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The relation between the intensity of chromospheric emissions and the photospheric magnetic field strength has been examined in several studies, but the effect of the magnetic field inclination on chromospheric emissions remains almost unexplored. We study how the inclination of the photospheric magnetic field, as measured by the full 3D magnetic vector from the Helioseismic and Magnetic Imager (HMI), affects the relationship between the magnetic field strength and the far-ultraviolet emission at around 1600 {\AA} observed by the Atmospheric Imaging Assembly (AIA). We also study how these parameters change spatially close to the active region perimeter. We analyzed the mutual dependence of 1168 co-temporal AIA and HMI observations from 2014 to 2017. We focused on magnetically active regions outside sunspots (e.g., plages and network) close to the solar disk center. We studied how the AIA and HMI parameters change with distance from the active region perimeter. The AIA 1600 emission typically decreases with increasing (more horizontal) inclination. For all inclinations, AIA 1600 emission increases with increasing magnetic field strength until saturating at some peak intensity, which depends on the cosine of the inclination, with horizontal regions saturating at lower intensities. In addition, we find that activity clusters have a narrow boundary (< 2 arcseconds) in which the AIA 1600 intensity, magnetic field strength, and inclination distributions and relations differ significantly from those in the inner layers. This study demonstrates the significant effect that magnetic field inclination and activity cluster border regions have on chromospheric emissions. Although the observed effects are likely reduced in low-resolution observations where different regions are averaged together, a detailed study is needed to examine the emission--magnetic field relation at different resolutions., Comment: 13 pages, 13 figures

We give a gentle introduction to solar imaging data, focusing on the challenges and opportunities of data-driven approaches for solar eruptions. The various solar phenomenon prediction problems that might benefit from statistical methods are presented. Available data products and software are described. State-of-the-art solar eruption forecasting models with data-driven approaches are summarized and discussed. Based on the characteristics of the datasets and state-of-the-art approaches, we point out several promising directions to explore from statistical modeling and computational perspectives.

Filaments are omnipresent features in the solar atmosphere. Their location, properties and time evolution can provide information about changes in solar activity and assist the operational space weather forecast. Therefore, filaments have to be identified in full disk images and their properties extracted from these images. Manual extraction is tedious and takes much time; extraction with morphological image processing tools produces a large number of false-positive detections. Automatic object detection, segmentation, and extraction in a reliable manner allows to process more data in a shorter time. The Chromospheric Telescope (ChroTel), Tenerife, Spain, the Global Oscillation Network Group (GONG), and the Kanzelh\"ohe Observatory (KSO), Austria, provide regular full-disk observations of the Sun in the core of the chromospheric H-alpha absorption line. We present a deep learning method that provides reliable extractions of filaments from H-alpha filtergrams. First, we train the object detection algorithm YOLOv5 with labeled filament data of ChroTel. We use the trained model to obtain bounding-boxes from the full GONG archive. In a second step, we apply a semi-supervised training approach, where we use the bounding boxes of filaments, to learn a pixel-wise classification of filaments with u-net. Here, we make use of the increased data set size to avoid overfitting of spurious artifacts from the generated training masks. Filaments are predicted with an accuracy of 92%. With the resulting filament segmentations, physical parameters such as the area or tilt angle can be easily determined and studied. This we demonstrate in one example, where we determine the rush-to-the pole for Solar Cycle 24 from the segmented GONG images. In a last step, we apply the filament detection to H-alpha observations from KSO which demonstrates the general applicability of our method to H-alpha filtergrams., Comment: 15 pages, 14 figures

This report provides a brief summary of the properties of new cameras selected for NSF's Global Oscillations Network Group (GONG) facilities operated by the NSO Integrated Synoptic Program (NISP). These camera replacements are part of a GONG refurbishment project aimed to extend GONG operations through roughly FY 2030. Testing has confirmed the suitability of the new cameras and that current data products would be largely unchanged. GONG magnetograms show approximately one-to-one scaling with old data, and the helioseismology data (l-nu diagrams) are nearly identical without any identifiable artifacts. A number of tests were also conducted for GONG processing pipelines and have demonstrated that the modified NISP data center pipelines can transition smoothly to processing observations taken with the new cameras, Comment: 8 pages

A combined uniform and long-time series of Ca-K images from the Kodaikanal Observatory (KO), Mount Wilson Observatory (MWO), and Mauna Loa Solar Observatory (MLSO) were used to identify and study the Ca-K small-scale features and their solar cycle variations over a century. The small scale features are classified into three distinct categories: enhanced network (EN), active network (AN), and quiet network (QN). All these features show that their areas vary according to the 11-year solar cycle. The relative amplitude of the Ca-K network variations agree with that of the sunspot cycle. The total area of these small-scale features varies from about 5% during the minimum phase of the solar cycle to about 20% during its maximum phase. Considering the average intensity and the amplitude of their area variations, we find that the total contribution of EN, AN and QN to the irradiance variation of the Sun is about 3%., Comment: Submitted article in RAA. It is under review. 15 pages, 8 figures

