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| Full disc solar maps obtained at (A) 230 GHz (A), (B) 100 GHz and (C) the NoRH map at 17 GHz. The blue rectangle in each panel indicates the selected area of the active region AR12470, taking into account solar rotation given timing differences relative to the HMI magnetogram.
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In recent decades our understanding of solar active regions (ARs) has improved substantially due to observations made with better angular resolution and wider spectral coverage. While prior AR observations have shown that these structures were always brighter than the quiet Sun at centimeter wavelengths, recent observations at millimeter and submil...
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We present observations of the rapid decay of a penumbral sector associated with a strong light bridge (LB) in the active region NOAA 12680 by analyzing the scattered light-corrected Solar Dynamics Observatory/Helioseismic and Magnetic Imager data. At the beginning of penumbral decay, some dark structures gradually broke away from the umbra to whic...
Citations
... We estimated wave propagation speed at different heights for all the loops as obtained for loop 6. These wave propagation speeds are estimated using time lags obtained from correlation analysis performed on Fourier-filtered light-curve pairs and corresponding height difference from the umbral atmospheric model of de Oliveira e Silva et al. ( 2022 ) and references therein, see details in Appendix D . ...
Sunspots host various oscillations and wave phenomena like umbral flashes, umbral oscillations, running penumbral waves, and coronal waves. All fan loops rooted in sunspot umbra constantly show a 3-min period propagating slow magnetoacoustic waves in the corona. However, their origin in the lower atmosphere is still unclear. In this work, we studied these oscillations in detail along a clean fan loop system rooted in active region AR12553 for a duration of 4-hour on June 16, 2016 observed by Interface Region Imaging Spectrograph (IRIS) and Solar Dynamics Observatory (SDO). We traced foot-points of several fan loops by identifying their locations at different atmospheric heights from the corona to the photosphere. We found presence of 3-min oscillations at foot-points of all the loops and at all atmospheric heights. We further traced origin of these waves by utilising their amplitude modulation characteristics while propagating in the solar atmosphere. We found several amplitude modulation periods in the range of 9–14 min, 20–24 min, and 30–40 min of these 3-min waves at all heights. Based on our findings, we interpret that 3-min slow magnetoacoustic waves propagating in coronal fan loops are driven by 3-min oscillations observed at the photospheric foot-points of these fan loops in the umbral region. We also explored any connection between 3-min and 5-min oscillations observed at the photospheric foot-points of these loops and found them to be weakly coupled. Results provide clear evidence of magnetic coupling of the solar atmosphere through propagation of 3-min waves along fan loops at different atmospheric heights.
... Semi-empirical models have also been used to investigate the observed correlation between the width of the H α line and the brightness temperatures at 3 mm in the plage-which was explained based on the mutual dependence of both diagnostics on the hydrogen atomlevel populations (Molnar et al., 2019). The agreement between empirical models and ALMA observations can also be improved in an automatic or iterative way using NLTE inversion codes (Section 3.1) when co-temporal spectroscopic observations at optical or ultraviolet (UV) wavelengths are likewise available (da Silva Santos et al., 2020a; de Oliveira e Silva et al., 2022;Hofmann et al., 2022;de Oliveira e Silva et al., 2022). Moreover, one-dimensional time-dependent simulations have been used to study the response of the solar atmosphere to flares. ...
... Semi-empirical models have also been used to investigate the observed correlation between the width of the H α line and the brightness temperatures at 3 mm in the plage-which was explained based on the mutual dependence of both diagnostics on the hydrogen atomlevel populations (Molnar et al., 2019). The agreement between empirical models and ALMA observations can also be improved in an automatic or iterative way using NLTE inversion codes (Section 3.1) when co-temporal spectroscopic observations at optical or ultraviolet (UV) wavelengths are likewise available (da Silva Santos et al., 2020a; de Oliveira e Silva et al., 2022;Hofmann et al., 2022;de Oliveira e Silva et al., 2022). Moreover, one-dimensional time-dependent simulations have been used to study the response of the solar atmosphere to flares. ...
The Atacama Large Millimeter/submillimeter Array (ALMA) offers new diagnostic possibilities that complement other commonly used diagnostics for the study of the Sun. In particular, ALMA’s ability to serve as an essentially linear thermometer of the chromospheric gas at unprecedented spatial resolution at millimeter wavelengths and future polarization measurements has great diagnostic potential. Solar ALMA observations are therefore expected to contribute significantly to answering long-standing questions about the structure, dynamics, and energy balance of the outer layers of the solar atmosphere. In this regard, current and future ALMA data are also important for constraining and further developing numerical models of the solar atmosphere, which in turn are often vital for the interpretation of observations. The latter is particularly important given the Sun’s highly intermittent and dynamic nature that involves a plethora of processes occurring over extended ranges in spatial and temporal scales. Realistic forward modeling of the Sun therefore requires time-dependent three-dimensional radiation magnetohydrodynamics that account for non-equilibrium effects and, typically as a separate step, detailed radiative transfer calculations, resulting in synthetic observables that can be compared to observations. Such artificial observations sometimes also account for instrumental and seeing effects, which, in addition to aiding the interpretation of observations, provide instructive tools for designing and optimizing ALMA’s solar observing modes. In the other direction, ALMA data in combination with other simultaneous observations enable the reconstruction of the solar atmospheric structure via data inversion techniques. This article highlights central aspects of the impact of ALMA for numerical modeling of the Sun and their potential and challenges, together with selected examples.
