ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O O O O B-Wavelength I-Wavelength O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Telescope O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Telescope O O B-CelestialObject O O O O O O O O O O O O O O O O O O O O B-Observatory B-Telescope O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O O O B-Wavelength O O B-Wavelength O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O B-Software O B-Software O O O O O O B-Software O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Software O O O O O O O O O B-Telescope O O O B-Software O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Telescope O B-Wavelength I-Wavelength O O O O O O O O
tokens: K, and purple: 7100 K. Table 1 summarizes the brightness temperatures of the sunspot umbra and quiet regions surrounded by the red and white rectangles, respectively, in Figures 2 – 4 . The brightness temperature of the 115 GHz image is also defined by the histogram of the observed level. The quiet region level at 115 GHz was set as 7500 K from Selhorst et al. ( 2005 ). Hence, the quiet-region levels of the radio emissions are defined by the data of the entire disk. Therefore, the brightness temperatures of the quiet regions surrounded by the white rectangles are different from the quiet-region values (9000 K at 34 GHz and 7500 K at 115 GHz). Table 1 Radio Brightness Temperatures of the Umbra and Quiet Regions Note . The umbra and quiet regions are surrounded by red and white rectangles, respectively, in Figures 2 – 4 . In this study, we selected an additional eight active regions observed by the NoRH at 34 GHz, in order to confirm the generality of this finding of low umbral contrast. The selected active regions contained relatively large and well-isolated sunspots including the sunspots observed on 2014 January 8 (see Figure 2 ) and 2014 October 24 (see Figure 3 ). We did not use the data from the sunspot observed on 2014 February 12 (Figure 4 ) because that sunspot was not large relative to the others. Although the NoRH observes the Sun every day, we limited the selection of regions to those with large sunspots located near the disk center for which SDO /AIA data are available. We also eliminated the data affected by flares and the spatial aliasing effect. We chose a 20′′ square region at the center of the umbra for each sunspot and derived the averaged brightness temperature of the center regions. Table 2 lists the 10 selected sunspots. Table 2 Radio Brightness Temperatures and UV/EUV Emissions at 1700 and 304 Å of the 10 Umbrae 4. DISCUSSION 4.1. Image Quality In this study, we applied two types of image synthesis, i.e., the CLEAN and Steer-CLEAN syntheses. Our result shows that the Steer-CLEAN synthesis provides a smoother image. This effect is more apparent in the quiet regions in Figures 1 (a) and (b). The standard deviations of the brightness temperatures of the sky region surrounded by the white rectangles in Figures 1 (a) and (b) are 377 and 284 K, respectively. Hence, the Steer-CLEAN synthesis provides a slightly better image from the same NoRH data. However, the Steer-CLEAN synthesis provides smoother structures in the peak brightness region. Hence, the dynamic ranges of these images (the ratio between the peak brightness and the standard deviation of the sky region) are almost the same (∼21 dB). It should be noted that the sky region surrounded by the white rectangles in Figure 1 partially contains the aliasing structures of the disk. Hence, the noise level of the sky region varies strongly with the region of our focus. Although the best spatial resolution of the NoRH at 34 GHz is about 5′′, the typical spatial resolutions in