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Submillimeter and millimeter wavelengths provide a unique view of the Universe, from the gas and dust that fills and surrounds galaxies to the chromosphere of our own Sun. Current single-dish facilities have presented a tantalising view of the brightest (sub-)mm sources, and interferometers have provided the exquisite resolution necessary to analyse the details in small fields, but there are still many open questions that cannot be answered with current facilities. In this report we summarise the science that is guiding the design of the Atacama Large Aperture Submillimeter Telescope (AtLAST). We demonstrate how tranformational advances in topics including star formation in high redshift galaxies, the diffuse circumgalactic medium, Galactic ecology, cometary compositions and solar flares motivate the need for a 50m, single-dish telescope with a 1-2 degree field of view and a new generation of highly multiplexed continuum and spectral cameras. AtLAST will have the resolution to drastically lower the confusion limit compared to current single-dish facilities, whilst also being able to rapidly map large areas of the sky and detect extended, diffuse structures. Its high sensitivity and large field of view will open up the field of submillimeter transient science by increasing the probability of serendipitous detections. Finally, the science cases listed here motivate the need for a highly flexible operations model capable of short observations of individual targets, large surveys, monitoring programmes, target of opportunity observations and coordinated observations with other observatories. AtLAST aims to be a sustainable, upgradeable, multipurpose facility that will deliver orders of magnitude increases in sensitivity and mapping speeds over current and planned submillimeter observatories., Comment: 47 pages, 12 figures. For further details on AtLAST see https://atlast.uio.no

We compile the catalog of Hvar Observatory solar observations in the time period corresponding to regular digitally stored chromospheric and photospheric observations 2010-2019. We make basic characterisation of observed phenomena and compare them to catalogs which are based on full disc solar images. We compile a catalog of observed ARs consisting of 1100 entries, where each AR is classified according to McIntosh and Mt Wilson classifications. We find that HVAR observations are biased towards more frequently observing more complex ARs and observing them in longer time periods, likely related to the small FOV not encompassing the whole solar disc. In H$\alpha$ observations we catalog conspicuous filaments/prominences and flares. We characterise filaments according to their location, chirality (if possible) and eruptive signatures. Analysis of the eruptive filaments reveals a slight bias in HVAR catalog towards observation of partial eruptions, possibly related to the observers tendency to observe filament which already showed some activity. In the flare catalog we focus on their observed eruptive signatures (loops or ribbons) and their shape. In addition, we associate them to GOES soft X-ray flares to determine their corresponding class. We find that HVAR observations seem biased towards more frequently observing stronger flares and observing them in longer time periods. We demonstrate the feasibility of the catalog on a case study of the flare detected on 2 August 2011 in HVAR H$\alpha$ observations and related Sun-to-Earth phenomena. Through flare-CME-ICME association we demonstrate the agreement of remote and in situ properties. The data used for this study, as well as the catalog, are made publicly available., Comment: Solar Physics, 24 pages, 14 figures

Observations at (sub-)millimeter wavelengths offer a complementary perspective on our Sun and other stars, offering significant insights into both the thermal and magnetic composition of their chromospheres. Despite the fundamental progress in (sub-)millimeter observations of the Sun, some important aspects require diagnostic capabilities that are not offered by existing observatories. In particular, simultaneously observations of the radiation continuum across an extended frequency range would facilitate the mapping of different layers and thus ultimately the 3D structure of the solar atmosphere. Mapping large regions on the Sun or even the whole solar disk at a very high temporal cadence would be crucial for systematically detecting and following the temporal evolution of flares, while synoptic observations, i.e., daily maps, over periods of years would provide an unprecedented view of the solar activity cycle in this wavelength regime. As our Sun is a fundamental reference for studying the atmospheres of active main sequence stars, observing the Sun and other stars with the same instrument would unlock the enormous diagnostic potential for understanding stellar activity and its impact on exoplanets. The Atacama Large Aperture Submillimeter Telescope (AtLAST), a single-dish telescope with 50\,m aperture proposed to be built in the Atacama desert in Chile, would be able to provide these observational capabilities. Equipped with a large number of detector elements for probing the radiation continuum across a wide frequency range, AtLAST would address a wide range of scientific topics including the thermal structure and heating of the solar chromosphere, flares and prominences, and the solar activity cycle. In this white paper, the key science cases and their technical requirements for AtLAST are discussed., Comment: 14 pages, 4 figures, submitted to Open Research Europe as part of a collection on the Atacama Large Aperture Submillimeter Telescope (AtLAST)