We explore model performance for the Alfven Wave Solar atmosphere Model (AWSoM) with near-real-time (NRT) synoptic maps of the photospheric vector magnetic field. These maps, produced by assimilating data from the Helioseismic Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO), use a different method developed at the National Solar Observatory (NSO) to provide a near contemporaneous source of data to drive numerical models. Here, we apply these NSO-HMI-NRT maps to simulate three Carrington rotations (CRs): 2107-2108 (centered on 2011/03/07 20:12 CME event), 2123 (integer CR) and 2218--2219 (centered on 2019/07/2 solar eclipse), which together cover a wide range of activity level for solar cycle 24. We show simulation results, which reproduce both extreme ultraviolet emission (EUV) from the low corona while simultaneously matching in situ observations at 1 au as well as quantify the total unsigned open magnetic flux from these maps.

This White Paper argues for the urgent need for the multi-vantage/multi-point observations of the Sun and the heliosphere in the framework of six (6) key science objectives. We further emphasize the critical importance of 5D-``space'': three spatial, one temporal and the magnetic field components. The importance of such observations cannot be overstated both for scientific research and the operational space weather forecast., Comment: White paper submitted to Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033, 6 pages

Long-term synoptic observations of the Sun are critical for advancing our understanding of Sun as an astrophysical object, understanding the solar irradiance and its role in solar-terrestrial climate, for developing predictive capabilities of solar eruptive phenomena and their impact on our home planet, and heliosphere in general, and as a data provider for the operational space weather forecast. We advocate for the development of a ground-based network of instruments provisionally called ngGONG to maintain critical observing capabilities for synoptic research in solar physics and for the operational space weather forecast., Comment: White paper submitted to Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033, 11 pages, 2 figures

This paper evaluates the quality of CME analysis that has been undertaken with the rare Faraday rotation observation of an eruption. Exploring the capability of the FETCH instrument hosted on the MOST mission, a four-satellite Faraday rotation radio sounding instrument deployed between the Earth and the Sun, we discuss the opportunities and challenges to improving the current analysis approaches., Comment: 33 pages, 24 figures

The bright regions in the solar chromosphere and temperature minimum have a good spatial correspondence with regions of intense photospheric magnetic field. Their observation started more than a hundred years ago with the invention of the spectroheliograph. While the historical spectroheliograms are essential for studying the long-term variability of the Sun, the modern satellite-borne observations can help us reveal the nature of chromospheric brightenings in previously unattainable detail. Our aim is to improve the understanding of the relation between magnetic fields and radiative structures by studying modern seeing-free observations of far-ultraviolet (FUV) radiation around 1600 \r{A} and photospheric magnetic fields. We used Helioseismic and Magnetic Imager (HMI) observations of photospheric magnetic fields and Atmospheric Imaging Assembly (AIA) observations of FUV contrast around 1600 \r{A}. We developed a robust method to find contrast thresholds defining bright and dark AIA 1600 \r{A} pixels, and we combine them to bright and dark clusters. We investigate the relation of magnetic fields and AIA 1600 \r{A} radiation in bright and dark clusters. We find that the percentage of bright pixels entirely explains the observed variability of 1600 \r{A} emission. We developed a multilinear regression model based on the percentages of bright and dark pixels, which can reliably predict the magnitude of the disk-averaged unsigned magnetic field. We find that bright and dark clusters closely correspond respectively to the populations of moderate (B > 55 G) and strong (B > 1365 G) magnetic field HMI clusters. The largest bright clusters have a constant mean unsigned magnetic field, as found previously for Ca II K plages. However, the magnetic field strength of bright clusters is 254.7$\pm$0.1 G, which is roughly 100 G larger than found earlier for Ca II K plages., Comment: 27 pages, 31 figures

Continuous observations play an important role in the studies of solar variability. While such observations can be achieved from space with almost 100% duty cycle, it is difficult to accomplish very high duty cycle from the ground. In this context, we assess the duty cycle that has been achieved from the ground by analyzing the observations of a six station network of identical instruments, Global Oscillation Network Group (GONG). We provide a detailed analysis of the duty cycle using GONG observations spanning over 18 years. We also discuss duty cycle of individual sites and point out various factors that may impact individual site or network duty cycle. The mean duty cycle of the network is 93%, however it reduces by about 5% after all images pass through the stringent quality-control checks. The standard deviations in monthly and yearly duty cycle values are found to be 1.9% and 2.2%, respectively. These results provide a baseline that can be used in the planning of future ground-based networks., Comment: Preprint contains 30 pages, 24 figures and 8 tables. It is published on an open access basis under a Creative Commons Attribution (CC BY) licence

