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

Abstract

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.

Transient brightenings in the quiet Sun detected by ALMA at 3 mm

“…The availability of millimeter-λ solar observations with Atacama Large Millimeter/submillimeter Array (ALMA) has the potential to provide new insights into the physics of the chromosphere thanks to the superior spatial and temporal resolution and sensitivity of the instrument. Observations of the quiet Sun with ALMA have been reported by Shimojo et al (2017a,b), White et al (2017), Alissandrakis et al (2017b), Bastian et al (2017), Yokoyama et al (2018), Nindos et al (2018), Loukitcheva et al (2019), Brajša et al (2018), Jafarzadeh et al (2019), Selhorst et al (2019), Molnar et al (2019), Patsourakos et al (2020), andWedemeyer et al (2020). Among those articles, detection of single weak transient events was reported by Shimojo et al (2017b), who studied a plasmoid ejection associated with an X-ray bright point, and by Yokoyama et al (2018) who reported jet-like activity at 3 mm.…”

The Sun at millimeter wavelengths

“…While the first regular ALMA observations of the Sun were only offered in Cycle 4 with a first solar campaign in December 2016, earlier observations from Commissioning and Science Verification (CSV) campaigns have been made publicly available. Both regular and CSV data are already used in publications: Alissandrakis et al (2017), Bastian et al (2017), Shimojo et al (2017b), Brajša et al (2018), Nindos et al (2018), Yokoyama et al (2018), Jafarzadeh et al (2019), Loukitcheva et al (2019), Molnar et al (2019), Rodger et al (2019), Selhorst et al (2019), Patsourakos et al (2020), da Silva Santos et al (2020.…”

The multi-thermal chromosphere

“…The Atacama Large Millimeter/Submillimeter Array (ALMA, Wootten & Thompson 2009) has overcome the spatial resolution limitations that observing at such long wavelengths entails. ALMA has shown that the landscape of the solar surface at 1 mm is marked by relatively cooler regions such as the QS and sunspot umbras, and hotter features such as plage regions (Loukitcheva et al 2017;Brajša et al 2018), and that the brightness of the mm continuum correlates with that of the Mg ii lines (Bastian et al 2017(Bastian et al , 2018Jafarzadeh et al 2019). However, the large scatter and offset between the two diagnostics is evidence for NLTE effects in the Mg ii lines that make them only partially sensitive to the chromospheric temperatures (e.g., Leenaarts et al 2013a), although there may also be systematic differences in their formation heights (e.g., da Silva Santos et al 2018).…”