Solar observations at sub-mm, mm and cm wavelengths offer a straightforward diagnostic of physical conditions in the solar atmosphere because they yield measurement of brightness temperature which, for optically thick features, equals intrinsic temperature - much unlike solar diagnostics in other spectral ranges. The Atacama Large Millimeter and sub-millimeter Array (ALMA) has therefore opened a new, hitherto underexplored, spectral window for studying the enigmatic solar chromosphere. In this review we discuss initial ALMA studies of the quiet chromosphere that used both single-dish and compact-array interferometric observing modes. We present results on the temperature structure of the chromosphere, comparison with classic empirical models of the chromosphere, and observations of the chromospheric network and spicules. Furthermore, we discuss what may be expected in the future, since the ALMA capabilities continuously expand and improve towards higher angular resolution, wavelength coverage, and polarization measurement for magnetometry.

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 submillimeter wavelengths have shown ARs with well defined dark umbrae. Given this new information, it is now necessary to update our understanding and models of the solar atmosphere in active regions. In this work, we present a data-constrained model of the AR solar atmosphere, in which we use brightness temperature measurements of NOAA 12470 at three radio frequencies: 17 (NoRH), 100 and 230 GHz (ALMA). Based on our model, which assumes that the radio emission originates from thermal free-free and gyroresonance processes, we calculate radio brightness temperature maps that can be compared with the observations. The magnetic field at distinct atmospheric heights was determined in our modelling process by force-free field extrapolation using photospheric magnetograms taken by HMI/SDO. In order to determine the best plasma temperature and density height profiles necessary to match the observations, the model uses a genetic algorithm that modifies a standard quiet Sun atmospheric model. Our results show that the height of the transition region (TR) of the modelled atmosphere varies with the type of region being modelled: for umbrae the TR is located at 1080 +/- 20 km above the solar surface; for penumbrae, the TR is located at 1800 +/- 50 km; and for bright regions outside sunspots, the TR is located at 2000 +/- 100 km. With these results, we find good agreement with the observed AR brightness temperature maps. Our modelled AR can be used to estimate the emission at frequencies without observational coverage., Comment: 20 pages, 8 figures, 4 tables, accepted for publication in Frontiers in Astronomy and Space Sciences

In September 1777, Ru{\dj}er Bo\v{s}kovi\'c observed sunspots for six days. Based on these measurements, he used his own methods to calculate the elements of the solar rotation, the longitude of the node, the inclination of the solar equator and the period. He published a description of the methods, the method of observation and detailed instructions for calculations in the second chapter of the fifth part of the Opera in 1785. In this paper, Bo\v{s}kovi\'c original calculations and repeated calculations by his procedure are published. By analysing the input quantities, procedures, and results, the input quantities of the error, and the calculation results are discussed. The reproduction of Bo\v{s}kovi\'c calculations is successfully reproduced and we obtained very similar results. The conclusion proposes a relationship of Bo\v{s}kovi\'c research with modern astronomy., Comment: 22 pages, 2 figures, 41 tables, published in The Mining-Geology-Petroleum Engineering Bulletin (https://www.scopus.com/sourceid/101730, https://hrcak.srce.hr/rgn-zbornik)