Full disk vector magnetic fields are used widely for developing better understanding of large-scale structure, morphology, and patterns of the solar magnetic field. The data are also important for modeling various solar phenomena. However, observations of vector magnetic fields have one important limitation that may affect the determination of the true magnetic field orientation. This limitation stems from our ability to interpret the differing character of the Zeeman polarization signals which arise from the photospheric line-of-sight vs. the transverse components of the solar vector magnetic field, and is likely exacerbated by unresolved structure (non-unity fill fraction) as well as the disambiguation of the 180$^\circ$ degeneracy in the transverse-field azimuth. Here we provide a description of this phenomenon, and discuss issues, which require additional investigation., Comment: 13 pages, 5 figure, Journal of Space Weather and Space Climate, accepted, 2021

Major solar eruptions occasionally direct interplanetary coronal mass ejections (ICMEs) to Earth and cause significant geomagnetic storms and low-latitude aurorae. While single extreme storms are of significant threats to the modern civilization, storms occasionally appear in sequence and, acting synergistically, cause 'perfect storms' at Earth. The stormy interval in January 1938 was one of such cases. Here, we analyze the contemporary records to reveal its time series on their source active regions, solar eruptions, ICMEs, geomagnetic storms, low-latitude aurorae, and cosmic-ray (CR) variations. Geomagnetic records show that three storms occurred successively on 17/18 January (Dcx ~ -171 nT) on 21/22 January (Dcx ~ -328 nT) and on 25/26 January (Dcx ~ -336 nT). The amplitudes of the cosmic-ray variations and sudden storm commencements show the impact of the first ICME as the largest (~ 6% decrease in CR and 72 nT in SSC) and the ICME associated with the storms that followed as more moderate (~ 3% decrease in CR and 63 nT in SSC; ~ 2% decrease in CR and 63 nT in SSC). Interestingly, a significant solar proton event occurred on 16/17 January and the Cheltenham ionization chamber showed a possible ground level enhancement. During the first storm, aurorae were less visible at mid-latitudes, whereas during the second and third storms, the equatorward boundaries of the auroral oval were extended down to 40.3{\deg} and 40.0{\deg} in invariant latitude. This contrast shows that the initial ICME was probably faster, with a higher total magnitude but a smaller southward component., Comment: main text 28 pages, references 11 pages, 2 tables, and 8 figures. Accepted for publication in the Astrophysical Journal

We argue that it is essential that the Astro2020 survey of the present state of American astronomy and the recommendations for the next decade address the issue of ensuring preservation of, and making more discoverable and accessible, the field's rich legacy materials. These include both archived observations of scientific value and items of historical importance. Much of this heritage likely will be lost if action is not taken in the next decade. It is proposed that the decadal plan include recommendations on (1) compiling a list of historic sites and development of models for their preservation, (2) carrying out a comprehensive inventory of astronomy's archival material, and (3) digitizing, with web-based publication, those photographs and papers judged to have the most value for scientific and historical investigations. The estimated cost for an example project on plate preservation is a one-time investment of less than $10 million over ten years plus the typical on-going costs to maintain and manage a medium-sized database., Comment: 11 pages, State of the Profession White Paper for Astro2020

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 continues till present. Aims. The sunspot field strength and polarity observations are critical in our project of reconstructing the solar magnetic field over the last hundred years. Here 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 develope d by us. It includes a semi-automatic selection of solar limbs and other features of the drawing, and a manual entry of the time of observations, the measured field strength and other notes hand-written on each drawing. The data are preserved in a 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 other instruments, and demonstrate that, if needed, the dataset could be continued using modern observations such as, for example, Vector Stokes Magnetograph (VSM) on Synoptic Optical Long-term Investigations of the Sun (SOLIS) platform., Comment: 14 pages, 12 figures, 4 tables, Astronomy and Astrophysics, accepted

Early telescopic observations of sunspots were conducted with instruments of relatively small aperture. These instruments also suffered from a higher level of scattered light, and the human eye served as a "detector". The eye's ability to resolve small details depends on image contrast, and on average the intensity variations smaller than $\approx$ 3\% contrast relative to background are not detected even if they are resolved by the telescope. Here we study the effect of these three parameters (telescope aperture, scattered light, and detection threshold of human vision) on sunspot number, group number, and area of sunspots. As an "ideal" dataset, we employ white-light (pseudo-continuum) observations from Helioseismic and Magnetic Imager (HMI) onboard of Solar Dynamics Observatory, and we model the appearance of sunspots by degrading the HMI images to corresponding telescope apertures with an added scattered light. We discuss the effects of different parameters on sunspot counts and derive functional dependencies, which could be used to normalize historical observations of sunspot counts to common denominator., Comment: Accepted for publication in MNRAS, 7 pages, 6 figures, 2 tables