... Improving the agreement between empirical models and ALMA observations can also be done in an automatic or iterative way using NLTE inversion codes (Sect. 3.1) when co-temporal spectroscopic observations at optical or ultraviolet (UV) wavelengths are likewise available (da Silva Santos et al., 2020aHofmann et al., 2022;de Oliveira e Silva et al., 2022). Moreover, one-dimensional time-dependent simulations have been used to study the response of the solar atmosphere to flares. ...
The Atacama Large Millimeter/submillimeter Array (ALMA) offers new diagnostic possibilities that complement other commonly used diagnostics for the study of our Sun. In particular, ALMA's ability to serve as an essentially linear thermometer of the chromospheric gas at unprecedented spatial resolution at millimetre wavelengths and future polarisation measurements have great diagnostic potential. Solar ALMA observations are therefore expected to contribute significantly to answering long-standing questions about the structure, dynamics and energy balance of the outer layers of the solar atmosphere. In this regard, current and future ALMA data are also important for constraining and further developing numerical models of the solar atmosphere, which in turn are often vital for the interpretation of observations. The latter is particularly important given the Sun's highly intermittent and dynamic nature that involves a plethora of processes occurring over extended ranges in spatial and temporal scales. Realistic forward modelling of the Sun therefore requires time-dependent three-dimensional radiation magnetohydrodynamics that account for non-equilibrium effects and, typically as a separate step, detailed radiative transfer calculations, resulting in synthetic observables that can be compared to observations. Such artificial observations sometimes also account for instrumental and seeing effects, which, in addition to aiding the interpretation of observations, provide instructive tools for designing and optimising ALMA's solar observing modes. In the other direction, ALMA data in combination with other simultaneous observations enables the reconstruction of the solar atmospheric structure via data inversion techniques. This article highlights central aspects of the impact of ALMA for numerical modelling for the Sun, their potential and challenges, together with selected examples.
... 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 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. ...
During the first few years of observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA), the scientific community has acquired a number of observational datasets targeting various structures in active regions, including sunspot umbra and penumbra, active region pores, and plages. In this paper we review the results obtained from the extensive analysis of these interferometric millimeter data, together with the coordinated observations from IRIS, SDO, IBIS, and Hinode, that reveal information on the chromospheric thermal structure above active regions and properties of small-scale heating events near magnetic field concentrations. We discuss the properties of waves (especially the three-minute oscillations) in sunspots, plage, and network. We speculate how high-resolution millimeter data can supplement spectral line observations in the visible and UV and can improve chromospheric spectroscopic inversions. We identify challenges in the interpretation of the millimeter continuum emission due to the complex, non-local and time-dependent processes that determine the electron density through the chromosphere. Finally we overview the prospects for future active regions observations with ALMA during the ascending phase of the solar cycle.
The Atacama Large Millimeter/submillimeter Array (ALMA) allows for solar observations in the wavelength range of 0.3–10 mm, giving us a new view of the chromosphere. The measured brightness temperature at various frequencies can be fitted with theoretical models of density and temperature versus height. We use the available ALMA and Metsähovi measurements of selected solar structures (quiet sun (QS), active regions (AR) devoid of sunspots, and coronal holes (CH)). The measured QS brightness temperature in the ALMA wavelength range agrees well with the predictions of the semiempirical Avrett–Tian–Landi–Curdt–Wülser (ATLCW) model, better than previous models such as the Avrett–Loeser (AL) or Fontenla–Avrett–Loeser model (FAL). We scaled the ATLCW model in density and temperature to fit the observations of the other structures. For ARs, the fitted models require 9%–13% higher electron densities and 9%–10% higher electron temperatures, consistent with expectations. The CH fitted models require electron densities 2%–40% lower than the QS level, while the predicted electron temperatures, although somewhat lower, do not deviate significantly from the QS model. Despite the limitations of the one‐dimensional ATLCW model, we confirm that this model and its appropriate adaptations are sufficient for describing the basic physical properties of the solar structures.
Sunspots host various oscillations and wave phenomena like umbral flashes, umbral oscillations, running penumbral waves, and coronal waves. All fan loops rooted in sunspot umbra constantly show a 3-min period propagating slow magnetoacoustic waves in the corona. However, their origin in the lower atmosphere is still unclear. In this work, we studied these oscillations in detail along a clean fan loop system rooted in active region AR12553 for a duration of 4-hour on June 16, 2016 observed by Interface Region Imaging Spectrograph (IRIS) and Solar Dynamics Observatory (SDO). We traced foot-points of several fan loops by identifying their locations at different atmospheric heights from the corona to the photosphere. We found presence of 3-min oscillations at foot-points of all the loops and at all atmospheric heights. We further traced origin of these waves by utilising their amplitude modulation characteristics while propagating in the solar atmosphere. We found several amplitude modulation periods in the range of 9-14 min, 20-24 min, and 30-40 min of these 3-min waves at all heights. Based on our findings, we interpret that 3-min slow magnetoacoustic waves propagating in coronal fan loops are driven by 3-min oscillations observed at the photospheric foot-points of these fan loops in the umbral region. We also explored any connection between 3-min and 5-min oscillations observed at the photospheric foot-points of these loops and found them to be weakly coupled. Results provide clear evidence of magnetic coupling of the solar atmosphere through propagation of 3-min waves along fan loops at different atmospheric heights.