Science Verification (SV) data of solar observations with Atacama Large Millimeter-submillimeter Array (ALMA) telescope were released to the scientific community. Understanding the centre to limb brightness function is necessary to compare features in full disk images. Our goals are to find the empirical centre to limb brightness functions in two available spectral bands and create flattened images with centre to limb brightness variations removed. We used second-order polynomial fit of the cosine of incidence angle to data points as a function of radial distance to the centre of the solar disk. The method also includes iterative removal of outliers based on the interquartile range. Fitting functions for all available images proved to adequately describe the data with comparatively small errors in the fitting coefficients. In both bands we found brightening towards the limb which is a consequence of increase in electron temperatures with radial distance in this region of the solar atmosphere. This study found that the $T_{b}$ of an active region has about 180 K difference between with and without the limb brightening at radial distance $\approx$0.75$R_{\odot}$ in Band 6. We also made flattened images with limb brightening removed. The limb brightening effect in ALMA images is significant enough (of the order of 10% for Band 3 and about 15% in Band 6) that it can not be neglected in further analyses. Since the effect of the side lobes was not included in this study, these values probably represent the lower limit of the limb brightening. The shape of the limb brightening function can also be used to constrain electron densities and temperatures in various layers of the solar atmosphere.

Polar brightening of the Sun at radio frequencies has been studied for almost fifty years and yet a disagreement persists between solar atmospheric models and observations. Some observations reported brightening values much smaller than the expected values obtained from the models, with discrepancies being particularly large at millimeter wavelengths. New clues to calibrate the atmospheric models can be obtained with the advent of the Atacama Large Millimeter/submillimeter Array (ALMA) radio interferometer. In this work, we analyzed the lower limit of the polar brightening observed at 100 and 230 GHz by ALMA, during its Science Verification period, 2015 December 16-20. We find that the average polar intensity is higher than the disk intensity at 100 and 230 GHz, with larger brightness intensities at the South pole in eight of the nine maps analyzed. The observational results were compared with calculations of the millimetric limb brightnening emission for two semi-empirical atmospheric models, FAL- C (Fontenla et al. 1993) and SSC (Selhorst et al. 2005a). Both models presented larger limb intensities than the averaged observed values. The intensities obtained with the SSC model were closer to the observations, with polar brightenings of 10.5% and 17.8% at 100 and 230 GHz, respectively. This discrepancy may be due to the presence of chromospheric features (like spicules) at regions close to the limb., Comment: 10 pages, 6 figures, 2 tables, accepted for publication in ApJ

Sunspot position data obtained from Kanzelh\"{o}he Observatory for Solar and Environmental Research (KSO) sunspot drawings and white light images in the period 1964 to 2016 were used to calculate the rotational and meridional velocities of the solar plasma. Velocities were calculated from daily shifts of sunspot groups and an iterative process of calculation of the differential rotation profiles was used to discard outliers. We found a differential rotation profile and meridional motions in agreement with previous studies using sunspots as tracers and conclude that the quality of the KSO data is appropriate for analysis of solar velocity patterns. By analysing the correlation and covariance of meridional velocities and rotation rate residuals we found that the angular momentum is transported towards the solar equator. The magnitude and latitudinal dependence of the horizontal component of the Reynolds stress tensor calculated is sufficient to maintain the observed solar differential rotation profile. Therefore, our results confirm that the Reynolds stress is the dominant mechanism responsible for transport of angular momentum towards the solar equator., Comment: 14 pages, 6 figures

The sunspot position from Greenwich Photoheliographic Results (GPR), US Air Force Solar Optical Observing Network and National Oceanic and Atmospheric Administration (USAF/NOAA), and Debrecen Photoheliographic Data (DPD) data bases in the period 1874 to 2016 were used to calculate yearly values of the solar differential-rotation parameters $A$ and $B$. The calculated differential-rotation parameters were compared with the solar-activity level. We found that the Sun rotates more differentially at the minimum than at the maximum of activity during the 1977 - 2016 epoch. An inverse correlation between equatorial rotation and solar activity was found using the recently revised sunspot number. The secular decrease of equatorial rotation rate accompanying the increase of activity stopped in the last part of the 20th century. It was noted that when a significant peak of equatorial rotation velocity is observed during minimum of activity, the strength of the next maximum is smaller than the previous one., Comment: 13pages, 3 figures

