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Full list of authors: Erdélyi, Robertus; Korsós, Marianna B.; Huang, Xin; Yang, Yong; Pizzey, Danielle; Wrathmall, Steven A.; Hughes, Ifan G.; Dyer, Martin J.; Dhillon, Vikram S.; Belucz, Bernadett; Brajša, Roman; Chatterjee, Piyali; Cheng, Xuewu; Deng, Yuanyong; Domínguez, Santiago Vargas; Joya, Raúl; Gömöry, Peter; Gyenge, Norbert G.; Hanslmeier, Arnold; Kucera, Ales; Kuridze, David; Li, Faquan; Liu, Zhong; Xu, Long; Mathioudakis, Mihalis; Matthews, Sarah; McAteer, James R. T.; Pevtsov, Alexei A.; Pötzi, Werner; Romano, Paolo; Shen, Jinhua; Temesváry, János; Tlatov, Andrey G.; Triana, Charles; Utz, Dominik; Veronig, Astrid M.; Wang, Yuming; Yan, Yihua; Zaqarashvili, Teimuraz; Zuccarello, Francesca.--This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited., The Solar Activity Magnetic Monitor (SAMM) Network (SAMNet) is a future UK-led international network of ground-based solar telescope stations. SAMNet, at its full capacity, will continuously monitor the Sun’s intensity, magnetic, and Doppler velocity fields at multiple heights in the solar atmosphere (from photosphere to upper chromosphere). Each SAMM sentinel will be equipped with a cluster of identical telescopes each with a different magneto-optical filter (MOFs) to take observations in K I, Na D, and Ca I spectral bands. A subset of SAMM stations will have white-light coronagraphs and emission line coronal spectropolarimeters. The objectives of SAMNet are to provide observational data for space weather research and forecast. The goal is to achieve an operationally sufficient lead time of e.g., flare warning of 2–8 h and provide many sought-after continuous synoptic maps (e.g., LoS magnetic and velocity fields, intensity) of the lower solar atmosphere with a spatial resolution limited only by seeing or diffraction limit, and with a cadence of 10 min. The individual SAMM sentinels will be connected to their master HQ hub where data received from all the slave stations will be automatically processed and flare warning issued up to 26 h in advance. © R. Erdélyi et al., Published by EDP Sciences 2022., RE is grateful to the Science and Technology Facilities Council (STFC, grant numbers ST/M000826/1 and ST/V005979/1) the UK and the Royal Society for enabling this research. RE also acknowledges the CAS Presidents International Fellowship Initiative, Grant No. 2019VMA052 and the TOP CLLD 7.1.1-16-H-ERFA-2019-00207-Magyar Napfizikai Alaptvány ID Nr 1-8-2019/P support.MBK acknowledges STFC grant ST/S000518/1 to Aberystwyth University. MBK also acknowledges the open research program of CAS Key Laboratory of Solar Activity, National Astronomical Observatories, No. KLSA201610. RE and MBK both acknowledge the CAS Key Laboratory of Solar Activity, National Astronomical Observatories Commission for Collaborating Research Program for the support received to carry out part of this work. IGH and DP acknowledge support from EPSRC (grant EP/R002061/1). RB acknowledges the support by the Croatian Science Foundation under project 7549 “Millimeter and submillimeter observations of the solar chromosphere with ALMA”. RB, AH, and AMV acknowledge the support from the Austrian-Croatian Bilateral Scientific Project “Comparison of ALMA observations with MHD-simulations of coronal waves interacting with coronal holes”. AK and PG acknowledge the support from grant VEGA 2/0048/20. AAP acknowledges NASA 80NSSC18K1242 grant and grant No. 19-02-00088 by RFBR. Sunspot Solar Observatory is a partnership between NSF (1945705), the National Solar Observatory, the state of New Mexico, and consortium partners. TZ acknowledges funding from the Austrian Science Fund (FWF, project P30695-N27). FZ acknowledges support by the Università degli Studi di Catania (Piano per la Ricerca Università di Catania 2016–2018 – Linea di intervento 1 “Chance”; Linea di intervento 2 “Dotazione ordinaria”; Fondi di Ateneo 2020–2022, Università di Catania, Linea Open Access), by the Italian MIUR-PRIN grant 2017APKP7T on “Circumterrestrial Environment: Impact of Sun-Earth Interaction”. YH acknowledges funding from NSFC (11790300,11790301). DK has received funding from the Sêr Cymru II scheme, part-funded by the European Regional Development Fund through the Welsh Government. © R. Erdélyi et al., Published by EDP Sciences 2022., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

