The chromosphere above sunspots at millimeter wavelengths

Loukitcheva, M.; Solanki, S. K.; White, S. M.

doi:10.1051/0004-6361/201321321

Cited by 25 publications

(36 citation statements)

References 27 publications

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“…alma_csv_obs_v1 than 1000 K for the analyzed active region. However, sunspots are seen in absorption, as localized dark areas, in agreement with some earlier studies for the wavelengths λ ≤ 3.5 mm (Lindsey & Kopp 1995;Loukitcheva et al 2014;Iwai & Shimojo 2015). As present analysis shows, the analyzed sunspot has brightness temperature lower by 90 K than a quiet Sun region at the same radial distance from the solar disc center allowing us to conclude that sunspots are darker not only in comparison with ambient active regions, but also to the quiet Sun regions.…”

Section: Summary Discussion and Conclusionsupporting

confidence: 92%

“…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: 70%

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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: Summary Discussion and Conclusionsupporting

confidence: 92%

Section: Introductionsupporting

confidence: 70%

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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“…The first is that it is an intrinsic property of sunspots. The observed umbral brightness temperature was around 900 K higher than cool quiet-Sun regions in the ALMA image, which is in fact consistent with the umbral brightening relative to the quiet Sun suggested by a number of existing models of the umbral atmosphere (e.g., see Loukitcheva et al 2014). This suggests that the discovery in this observation might not be of an "umbral brightness enhancement" but rather of a "penumbral darkening".…”

Section: Origin Of the Umbral Brightness Enhancementsupporting

confidence: 88%

“…Hence, the observational results require non-LTE radiative-transfer simulations to facilitate their interpretation. In fact, available atmospheric models for sunspot umbrae show a large scatter of the predicted temperature at the chromosphere (see Figure 6 of Loukitcheva et al 2014). …”

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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“…Thus, Kundu & McCullough (1971) detected polarized emission in an AR at λ = 9 mm with P in the range from about 1 to 4%. We have investigated the expected circular polarization from active regions at millimeter wavelengths using two sets of umbral models: the umbral model by Severino et al (1994), which provides the best agreement with the observed mm brightness in umbrae according to Loukitcheva et al (2014), and a more recent semiempirical um- bral model by Fontenla et al (2011). The magnetic field was modeled by a vertical dipole buried under the photosphere, following Zlotnik (1968).…”

Section: Discussionmentioning

confidence: 99%

Millimeter radiation from a 3D model of the solar atmosphere

Loukitcheva

White

Solanki

et al. 2017

A&A

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Aims. We use state-of-the-art, three-dimensional non-local thermodynamic equilibrium (non-LTE) radiative magnetohydrodynamic simulations of the quiet solar atmosphere to carry out detailed tests of chromospheric magnetic field diagnostics from free-free radiation at millimeter and submillimeter wavelengths (mm/submm). Methods. The vertical component of the magnetic field was deduced from the mm/submm brightness spectra and the degree of circular polarization synthesized at millimeter frequencies. We used the frequency bands observed by the Atacama Large Millimeter/Submillimeter Array (ALMA) as a convenient reference. The magnetic field maps obtained describe the longitudinal magnetic field at the effective formation heights of the relevant wavelengths in the solar chromosphere.Results. The comparison of the deduced and model chromospheric magnetic fields at the spatial resolution of both the model and current observations demonstrates a good correlation, but has a tendency to underestimate the model field. The systematic discrepancy of about 10% is probably due to averaging of the restored field over the heights contributing to the radiation, weighted by the strength of the contribution. On the whole, the method of probing the longitudinal component of the magnetic field with free-free emission at mm/submm wavelengths is found to be applicable to measurements of the weak quiet-Sun magnetic fields. However, successful exploitation of this technique requires very accurate measurements of the polarization properties (primary beam and receiver polarization response) of the antennas, which will be the principal factor that determines the level to which chromospheric magnetic fields can be measured. Conclusions. Consequently, high-resolution and high-precision observations of circularly polarized radiation at millimeter wavelengths can be a powerful tool for producing chromospheric longitudinal magnetograms.

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The chromosphere above sunspots at millimeter wavelengths

Cited by 25 publications

References 27 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

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

Millimeter radiation from a 3D model of the solar atmosphere

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