We propose a novel approach to reconstruct the surface magnetic helicity density on the Sun or sun-like stars. The magnetic vector potential is determined via decomposition of vector magnetic field measurements into toroidal and poloidal components. The method is verified using data from a non-axisymmetric dynamo model. We apply the method to vector field synoptic maps from Helioseismic and Magnetic Imager (HMI) onboard of Solar Dynamics Observatory (SDO) to study evolution of the magnetic helicity density during solar cycle 24. It is found that the mean helicity density of the non-axisymmetric magnetic field of the Sun evolves in a way which is similar to that reported for the current helicity density of the solar active regions. It has predominantly the negative sign in the northern hemisphere, and it is positive in the southern hemisphere. Also, the hemispheric helicity rule for the non-axisymmetric magnetic field showed the sign inversion at the end of cycle 24. Evolution of magnetic helicity density of large-scale axisymmetric magnetic field is different from that expected in dynamo theory. On one hand, the mean large- and small-scale components of magnetic helicity density display the hemispheric helicity rule of opposite sign at the beginning of cycle 24. However, later in the cycle, the two helicities exhibit the same sign in contrast with the theoretical expectations., Comment: 8 pages 5 figures

The line-of-sight (LOS) component of the large-scale photospheric magnetic field has been observed since the 1950s, but the daily full-disk observations of the full vector magnetic field started only in 2010 using the SOLIS Vector Stokes Magnetograph (VSM) and the SDO helioseismic and magnetic imager (HMI). Traditionally, potential field extrapolations are based on the assumption that the magnetic field in the photosphere is approximately radial. The validity of this assumption has not been tested yet. We investigate here the structure and evolution of the three components of the solar large-scale magnetic field in 2010 - 2017, covering the ascending to mid-declining phase of solar cycle 24, using SOLIS/VSM vector synoptic maps of the photospheric magnetic field. We compare the observed VSM vector magnetic field to the potential vector field derived using the VSM LOS magnetic field observations as an input. The new vector field data allow us to derive the meridional inclination and the azimuth angle of the magnetic field and to investigate their solar cycle evolution and latitudinal profile of these quantities. SOLIS/VSM vector data show that the photospheric magnetic field is in general fairly non-radial. In the meridional plane the field is inclined toward the equator, reflecting the dipolar structure of the solar magnetic field. Rotationally averaged meridional inclination does not have significant solar cycle variation. While the vector radial component $B_r$ and the potential radial component $B_r^{PFSS}$ are fairly similar, the meridional and zonal components do not agree very well. We find that SOLIS/VSM vector observations are noisy at high latitudes and suffer from the vantage point effect more than LOS observations. This is due to different noise properties in the LOS and transverse components of the magnetic field, which needs to be addressed in future studies.

Over the past decades and even centuries, the astronomical community has accumulated a signif-icant heritage of recorded observations of a great many astronomical objects. Those records con-tain irreplaceable information about long-term evolutionary and non-evolutionary changes in our Universe, and their preservation and digitization is vital. Unfortunately, most of those data risk becoming degraded and thence totally lost. We hereby call upon the astronomical community and US funding agencies to recognize the gravity of the situation, and to commit to an interna-tional preservation and digitization efforts through comprehensive long-term planning supported by adequate resources, prioritizing where the expected scientific gains, vulnerability of the origi-nals and availability of relevant infrastructure so dictates. The importance and urgency of this issue has been recognized recently by General Assembly XXX of the International Astronomical Union (IAU) in its Resolution B3: "on preservation, digitization and scientific exploration of his-torical astronomical data". We outline the rationale of this promotion, provide examples of new science through successful recovery efforts, and review the potential losses to science if nothing it done., Comment: 8 pages, White Paper submitted to ASTRO2020 Decadal Survey

The authors of this report met on 28-30 March 2018 at the New Jersey Institute of Technology, Newark, New Jersey, for a 3-day workshop that brought together a group of data providers, expert modelers, and computer and data scientists, in the solar discipline. Their objective was to identify challenges in the path towards building an effective framework to achieve transformative advances in the understanding and forecasting of the Sun-Earth system from the upper convection zone of the Sun to the Earth's magnetosphere. The workshop aimed to develop a research roadmap that targets the scientific challenge of coupling observations and modeling with emerging data-science research to extract knowledge from the large volumes of data (observed and simulated) while stimulating computer science with new research applications. The desire among the attendees was to promote future trans-disciplinary collaborations and identify areas of convergence across disciplines. The workshop combined a set of plenary sessions featuring invited introductory talks and workshop progress reports, interleaved with a set of breakout sessions focused on specific topics of interest. Each breakout group generated short documents, listing the challenges identified during their discussions in addition to possible ways of attacking them collectively. These documents were combined into this report-wherein a list of prioritized activities have been collated, shared and endorsed., Comment: Workshop Report