We briefly review the history of observations of magnetic fields on the Sun, and describe early magnetograps for full disk measurements. Changes in instruments and detectors, the cohort of observers, the knowledge base etc may result in non-uniformity of the long-term synoptic datasets. Still, such data are critical for detecting and understanding the long-term trends in solar activity. We demonstrate the value of historical data using studies of active region tilt (Joy’s law) and the evolution of polar field and its reversal. Using the longest dataset of sunspot field strength measurements from Mount Wilson Observatory (1917-present) supplemented by shorter datasets from Pulkovo (1956–1997) and Crimean (1956-present) observatories we demonstrate that the magnetic properties of sunspots did not change over the last hundred years. We also show that the relationship between the sunspot area and its magnetic flux can be used to extend the studies of magnetic field in sunspots to periods with no direct magnetic field measurements. Finally, we show how more recent full disk observations of the vector magnetic field can be used to study the long-term (solar cycle) variations in magnetic helicity on the Sun.

Context: Systematic observations of magnetic field strength and polarity in sunspots began at Mount Wilson Observatory (MWO), USA in early 1917. Except for a few brief interruptions, this historical dataset has continued until the present. Aims: Sunspot field strength and polarity observations are critical in our project of reconstructing the solar magnetic field over the last hundred years. We provide a detailed description of the newly digitized dataset of drawings of sunspot magnetic field observations. Methods: The digitization of MWO drawings is based on a software package that we developed. It includes a semiautomatic selection of solar limbs and other features of the drawing, and a manual entry of the time of observations, measured field strength, and other notes handwritten on each drawing. The data are preserved in an MySQL database. Results: We provide a brief history of the project and describe the results from digitizing this historical dataset. We also provide a summary of the final dataset and describe its known limitations. Finally, we compare the sunspot magnetic field measurements with those from other instruments, and demonstrate that, if needed, the dataset could be continued using modern observations such as, for example, the Vector Stokes Magnetograph on the Synoptic Optical Long-term Investigations of the Sun platform.

Context. We discuss variations of polarized radio emission using the correlation data obtained with the Nobeyama Radioheliograph at 1.76 cm wavelength. We focus on the radio 3-min oscillations. Aims. We consider the daily averaged correlation data with right and the left hand circular polarizations in order to study how the power spectrum and the basic periods of the 3-min oscillations changed during 1992–2006. Methods. We use spectral analysis to obtain the power spectrum of oscillations, excluding the solar flares. Results. It was found that oscillations with a period of about 3 min occur during all phases of the solar activity cycle when at least one sunspot is present on the solar disk, and that there is a linear correlation between the strength of the 3-min oscillations and the monthly mean of the sunspot areas. The basic period of these oscillations changes, beginning from 150–160 s to 170–180 s with the level of solar activity. Also, we found that there is a linear correlation between the basic period and the monthly mean of the sunspot areas.

An analysis of solar rotation as a function of heliographic latitude and time is made using daily radio maps of the Sun at the wavelength of 1.76 cm. Variations of the velocity as a function of the latitude during the period 1992{2001 have been studied. The mean synodical rotation rate of the intensity features is best t by ! =1 3:41 1:66 sin 2 2:19 sin 4 (deg/day) where is the latitude. We have found alternating bands of faster and slower rotation. They travel from higher latitudes towards the equator during the current solar cycle. Radio observations with high accuracy and reliability thus conrm the reality of torsional oscillations in the higher levels of the solar atmosphere.

Daily radio maps of the Sun at the wavelength of 1.76 cm were used to analyze the parameters of solar activity at all heliographic latitudes for the period 1992{2001. As a criterion of the level of solar activity, we analyzed the area/number of regions with an excess of brightness above a certain xed level as well as regions with brightness below a certain level. The distribution of such \bright" and \dark" regions with heliographic latitude as function of time was found. Special attention was paid to the high latitude polar regions where the ways of analyzing solar activity are rather limited and have no generally accepted methods. The results are compared with some other indices of high latitude solar activity, such as polar faculae and magnetic eld measurements. They appear to be in general agreement with the radio observations. The advantage of using radio observations is a more homogeneous database and a stable method of analysis.

