Center-to-limb observations of the Sun with ALMA

Alissandrakis, C. E.; Patsourakos, S.; Nindos, A.; Bastian, T. S.

doi:10.1051/0004-6361/201730953

Cited by 50 publications

(49 citation statements)

References 20 publications

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“…In present analysis we have determined the brightness temperature of the quiet Sun region at the disc center to be 6040 K at 1.21 mm, which is fully consistent with the central brightness temperature determined by White et al (2017) for the 2015 data (6040 ± 250 K). Also the estimated limb brightening at 1.21 mm is up to 10 %, consistent with the results of Alissandrakis et al (2017).…”

Section: Summary Discussion and Conclusionsupporting

confidence: 89%

“…The quiet Sun brightness temperature increases steadily with increasing radial distance from the disc center consistent with a pronounced limb brightening. The amount of this limb brightening is in the order of 10 % in agreement with recent results of Alissandrakis et al (2017). The central brightness temperature of the quiet Sun is measured as the average value of all intensities within the small circle located at the center of the solar disc in Fig.…”

Section: Quiet Sunsupporting

confidence: 89%

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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: 89%

Section: Quiet Sunsupporting

confidence: 89%

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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“…Once prominence observations are obtained, it will also be important to understand how to use such measurements to infer information about the temperature and plasma structures in question. The visibility of prominences beyond the limb in both ALMA Bands 3 and 6 has recently been indicated by recent ALMA science verification full-disk imaging results (White et al, 2017;Alissandrakis et al, 2017).…”

Section: Introductionmentioning

confidence: 79%

Solar Prominence Modelling and Plasma Diagnostics at ALMA Wavelengths

Rodger

Labrosse

2017

Sol Phys

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Our aim is to test potential solar prominence plasma diagnostics as obtained with the new solar capability of the Atacama Large Millimeter/submillimeter Array (ALMA). We investigate the thermal and plasma diagnostic potential of ALMA for solar prominences through the computation of brightness temperatures at ALMA wavelengths. The brightness temperature, for a chosen line of sight, is calculated using the densities of electrons, hydrogen, and helium obtained from a radiative transfer code under non-local thermodynamic equilibrium (non-LTE) conditions, as well as the input internal parameters of the prominence model in consideration. Two distinct sets of prominence models were used: isothermal-isobaric fine-structure threads, and large-scale structures with radially increasing temperature distributions representing the prominence-to-corona transition region. We compute brightness temperatures over the range of wavelengths in which ALMA is capable of observing (0.32 -9.6 mm), however, we particularly focus on the bands available to solar observers in ALMA cycles 4 and 5, namely 2.6 -3.6 mm (Band 3) and 1.1 -1.4 mm (Band 6). We show how the computed brightness temperatures and optical thicknesses in our models vary with the plasma parameters (temperature and pressure) and the wavelength of observation. We then study how ALMA observables such as the ratio of brightness temperatures at two frequencies can be used to estimate the optical thickness and the emission measure for isothermal and non-isothermal prominences. From this study we conclude that for both sets of models, ALMA presents a strong thermal diagnostic capability, provided that the interpretation of observations is supported by the use of non-LTE simulation results.

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“…The contribution of H − absorption becomes non-negligible at wavelengths shorter than about 1 mm, where radiation is formed in cooler layers. Alissandrakis et al (2017) have estimated that at around electron temperature of 6,000 K, the H − opacity is about 10% of the total opacity (see also Wedemeyer et al, 2016).…”

Section: H − Free-free Mechanismmentioning

confidence: 99%

Incoherent Solar Radio Emission

Nindos

2020

Front. Astron. Space Sci.

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Incoherent solar radio radiation comes from the free-free, gyroresonance, and gyrosynchrotron emission mechanisms. Free-free is primarily produced from Coulomb collisions between thermal electrons and ions. Gyroresonance and gyrosynchrotron result from the acceleration of low-energy electrons and mildly relativistic electrons, respectively, in the presence of a magnetic field. In the non-flaring Sun, free-free is the dominant emission mechanism with the exception of regions of strong magnetic fields which emit gyroresonance at microwaves. Due to its ubiquitous presence, free-free emission can be used to probe the non-flaring solar atmosphere above temperature minimum. Gyroresonance opacity depends strongly on the magnetic field strength and orientation; hence it provides a unique tool for the estimation of coronal magnetic fields. Gyrosynchrotron is the primary emission mechanism in flares at frequencies higher than 1-2 GHz and depends on the properties of both the magnetic field and the accelerated electrons, as well as the properties of the ambient plasma. In this paper we discuss in detail the above mechanisms and their diagnostic potential.

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Center-to-limb observations of the Sun with ALMA

Cited by 50 publications

References 20 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 Prominence Modelling and Plasma Diagnostics at ALMA Wavelengths

Incoherent Solar Radio Emission

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