Chromospheric Sunspots in the Millimeter Range as Observed by the Nobeyama Radioheliograph

Iwai, Kazumasa; Koshiishi, H.; Shibasaki, Κ.; Nozawa, Satoshi; Miyawaki, Shun; Yoneya, Takuro

doi:10.3847/0004-637x/816/2/91

Cited by 14 publications

(14 citation statements)

References 24 publications

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“…As expected, active regions at mm and sub-mm wavelengths generally appear as bright areas, although sunspot umbrae, if resolved, have lower brightness temperatures than the quiet Sun level at λ ≤ 3.5 mm (Lindsey & Kopp 1995;Loukitcheva et al 2014;Iwai & Shimojo 2015) or have almost equal brightness temperature as the quiet Sun at λ = 8.8 mm (Iwai et al 2016).…”

Section: Introductionsupporting

confidence: 71%

First analysis of solar structures in 1.21 mm full-disc ALMA image of the Sun

Brajša

Sudar

Benz

et al. 2018

A&A

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Context. Various solar features can be seen in emission or absorption on maps of the Sun in the millimeter and submillimeter wavelength range. The recently installed Atacama Large Millimeter/submillimeter Array (ALMA) is capable of observing the Sun in that wavelength range with an unprecedented spatial, temporal and spectral resolution. To interpret solar observations with ALMA the first important step is to compare solar ALMA maps with simultaneous images of the Sun recorded in other spectral ranges. Aims. The first aim of the present work is to identify different structures in the solar atmosphere seen in the optical, infrared and EUV parts of the spectrum (quiet Sun, active regions, prominences on the disc, magnetic inversion lines, coronal holes and coronal bright points) in a full disc solar ALMA image. The second aim is to measure the intensities (brightness temperatures) of those structures and to compare them with the corresponding quiet Sun level. Methods. A full disc solar image at 1.21 mm obtained on December 18, 2015 during a CSV-EOC campaign with ALMA is calibrated and compared with full disc solar images from the same day in Hα line, in He I 1083 nm line core, and with various SDO images (AIA at 170 nm, 30.4 nm, 21.1 nm, 19.3 nm, and 17.1 nm and HMI magnetogram). The brightness temperatures of various structures are determined by averaging over corresponding regions of interest in the calibrated ALMA image. Results. Positions of the quiet Sun, active regions, prominences on the disc, magnetic inversion lines, coronal holes and coronal bright points are identified in the ALMA image. At the wavelength of 1.21 mm active regions appear as bright areas (but sunspots are dark), while prominences on the disc and coronal holes are not discernible from the quiet Sun background, although having slightly less intensity than surrounding quiet Sun regions. Magnetic inversion lines appear as large, elongated dark structures and coronal bright points correspond to ALMA bright points. Conclusions. These observational results are in general agreement with sparse earlier measurements at similar wavelengths. The identification of coronal bright points represents the most important new result. By comparing ALMA and other maps, it was found that the ALMA image was oriented properly and that the procedure of overlaying the ALMA image with other images is accurate at the 5 arc sec level. The potential of ALMA for physics of the solar chromosphere is emphasized.

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Section: Introductionsupporting

confidence: 71%

First analysis of solar structures in 1.21 mm full-disc ALMA image of the Sun

Brajša

Sudar

Benz

et al. 2018

A&A

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“…12) are best suited for comparison with the brightness differences calculated from the models. For completeness, we also plot the observational values obtained from BIMA maps at 3.5 mm by Loukitcheva et al (2014) for the big and small umbrae (triangles), respectively, at a resolution of 12 ; the measurements from JCMT at 0.35, 0.85 and 1.2 mm (diamonds) made by Lindsey & Kopp (1995) at a resolution of 14-17 ; brightness observations at 2.6 and 3.5 mm obtained from the Nobeyama 45-m telescope by Iwai & Shimojo (2015) at a resolution of 15 , and at 8.8 mm from the NoRH by Iwai et al (2016) at a resolution of 5-10 (plus signs). Figure 12 shows that, at wavelengths longer than 1.3 mm, the observed umbral brightness, either averaged over the whole umbra (green circles at 1.3 mm and 3 mm), or measured with moderate spatial resolution (at 2.6, 3.5, and 8.8 mm), is not very different from the QS brightness.…”

