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We compile the catalog of Hvar Observatory (HVAR) solar observations in the time period corresponding to regular digitally stored chromospheric and photospheric observations 2010 – 2019. We make basic characterization of observed phenomena and compare them to catalogs that are based on full-disk solar images. We compile a catalog of observed active regions (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 field of view not encompassing the whole solar disk. In H α observations, we catalog conspicuous filaments/prominences and flares. We characterize filaments according to their location, chirality (if possible), and eruptive signatures. Analysis of the eruptive filaments reveals a slight bias in the HVAR catalog towards observation of partial eruptions, possibly related to the observers’ tendency to observe filaments that 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 α 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. [ABSTRACT FROM AUTHOR]

The rotation of the solar corona is not a fully resolved issue. Coronal holes (CHs) reveal sometimes more rigid, but in other cases more differential rotation profiles. We used two datasets of coronal bright points (CBPs), one within CHs, and one outside of CHs. We analyzed rotation profiles in the two datasets of CBPs and compared them to check if there is any difference between rotation profiles for CBPs within CHs and those outside of CHs. The reported rigidity of the CHs rotational profiles implies that Reynolds stresses, which are considered to be the main drivers of the solar differential profile, should also be different between the two datasets. Therefore, we analyzed the horizontal Reynolds stress for the two datasets as well. We also compared the meridional motion of the two datasets. In all cases the results between the two datasets were the same within the observational error. In both datasets the solar rotation profile is significantly differential and similar to the photospheric profile. [ABSTRACT FROM AUTHOR]

In 1777 Ruđer Bošković observed sunspots, determined their positions and the solar rotation elements by his own methods briefly described here. We repeat his calculations of the mean solar time, sunspot positions, and solar rotation elements using both the Bošković's original equations and equations adapted for modern computers. We repeat the calculations using two values of the obliquity of the ecliptic, Bošković's, and an interpolated one. Using his 1777 observations, Bošković obtained the solar equator's inclination, ecliptic longitude of the ascending node, and sidereal and synodic rotation periods. We analyzed and compared these original Bošković results with our repeated calculations. Our results confirm the validity of Bošković's methods and the precision of his calculations. Additionally, the paper presents the solar differential rotation determination using all 1777 observations by Ruđer Bošković. [ABSTRACT FROM AUTHOR]

Analyses of solar rotation and activity, and their variations, can improve the understanding of the formation and interaction of the solar-activity bands and therefore give important observational constraints on the modelling of the solar dynamo and improve the forecast capability in solar activity and space weather. Sunspot-position data obtained from the Greenwich and Debrecen Photoheliographic data for the years 1874 to 2016 were used to calculate the rotational and meridional velocities of the solar plasma. The velocities were calculated from daily shifts of sunspot groups. By analysing the rotation-rate residuals, covariance of meridional velocities and rotation-rate residuals, as well as daily total sunspot number, taken from WDC-SILSO, for periodicity in the range of periods larger than two days, significant periods of their changes were found. It was demonstrated that the frequencies of the quasi-annual variations of solar activity, with periods from 230 to 400 days, match those of classical Rossby modes with m ≥ 17 . There is no match between the frequencies of Rossby modes and the peaks in the rotation-rate residual and horizontal component of the Reynolds-stress-tensor periodograms. These variations are most probably connected with the existence of active longitudes. It was demonstrated that the algorithms for detecting periodic signals should be used with caution when analysing complex periodic data. [ABSTRACT FROM AUTHOR]

Recently Arlt et al. (Astron. Astrophys. 595, A104, 2016) published archival sunspot data measured by Cristoph Scheiner in the period from 1611 to 1631. Scheiner's data precede Maunder minimum. The main goal of this paper is to study several aspects of sunspot motion across the solar disk (rotation, meridional motion, Reynolds stress) to investigate possible clues about the grand solar minimum that started soon after these observations were made. Another objective is to estimate quality and usefulness of the archival sunspot data for scientific analysis in present times. We used the daily shift method to calculate velocities from positional and temporal data of sunspots measured by Scheiner. Considering the errors of the calculated parameters, solar rotation profile and meridional motions are the same as the modern data show. Correlation and covariance of the solar rotation residual and meridional velocities are different than observed today by tracing sunspot motion. Correlation and covariance of the solar rotation residual and meridional velocities show some signs that might indicate changes in the solar dynamo mechanism before the Maunder minimum. Archival sunspot data, in this case, are of good quality for scientific analysis and provide a valuable resource for modern scientific research. [ABSTRACT FROM AUTHOR]

This document is a correction notice for an article titled "Bošković's Method for Determining the Axis and Rate of Solar Rotation by Observing Sunspots in 1777" published in the journal Solar Physics. The original article contained grammatical, syntactical, and spelling mistakes, which have been corrected. Additionally, a purple line that was mistakenly included in Figure 6 has been removed. The correction notice also mentions changes made to page numbers in citations and the heading of Figure 6. The authors of the article declare no competing interests, and the publisher remains neutral regarding jurisdictional claims and institutional affiliations. [Extracted from the article]

Science Verification (SV) data of solar observations with the Atacama Large Millimeter-submillimeter Array (ALMA) telescope have been 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 a 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 inter-quartile range. Fitting functions for all available images proved to describe the data adequately with comparatively small errors in the fitting coefficients. In both bands we found brightening towards the limb, which is a consequence of the increase in electron temperature with radial distance in this region of the solar atmosphere. This study found that the brightness temperature [ T b ] of an active region has about 180 K difference between estimates with and without the limb brightening at radial distance ≈ 0.75 R ⊙ 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 cannot 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. [ABSTRACT FROM AUTHOR]

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 focussing 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°, where there is a possible maximum in absolute value. [ABSTRACT FROM AUTHOR]

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 above 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. [ABSTRACT FROM AUTHOR]