ALMA small-scale features in the quiet Sun and active regions

Abstract

Aims. The main aim of the present analysis is to decipher (i) the small-scale bright features in solar images of the quiet Sun and active regions obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) and (ii) the ALMA correspondence of various known chromospheric structures visible in the Hα images of the Sun. Methods. Small-scale ALMA bright features in the quiet Sun region were analyzed using single-dish ALMA observations (1.21 mm, 248 GHz) and in an active region using interferometric ALMA measurements (3 mm, 100 GHz). With the single-dish observations, a full-disk solar image is produced, while interferometric measurements enable the high-resolution reconstruction of part of the solar disk, including the active region. The selected quiet Sun and active regions are compared with the Hα (core and wing sum), EUV, and soft X-ray images and with the magnetograms. Results. In the quiet Sun region, enhanced emission seen in the ALMA is almost always associated with a strong line-of-sight magnetic field. Four coronal bright points were identified, while other small-scale ALMA bright features are most likely associated with magnetic network elements and plages. In the active region, in 14 small-scale ALMA bright features randomly selected and compared with other images, we found five good candidates for coronal bright points, two for plages, and five for fibrils. Two unclear cases remain: a fibril or a jet, and a coronal bright point or a plage. A comparison of the Hα core image and the 3 mm ALMA image of the analyzed active region showed that the sunspot appears dark in both images (with a local ALMA radiation enhancement in sunspot umbra), the four plage areas are bright in both images and dark small Hα filaments are clearly recognized as dark structures of the same shape also in ALMA.

First looks at solar active regions with ALMA

“…Surprisingly, no clear cospatial counterpart of the mm umbral enhancement was found in the AIA 1700A, 304A, or 171A channels, although they showed generally similar large-scale structure of the AR plage as in the mm image. Brajša et al (2021) analyzed the same active region mosaic and confirmed the umbral brightening. These authors also identified several correspondences between temperature enhancements in the region surrounding the sunspot and chromospheric fibrils and coronal bright points seen in several AIA/SDO channels (Figure 2).…”

“…Mapping of the Sun with spatial resolution of order of arcseconds has revealed the fine structure of the solar radio emission from ARs. With ALMA it became possible to distinguish and describe sunspot umbra and penumbra in the mm images (Iwai et al, 2017;Loukitcheva et al, 2017), to study signatures of the three-minute oscillations and wave propagation in the mm brightness of sunspot umbra (Chai et al, 2022a), to detect oscillations of mm brightness in small bright features in a plage (Guevara Gómez et al, 2021), to study spatial association of the observed oscillations through the atmosphere mapped by the different passbands (Narang et al, 2022), and evaluate the contribution from detected high-frequency acoustic waves to the chromospheric heating in plages (Molnar et al, 2021), to explore AR small-scale transient brightenings as signatures of heating linked to magnetic reconnection events (da Silva Santos et al, 2020b) and to provide further evidence for heating in the upper chromosphere through current dissipation (da Silva Santos et al, 2022a), to image a solar active region filament at sub-arcsecond resolution with the Interface Region Imaging Spectrograph (IRIS, De Pontieu et al, 2014) and ALMA (da Silva Santos et al, 2022b), to explore the structure and the dynamics of chromospheric plages, namely, of fibrils (Chintzoglou et al, 2021a) and jet-like features (Type II spicules) (Chintzoglou et al, 2021b), to perform quantitative comparison between the mm continuum brightness and the chromospheric diagnostics from observations, e.g., with Hα (Molnar et al, 2019), and Mg II line Bastian et al, 2018;Jafarzadeh et al, 2019), to constrain the thermodynamical properties of the plage atmosphere using ALMA and IRIS data from a NLTE inversion code (da Silva Santos et al, 2020a), and finally, to identify small-scale bright features in the ALMA images and study their relation to the chromospheric structures seen in the EUV, UV and Hα images (Brajša et al, 2018(Brajša et al, , 2021, and to employ ALMA measurements in a data-constrained model of the sunspot's atmosphere (de Oliveira e Silva et al, 2022). Below we provide more details about these observational results.…”

The quiet sun at mm wavelengths as seen by ALMA

“…A good dark-dark correspondence (sunspot and small filaments) and bright-bright correspondence (plages) between the two images was indeed found by Brajša et al (2021). However, the relationship between Hα core and highresolution ALMA images in the millimeter wavelength range is more complicated, since these authors found examples of both types of behavior, correlation and anticorrelation.…”

“…Moreover, all ALMA 1.21 mm bright points show a well-defined relationship with magnetic structures (100% correspondence), mostly with small-scale bipolar magnetic regions and in some cases with small unipolar magnetic regions. Brajša et al (2021) investigated small-scale ALMA bright features in the quiet Sun region using a full-disk solar image produced with single-dish ALMA measurements (1.21 mm, 248 GHz) performed on 18 December 2015. The selected quiet Sun region was compared with the EUV and soft X-ray images and with the magnetograms (Figure 7).…”

Flares detected in ALMA single-dish images of the Sun

“…Period UT Project code Freq. N GHz 2017- 04-23 14:06 -16:30 2016.1.01129.S 230 9 2017-04-26 14:17 -16:19 2016.1.00070.S 107 11 2018-04-03 13:47 -17:39 2017.1.00072.S 95 23 2018-04-19 15:26 -17:47 2017.1.01138.S 95 14 2018-12-15 13:08 -15:12 2018.1.01879.S 95 10 For this reason, solar ALMA observations in the past have focused on stationary phenomena such as radius, center-to-limb and center -to-pole brightnening measurements (Alissandrakis et al 2017;Selhorst et al 2019;Sudar et al 2019a,b;Menezes et al 2021Menezes et al , 2022, identification and analysis of various stationary structures in full disk (Brajša et al 2018;Skokić et al 2020) and interferometric ALMA images (Iwai et al 2017;Bastian et al 2017;Nindos et al 2018;Jafarzadeh et al 2019;Rodger et al 2019;Loukitcheva et al 2019;Molnar et al 2019;Shimojo et al 2020;Wedemeyer et al 2020;Brajša et al 2020Brajša et al , 2021.…”