We use recently digitized sunspot drawings from Mount Wilson Observatory to investigate the latitudinal dependence of tilt angles of active regions and its change with solar cycle. The drawings cover the period from 1917 to present and contain information about polarity and strength of magnetic field in sunspots. We identify clusters of sunspots of same polarity, and used these clusters to form ``bipole pairs''. The orientation of these bipole pairs was used to measure their tilts. We find that the latitudinal profile of tilts does not monotonically increase with latitude as most previous studies assumed, but instead, it shows a clear maximum at about 25--30 degree latitudes. Functional dependence of tilt ($\gamma$) on latitude ($\varphi$) was found to be $\gamma = (0.20\pm 0.08) \sin (2.80 \varphi) + (-0.00\pm 0.06)$. We also find that latitudinal dependence of tilts varies from one solar cycle to another, but larger tilts do not seem to result in stronger solar cycles. Finally, we find the presence of a systematic offset in tilt of active regions (non-zero tilts at the equator), with odd cycles exhibiting negative offset and even cycles showing the positive offset., Comment: 14 pages, 5 figures, accepted for publication in Solar Physics

We investigate the restructuring of the magnetic field in sunspots associated with two flares: the X6.5 flare on 6 December 2006 and the X2.2 flare on 15 February 2011. The observed changes were evaluated with respect to the so-called twist-removal model, in which helicity (twist) is removed from the corona as the result of an eruption. Since no vector magnetograms were available for the X6.5 flare, we applied the azimuthal symmetry approach to line-of-sight magnetograms to reconstruct the pseudo-vector magnetic field and investigate the changes in average twist and inclination of magnetic field in the sunspot around the time of the flare. For the X2.2 flare, results from the full vector magnetograms were compared with the pseudo-vector field data. For both flares, the data show changes consistent with the twist-removal scenario. We also evaluate the validity of the azimuthal symmetry approach on simple isolated round sunspots. In general, the derivations based on the azimuthal symmetry approach agree with true-vector field data though we find that even for symmetric sunspots the distribution of the magnetic field may deviate from an axially symmetric distribution., Comment: accepted for publication in the ApJ

We describe here a procedure to combine measurements in the 393.37 nm Ca II K spectral line taken at different observatories. Measurements from the National Solar Observatory (NSO) Integrated Sunlight Spectrometer (ISS) on the Synoptic Optical Long-term Investigations of the Sun (SOLIS) telescope, the NSO/Sac Peak Ca II K-Line Monitoring Program, and Ca II K filtergrams from Kodaikanal Solar Observatory (KKL) are merged together to create a pair of composites of the Ca II K 1-A emission index. These composites are publicly available from the SOLIS website at http://solis.nso.edu/0/iss/., Comment: 11 pages, 9 figures. Technical Report:NSO/NISP-2017-001

The most commonly used index of stellar magnetic activity is the instrumental flux scale of singly-ionized calcium H & K line core emission, S, developed by the Mount Wilson Observatory (MWO) HK Project, or the derivative index R'_HK. Accurately placing the Sun on the S scale is important for comparing solar activity to that of the Sun-like stars. We present previously unpublished measurements of the reflected sunlight from the Moon using the second-generation MWO HK photometer during solar cycle 23 and determine cycle minimum S_min,23 = 0.1634 +/- 0.0008, amplitude Delta S_23 = 0.0143 +/- 0.0012, and mean = 0.1701 +/- 0.0005. By establishing a proxy relationship with the closely related National Solar Observatory Sacramento Peak calcium K emission index, itself well-correlated with the Kodaikanal Observatory plage index, we extend the MWO S time series to cover cycles 15-24 and find on average = 0.1621 +/- 0.0008, = 0.0145 +/- 0.0012, = 0.1694 +/- 0.0005. Our measurements represent an improvement over previous estimates which relied on stellar measurements or solar proxies with non-overlapping time series. We find good agreement from these results with measurements by the Solar-Stellar Spectrograph at Lowell Observatory, an independently calibrated instrument, which gives us additional confidence that we have accurately placed the Sun on the S-index flux scale., Comment: 17 pages, 7 figures, 3 tables. Accepted for publication in the Astrophysical Journal

In the United States, scientific research in space weather is funded by several Government Agencies including the National Science Foundation (NSF) and the National Aeronautics and Space Agency (NASA). For commercial purposes, space weather forecast is made by the Space Weather Prediction Center (SWPC) of the National Oceanic and Atmospheric Administration (NOAA). Observations come from the network of groundbased observatories funded via various sources, as well as from the instruments on spacecraft. Numerical models used in forecast are developed in the framework of individual research projects. Later, the most promising models are selected for additional testing at SWPC. In order to increase the application of models in research and education, NASA in collaboration with other agencies created Community Coordinated Modeling Center (CCMC). In mid-1990, US scientific community presented compelling evidence for developing the National Program on Space Weather, and in 1995, such program has been formally created. In 2015, the National Council on Science and Technology issued two documents: the National Space Weather Strategy [1] and the Action Plan [2]. In the near future, these two documents will define the development of Space Weather research and forecasting activity in USA. Both documents emphasize the need for close international collaboration in area of space weather., Comment: 6 pages, 2 figures, submitted for publication in Proceedings of XX annual Conference on Solar and Solar-Terrestrial Physics held at the Central (Pulkovo) Astronomical Observatory, 10-14 Oct. 2016