The Debrecen Photoheliographic Data catalogue is a continuation of the Greenwich Photoheliographic Results providing daily positions of sunspots and sunspot groups. We analyse the data for sunspot groups focusing on meridional motions and transfer of angular momentum towards the solar equator. Velocities are calculated with a daily shift method including an automatic iterative process of removing the outliers. Apart from the standard differential rotation profile, we find meridional motion directed towards the zone of solar activity. The difference in measured meridional flow in comparison to Doppler measurements and some other tracer measurements is interpreted as a consequence of different flow patterns inside and outside of active regions. We also find a statistically significant dependence of meridional motion on rotation velocity residuals confirming the transfer of angular momentum towards the equator. Analysis of horizontal Reynolds stress reveals that the transfer of angular momentum is stronger with increasing latitude up to about 40$^{\circ}$ where there is a possible maximum in absolute value., Comment: 16 pages, 5 figures

Context. It is possible to detect and track coronal bright points (CBPs) in SDO/AIA images. Combination of high resolution and high cadence provides a wealth of data that can be used to determine velocity flows on the solar surface with very high accuracy. Aims. We derived a very accurate solar rotation profile and investigated meridional flows, torsional oscillations and horizontal Reynolds stress based on $\approx$6 months of SDO/AIA data. Methods. We used a segmentation algorithm to detect CBPs in SDO/AIA images. We also used invariance of the solar rotation profile with central meridian distance (CMD) to determine the height of CBPs in 19.3 nm channel. Results. Best fit solar rotation profile is given by $\omega(b)=(14.4060\pm0.0051 + (-1.662\pm0.050)\sin^{2}b + (-2.742\pm0.081)\sin^{4}b)${\degr} day$^{-1}$. Height of CBPs in SDO/AIA 19.3 nm channel was found to be $\approx$6500 km. Meridional motion is predominantly poleward for all latitudes, while solar velocity residuals show signs of torsional oscillations. Horizontal Reynolds stress was found to be small compared to similar works, but still showing transfer of angular momentum towards the solar equator. Conclusion. Most of the results are consistent with Doppler measurements rather than tracer measurements. Fairly small calculated value of horizontal Reynolds stress might be due to the particular phase of the solar cycle. Accuracy of the calculated rotation profile indicates that it is possible to measure changes in the profile as the solar cycle evolves. Analysis of further SDO/AIA CBP data will also help in better understanding of the temporal behaviour of the rotation velocity residuals, meridional motions and Reynolds stress.

ALMA provides the necessary spatial, temporal and spectral resolution to explore central questions in contemporary solar physics with potentially far-reaching implications for stellar atmospheres and plasma physics. It can uniquely constraint the thermal and magnetic field structure in the solar chromosphere with measurements that are highly complementary to simultaneous observations with other ground-based and space-borne instruments. Here, we highlight selected science cases., Comment: 2 pages, 1 figure; to appear in ASP Conference Series, proceedings of "Revolution in Astronomy with ALMA: The 3rd year" (Tokyo, Japan, December 2014), eds. D. Iono, A. Wootten, L. Testi (submitted version)

The Atacama Large Millimeter/submillimeter Array (ALMA) will be a valuable tool for observing the chromosphere of our Sun at (sub-)millimeter wavelengths at high spatial, temporal and spectral resolution and as such has great potential to address long-standing scientific questions in solar physics. In order to make the best use of this scientific opportunity, the Solar Simulations for the Atacama Large Millimeter Observatory Network has been initiated. A key goal of this international collaboration is to support the preparation and interpretation of future observations of the Sun with ALMA., Comment: 2 pages; to appear in ASP Conference Series, proceedings of "Revolution in Astronomy with ALMA: The 3rd year" (Tokyo, Japan, December 2014), eds. D. Iono, A. Wootten, L. Testi (submitted version)