Section: Penumbral Modelsmentioning

confidence: 99%

Solar ALMA Observations: Constraining the Chromosphere above Sunspots

Loukitcheva

Iwai

Solanki

et al. 2017

ApJ

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We present the first high-resolution Atacama Large Millimeter/Submillimeter Array (ALMA) observations of a sunspot at wavelengths of 1.3 mm and 3 mm, obtained during the solar ALMA Science Verification campaign in 2015, and compare them with the predictions of semi-empirical sunspot umbral/penumbral atmosphere models. For the first time millimeter observations of sunspots have resolved umbral/penumbral brightness structure at the chromospheric heights, where the emission at these wavelengths is formed. We find that the sunspot umbra exhibits a radically different appearance at 1.3 mm and 3 mm, whereas the penumbral brightness structure is similar at the two wavelengths. The inner part of the umbra is ∼600 K brighter than the surrounding quiet Sun (QS) at 3 mm and is ∼700 K cooler than the QS at 1.3 mm, being the coolest part of sunspot at this wavelength. On average, the brightness of the penumbra at 3 mm is comparable to the QS brightness, while at 1.3 mm it is ∼1000 K brighter than the QS. Penumbral brightness increases towards the outer boundary in both ALMA bands. Among the tested umbral models, that of Severino et al. (1994) provides the best fit to the observational data, including both the ALMA data analyzed in this study and data from earlier works. No penumbral model amongst those considered here gives a satisfactory fit to the currently available measurements. ALMA observations at multiple mm wavelengths can be used for testing existing sunspot models, and serve as an important input to constrain new empirical models.

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“…In both cases, the umbra was darker than the surrounding atmosphere, albeit with a spatial resolution of [12][13][14][15][16] arcsec that limited their ability to separate typical sunspot umbrae from the surrounding bright penumbra and active region. At a wavelength of 8.8 mm, Iwai et al (2016) found that sunspots are generally not distinguishable from the surrounding atmosphere using the Nobeyama Radioheliograph (NoRH).…”

Section: Introductionmentioning

confidence: 99%

ALMA Discovery of Solar Umbral Brightness Enhancement at λ = 3 mm

Iwai

Loukitcheva

Shimojo

et al. 2017

ApJL

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We report the discovery of a brightness enhancement in the center of a large sunspot umbra at a wavelength of 3 mm using the Atacama Large Millimeter/sub-millimeter Array (ALMA). Sunspots are amongst the most prominent features on the solar surface, but many of their aspects are surprisingly poorly understood. We analyzed a λ=3 mm (100 GHz) mosaic image obtained by ALMA, which includes a large sunspot within the active region AR12470 on December 16, 2015. The 3 mm map has a 300"×300" field-of-view and 4".9×2".2 spatial resolution, which is the highest spatial-resolution map of an entire sunspot in this frequency range. We find a gradient of 3 mm brightness from a high value in the outer penumbra to a low value in the inner penumbra/outer umbra. Within the inner umbra, there is a marked increase in 3mm brightness temperature, which we call an umbral brightness enhancement. This enhanced emission corresponds to a temperature excess of 800 K relative to the surrounding inner penumbral region and coincides with excess brightness in the 1330 and 1400 Å slitjaw images of the Interface Region Imaging Spectrograph (IRIS), adjacent to a partial lightbridge. This λ=3 mm brightness enhancement may be an intrinsic feature of the sunspot umbra at chromospheric heights, such as a manifestation of umbral flashes, or it could be related to a coronal plume since the brightness enhancement was coincident with the footpoint of a coronal loop observed at 171 Å.

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Chromospheric Sunspots in the Millimeter Range as Observed by the Nobeyama Radioheliograph

Cited by 14 publications

References 24 publications

First analysis of solar structures in 1.21 mm full-disc ALMA image of the Sun

First analysis of solar structures in 1.21 mm full-disc ALMA image of the Sun

Solar ALMA Observations: Constraining the Chromosphere above Sunspots

ALMA Discovery of Solar Umbral Brightness Enhancement at λ = 3 mm

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