The Sun has a steady 11-year cycle in magnetic activity most well-known by the rising and falling in the occurrence of dark sunspots on the solar disk in visible bandpasses. The 11-year cycle is also manifest in the variations of emission in the Ca II H & K line cores, due to non-thermal (i.e. magnetic) heating in the lower chromosphere. The large variation in Ca II H & K emission allows for study of the patterns of long-term variability in other stars thanks to synoptic monitoring with the Mount Wilson Observatory HK photometers (1966-2003) and Lowell Observatory Solar-Stellar Spectrograph (1994-present). Overlapping measurements for a set of 27 nearby solar-analog (spectral types G0-G5) stars were used to calibrate the two instruments and construct time series of magnetic activity up to 50 years in length. Precise properties of fundamental importance to the dynamo are available from Hipparcos, the Geneva-Copenhagen Survey, and CHARA interferometry. Using these long time series and measurements of fundamental properties, we do a comparative study of stellar "twins" to explore the sensitivity of the stellar dynamo to small changes to structure, rotation, and composition. We also compare this sample to the Sun and find hints that the regular periodic variability of the solar cycle may be rare among its nearest neighbors in parameter space., Comment: 5 pages, 2 figures. To appear in the proceedings of "The 19th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun" (ed. G. A. Feiden), Uppsala, Sweden, 06-10 June 2016

Using observations of sunspot magnetic field strengths (H) from the Crimean Astrophysical Observatory (CrAO) and area (S) of sunspots from the Kislovodsk Mountain Astronomical Station of Pulkovo Observatory, we investigate the changes in the relation between H and S over the period of about two solar cycles (1994-2013). The data were fitted by H = A + B log S, where A = (778+/-46) and B = (778+/-25). We show that the correlation between H and S varies with the phase of solar cycle, and $A$ coefficient decreases significantly after year 2001, while B coefficient does not change significantly. Furthermore, our data confirm the presence of two distinct populations in distribution of sunspots (small sunspots with weaker field strength and large sunspots with stronger field). We show that relative contribution of each component to the distribution of sunspots by their area changes with the phase of solar cycle and on longer-then-cycle periods. We interpret these changes as a signature of a long-term (centennial) variations in properties of sunspots., Comment: 8 pages, 7 figures, accepted for publication in Astron. Nachrichten

Long-term synoptic observations in the resonance line of Ca II K constitute a fundamental database for a variety of retrospective analyses of the state of the solar magnetism. Synoptic Ca II K observations began in late 1904 at the Kodaikanal Observatory, in India. In early 1970s, the National Solar Observatory (NSO) at Sacramento Peak (USA) started a new program of daily Sun-as-a-star observations in the Ca II K line. Today the NSO is continuing these observations through its Synoptic Optical Long-term Investigations of the Sun (SOLIS) facility. These different data sets can be combined into a single disk-integrated Ca II K index time series that describes the average properties of the chromospheric emission over several solar cycles. We present such a Ca II K composite and discuss its correlation with the new entirely revised sunspot number data series. For this preliminary investigation, the scaling factor between pairs of time series was determined assuming a simple linear model for the relationship between the monthly mean values during the duration of overlapping observations., Comment: 16 pages, 8 figures, accepted for publication in Solar Physics

Recently, the National Solar Observatory (NSO) Solar-atmosphere Pipeline Working Group has undertaken the production of synoptic maps from Helioseismic and Magnetic Imager (HMI) magnetograms. A set of maps has been processed spanning the data available for 2010-2015 using twice daily images (taken at UT midnight and noon) and running them through the same algorithms used to produce SOLIS/VSM 6302l mean-magnetic and spatial-variance maps. The contents of this document provide an overview of what these maps look like, and the processing steps used to generate them from the original HMI input data., Comment: 16 pages, 8 figures

Analysis of synoptic data from the Vector Stokes Magnetograph (VSM) of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) and the NASA/NSO Spectromagnetograph (SPM) at the NSO/Kitt Peak Vacuum Telescope facility shows that the reversals of solar polar magnetic fields exhibit elements of a stochastic process, which may include the development of specific patterns of emerging magnetic flux, and the asymmetry in activity between northern and southern hemispheres. The presence of such irregularities makes the modeling and prediction of polar field reversals extremely hard if possible. In a classical model of solar activity cycle, the unipolar magnetic regions (UMRs) of predominantly following polarity fields are transported polewards due to meridional flows and diffusion. The UMRs gradually cancel out the polar magnetic field of the previous cycle, and re-build the polar field of opposite polarity setting the stage for the next cycle. We show, however, that this deterministic picture can be easily altered by the developing of a strong center of activity, or by the emergence of an extremely large active region, or by a "strategically placed" coronal hole. We demonstrate that the activity occurring during the current cycle 24 may be the result of this randomness in the evolution of the solar surface magnetic field., Comment: 19 pages, 5 figures, 1 table, accepted for publication in "The Journal Solar-Terrestrial Physics"

