the adjacent umbral background. The UDs mean intensity obtained by Watanabe et al (2012) is 1.46-1.50 I ud /I bg , where I ud is the maximum umbral dot intensity and I bg is the adjacent umbral background, while the results obtained by Ji et al (2016) indicate that the UDs intensity is in the range between 1.05 I ud /I bg and 1.15 I ud /I bg . Among the physical properties, the study of Doppler velocities inside UDs was carried out by several authors.…”

“…These observations have unveiled some physical properties of several fine structures of the sunspots such as umbral dots (UDs), the small and isolated bright features often Feng et al (2014) observed in the umbra. UDs are convective phenomena and observations are important in order to know whether or not the properties depend on umbral magnetic field strengths (Ji et al 2016). Different researchers tried to study the UDs physical parameters, but this investigation is complicated due to their size at the limit of the spatial resolution achieved by current telescopes.…”

“…55. Recently, investigations on UDs properties carried out by Ji et al (2016) show diameter values between 0 . 2 and 0 .…”

The Formation of an Inverse S-Shaped Active-Region Filament Driven by Sunspot Motion and Magnetic Reconnection

“…Then, we co-aligned the SDO and NVST images using the subpixel registration algorithm, respectively (Feng et al 2012;Yang et al 2014;Ji et al 2015). Because 304Å and Hα images have similar structures, we used a cross-correlation method to align the two images and then the other wavelength images of AIA were co-aligned with the 304Å images.…”