Coronal bright points (CBP) are ubiquitous small brightenings in the solar corona associated with small magnetic bipoles. We derive the solar differential rotation profile by tracing the motions of CBPs detected by the Atmospheric Imaging Assembly (AIA) instrument aboard the Solar Dynamics Observatory (SDO). We also investigate problems related to detection of coronal bright points resulting from instrument and detection algorithm limitations. To determine the positions and identification of coronal bright points we used a segmentation algorithm. A linear fit of their central meridian distance and latitude versus time was utilised to derive velocities. We obtained 906 velocity measurements in a time interval of only 2 days. The differential rotation profile can be expressed as $\omega_{rot} = (14.47\pm 0.10 + (0.6\pm 1.0)\sin^{2}(b) + (-4.7\pm 1.7)\sin^{4}(b))$\degr day$^{-1}$. Our result is in agreement with other work and it comes with reasonable errors in spite of the very short time interval used. This was made possible by the higher sensitivity and resolution of the AIA instrument compared to similar equipment as well as high cadence. The segmentation algorithm also played a crucial role by detecting so many CBPs, which reduced the errors to a reasonable level. Data and methods presented in this paper show a great potential to obtain very accurate velocity profiles, both for rotation and meridional motion and, consequently, Reynolds stresses. The amount of coronal bright point data that could be obtained from this instrument should also provide a great opportunity to study changes of velocity patterns with a temporal resolution of only a few months. Other possibilities are studies of evolution of CBPs and proper motions of magnetic elements on the Sun.

Context. We compare solar differential rotation of subphotospheric layers derived from local helioseismology analysis of GONG++ dopplergrams and the one derived from tracing small bright coronal structures (SBCS) using EIT/SOHO images for the period August 2001 - December 2006, which correspond to the declining phase of solar cycle 23. Aims. The study aims to find a relationship between the rotation of the SBCS and the subphotospheric angular velocity. The northsouth asymmetries of both rotation velocity measurements are also investigated. Methods. Subphotospheric differential rotation was derived using ring-diagram analysis of GONG++ full-disk dopplergrams of 1 min cadence. The coronal rotation was derived by using an automatic method to identify and track the small bright coronal structures in EIT full-disk images of 6 hours cadence. Results. We find that the SBCS rotate faster than the considered upper subphotospheric layer (3Mm) by about 0.5 deg/day at the equator. This result joins the results of several other magnetic features (sunspots, plages, faculae, etc.) with a higher rotation than the solar plasma. The rotation rate latitudinal gradients of the SBCS and the subphotospheric layers are very similar. The SBCS motion shows an acceleration of about 0.005 deg/day/month during the declining phase of solar cycle 23, whereas the angular velocity of subsurface layers does not display any evident variation with time, except for the well known torsional oscillation pattern. Finally, both subphotospheric and coronal rotations of the southern hemisphere are predominantly larger than those of the northern hemisphere. At latitudes where the north-south asymmetry of the angular velocity increases (decreases) with activity for the SBCS, it decreases (increases) for subphotospheric layers., Comment: 6pages, 8 figures, Accepted for publication in Astronomy and Astrophysics

Some 15% of solar flares having a soft X-ray flux above GOES class C5 are reported to lack coherent radio emission in the 100 - 4000 MHz range (type I - V and decimetric emissions). A detailed study of 29 such events reveals that 22 (76%) of them occurred at a radial distance of more than 800'' from the disk center, indicating that radio waves from the limb may be completely absorbed in some flares. The remaining seven events have statistically significant trends to be weak in GOES class and to have a softer non-thermal X-ray spectrum. All of the non-limb flares that were radio-quiet > 100 MHz were accompanied by metric type III emission below 100 MHz. Out of 201 hard X-ray flares, there was no flare except near the limb (R>800'') without coherent radio emission in the entire meter and decimeter range. We suggest that flares above GOES class C5 generally emit coherent radio waves when observed radially above the source., Comment: Solar Physics, in press