In this work we introduce a new way of binning sunspot group data with the purpose of better understanding the impact of the solar cycle on sunspot properties and how this defined the characteristics of the extended minimum of cycle 23. Our approach assumes that the statistical properties of sunspots are completely determined by the strength of the underlying large-scale field and have no additional time dependencies. We use the amplitude of the cycle at any given moment (something we refer to as activity level) as a proxy for the strength of this deep-seated magnetic field. We find that the sunspot size distribution is composed of two populations: one population of groups and active regions and a second population of pores and ephemeral regions. When fits are performed at periods of different activity level, only the statistical properties of the former population, the active regions, is found to vary. Finally, we study the relative contribution of each component (small-scale versus large-scale) to solar magnetism. We find that when hemispheres are treated separately, almost every one of the past 12 solar minima reaches a point where the main contribution to magnetism comes from the small-scale component. However, due to asymmetries in cycle phase, this state is very rarely reached by both hemispheres at the same time. From this we infer that even though each hemisphere did reach the magnetic baseline, from a heliospheric point of view the minimum of cycle 23 was not as deep as it could have been.

We apply a new technique, the mutual information (MI) from information theory, to time-distance helioseismology, and demonstrate that it can successfully reproduce several classic results based on the widely used cross-covariance method. MI quantifies the deviation of two random variables from complete independence, and represents a more general method for detecting dependencies in time series than the cross-covariance function, which only detects linear relationships. We provide a brief description of the MI-based technique and discuss the results of the application of MI to derive the solar differential rotation profile, a travel-time deviation map for a sunspot and a time-distance diagram from quiet Sun measurements., Comment: 14 pages, 5 figures, accepted for publication in Solar Physics

Unlike other stars, the surface of the Sun can be spatially resolved to a high degree of detail. But the Sun can also be observed as if it was a distant star. The availability of solar disk-resolved and disk-integrated spectra offers an opportunity to devise methods to derive information about the spatial distribution of solar features from Sun-as-a-star measurements. Here, we present an update on work done at the National Solar Observatory to reconcile disk-integrated and disk-resolved solar spectra from the Synoptic Optical Long-term Investigation of the Sun (SOLIS) station. The results of this work will lead to a new approach to infer the information about the spatial distribution of features on other stars, from the overall filling factor of active regions to, possibly, the latitude/longitude distribution of features., Comment: 8 pages, 6 figures, 18th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun

In this work we take advantage of eleven different sunspot group, sunspot, and active region databases to characterize the area and flux distributions of photospheric magnetic structures. We find that, when taken separately, different databases are better fitted by different distributions (as has been reported previously in the literature). However, we find that all our databases can be reconciled by the simple application of a proportionality constant, and that, in reality, different databases are sampling different parts of a composite distribution. This composite distribution is made up by linear combination of Weibull and log-normal distributions -- where a pure Weibull (log-normal) characterizes the distribution of structures with fluxes below (above) $10^{21}$Mx ($10^{22}$Mx). We propose that this is evidence of two separate mechanisms giving rise to visible structures on the photosphere: one directly connected to the global component of the dynamo (and the generation of bipolar active regions), and the other with the small-scale component of the dynamo (and the fragmentation of magnetic structures due to their interaction with turbulent convection). Additionally, we demonstrate that the Weibull distribution shows the expected linear behavior of a power-law distribution (when extended into smaller fluxes), making our results compatible with the results of Parnell et al. (2009).

The Vector Spectromagnetograph (VSM) instrument on the Synoptic Optical Longterm Investigations of the Sun (SOLIS) telescope is capable of obtaining spectropolarimetry for the full Sun (or a select latitudinal range) with one arcsecond spatial resolution and 0.05 Angstrom spectral resolution. This is achieved by scanning the Sun in declination and building up spectral cubes for multiple polarization states, utilizing a beamsplitter and two separate 2k x 2k CCD cameras. As a result, the eastern and western hemispheres of the Sun are separated in preliminary VSM images by a vertical gap with soft edges and variable position and width. Prior to the comprehensive analysis presented in this document, a trial-and-error approach to removing the gap had yielded an algorithm that was inconsistent, undocumented, and responsible for incorrectly eliminating too many image columns. Here we describe, in detail, the basis for a new, streamlined, and properly calibrated prescription for locating and removing the gap that is correct to within approximately one arcsecond (one column).