In this work we study different manifestations of activity in the polar zones of the Sun in order to gain understanding on the phenomenon of enhanced radio temperature regions (ETR) at high solar latitudes. We have obtained simultaneous radio and optical data during 9 days in 1997. The radio data from the Metsahovi radio telescope, Finland, consisted of 37 GHz and 87 GHz solar maps. White light observations at the Kislovodsk solar mountain station in Russia were used to measure the coordinates of polar faculae groups and diffuse bright structures. We also compared our data with the 171 AEUV SOHO/EIT images for the same time periods. We find the ETRs in general coincide with the relatively dark areas seen in the SOHO/EIT images. Bright structures in the SOHO/EIT maps are, in general, encircled by the polar faculae groups and diffuse bright structures visible in white light. Some of the EUV bright structures appear to be bases of solar plumes. Connections between ETRs and polar faculae are complicated; sometimes polar faculae groups and diffuse bright structures are situated around the ETR maxima or at their borders, sometimes we see the polar faculae groups distributed over the whole ETR area. Some faint ETRs appear to have no associated polar faculae. However, in general there is a correlation between the ETRs, the polar faculae groups, and the bright structures (bases of the plumes and some other features), indicating that they are different manifestations of the same underlying activity. It is possible that magnetic loops in the active areas, traced by the polar faculae, are responsible for the observed radio enhancements.

We report on the MASTER Global Robotic Net discovery of an eclipsing binary, MASTER OT J095310.04+335352.8, previously known as unremarkable star TYC 2505-672-1, which displays extreme orbital parameters. The orbital period P=69.1 yr is more than 2.5 times longer than that of epsilon-Aurigae, which is the previous record holder. The light curve is characterized by an extremely deep total eclipse with a depth of more than 4.5 mag, which is symmetrically shaped and has a total duration of 3.5 yrs. The eclipse is essentially gray. The spectra acquired with the Russian 6 m BTA telescope both at minimum and maximum light mainly correspond to an M0-1III--type red giant, but the spectra taken at the bottom of eclipse show small traces of a sufficiently hot source. The observed properties of this system can be better explained as the red giant eclipsed by a large cloud (the disk) of small particles surrounding the invisible secondary companion., 8 figures, 9 pages, Astronomy and astrophysics in print

Aims. The present work is the first in a series of articles that develop a new proxy to represent the evolution of magnetic activity in past solar cycles by combining the information from historical Ca II K line spectroheliograms and sunspot magnetic field measurements.Methods. We use synoptic (Carrington) maps from 1915–1985 that were derived from daily Ca K line observations at Mount Wilson Observatory to identify the chromospheric plages and to create synoptic pseudo-magnetograms. We use historical observations of sunspot magnetic fields from 1917 to the present to assign polarity to pixels situated within plages. The original Ca K spectroheliograms are nonuniform in their brightness, and we develop a novel approach to re-normalize their intensities.Results. We show that a homogeneous long-term series of pseudo-magnetograms can be successfully constructed by combining sunspot field measurements and plages with renormalized intensities. In our tests, about 80% of pixels situated within plages showed the same magnetic polarity as the synoptic magnetograms taken with the Kitt Peak Vacuum Telescope. Finally, we discuss possible approaches to further improve the agreement between observed and pseudo-magnetograms.

Aims. We investigate the long-term change in coronal large-scale structure at periods of minimum solar activity from 1878 to 2008. Methods. A parameter γ that characterizes the angle between high-latitude boundaries of the large coronal streamers at a distance of 2 Ris used to quantify the large-scale coronal structure at minimum solar activity. The comparative analysis of the solar corona during the minimum epochs in activity cycles 12 to 24 shows that the index has been slow and steadily changing during the past 130 years. Results. The maximum value of the index occurred during activity cycles 17 to 19, i.e. around 1950, which was the period when the global magnetic field of the Sun was closest to a dipole configuration. During activity minima close to the years 1900 and 2000, the large-scale coronal structure corresponded to a quadrupole configuration. The reasons for the variations in the solar coronal structure and its relation with long-term variations in the geomagnetic indices and Gleissberg cycle are discussed.