The purpose of the SOLIS Zeemanfit Code is to provide a straight-forward, easily checked measure of the total magnetic-field strength in the high-strength umbral regions of the solar disk. In the highest-strength regions, the Zeeman splitting of the 6302-angstrom Fe line becomes wide enough for the triplet nature of the line to be visible by eye in non-polarized light. Therefore, a three-line fit to the spectra should, in principle, provide a fairly robust measure of the total magnetic-field strength. The code uses the Level-1.5 spec-cube data of the SOLIS VSM 6302-vector observations (specifically the Stokes-I and Stokes-V components) to fit the line profiles at each appropriate pixel and calculate the magnetic-field-strength from the line-center separation of the two fit 6302.5 sigma-components. The 6301.5-angstrom Fe line is also present and fit in the VSM 6302-vector data, but it is an anomalous-Zeeman line with a weaker response to magnetic fields. Therefore, no magnetic- field measure is derived from this portion of the spectral fit., Comment: 39 pages containing 30 figures. Represents the primary documentation for the new SOLIS VSM Zeemanfit data product; corrected confusing variable units listed below Equation 9

The magnetic field permeating the solar atmosphere is generally thought to provide the energy for much of the activity seen in the solar corona, such as flares, coronal mass ejections (CMEs), etc. To overcome the unavailability of coronal magnetic field measurements, photospheric magnetic field vector data can be used to reconstruct the coronal field. Currently there are several modelling techniques being used to calculate three-dimension of the field lines into the solar atmosphere. For the first time, synoptic maps of photospheric vector magnetic field synthesized from Vector Spectromagnetograph (VSM) on Synoptic Optical Long-term Investigations of the Sun (SOLIS) are used to model the coronal magnetic field and estimate free magnetic energy in the global scale. The free energy (i.e., the energy in excess of the potential field energy) is one of the main indicators used in space weather forecasts to predict the eruptivity of active regions. We solve the nonlinear force-free field equations using optimization principle in spherical geometry. The resulting three-dimensional magnetic fields are used to estimate the magnetic free energy content E_{free}=E_{nlfff}-E_{pot}, i.e., the difference of the magnetic energies between the nonpotential field and the potential field in the global solar corona. For comparison, we overlay the extrapolated magnetic field lines with the extreme ultraviolet (EUV) observations by the Atmospheric Imaging Assembly on board SDO. For a single Carrington rotation 2121, we find that the global NLFFF magnetic energy density is 10.3% higher than the potential one. Most of this free energy is located in active regions., Comment: Submitted to Astronomy and Astrophysics Journal

We applied automatic identification of sunspot umbrae and penumbrae to daily observations from the Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) to study their magnetic flux (B) and area (A). The results confirm a previously known logarithmic relationship between the area of sunspots and their maximum flux density. In addition, we find that the relation between average magnetic flux (Bavg) and sunspot area shows a bimodal distribution: for small sunspots and pores (A < 20 millionth of solar hemisphere, MSH), Bavg = 800 G (gauss), and for large sunspots (A > 100 MSH), Bavg is about 600 G. For intermediate sunspots, average flux density linearly decreases from about 800 G to 600 G. A similar bimodal distribution was found in several other integral parameters of sunspots. We show that this bimodality can be related to different stages of sunspot penumbra formation and can be explained by the difference in average inclination of magnetic fields at the periphery of small and large sunspots., Comment: 13 pages, 5 figures, accepted for publication in Solar Physics

We investigate gradient of magnetic field across neutral lines (NLs), and compare their properties for NLs with and without the chromospheric filaments. Our results show that there is a range of preferred magnetic field gradients where the filament formation is enhanced. On the other hand, a horizontal gradient of magnetic field across a NL alone does not appear to be a single factor that determines if a filament will form (or not) in a given location., Comment: 11 pages, 8 figures

Solar atmosphere is a single system unified by the presence of large-scale magnetic fields. Topological changes in magnetic fields that occur in one place may have consequences for coronal heating and eruptions for other, even remote locations. Coronal magnetic fields also play role in transport of magnetic helicity from Sun's subphotosphere/upper convection zone to the interplanetary space. We discuss observational evidence pertinent to some aspects of the solar corona being a global interconnected system, i.e., large-scale coronal heating due to new flux emergence, eruption of chromospheric filament resulting from changes in magnetic topology triggered by new flux emergence, sunspots rotation as manifestation of transport of helicity through the photosphere, and potential consequences of re-distribution of energy from solar luminosity to the dynamo for solar cycle variations of solar irradiance., Comment: 9 pages, 3 figures

Measurements from the Mount Wilson Observatory (MWO) are used to study the long-term variations of sunspot field strengths from 1920 to 1958. Following a modified approach similar to that in Pevtsov et al. (2011), for each observing week we select a single sunspot with the strongest field strength measured that week and then compute monthly averages of these weekly maximum field strengths. The data show the solar cycle variation of the peak field strengths with an amplitude of about 500-700 gauss (G), but no statistically significant long-term trends. Next, we use the sunspot observations from the Royal Greenwich Observatory (RGO) to establish a relationship between the sunspot areas and the sunspot field strengths for Cycles 15-19. This relationship is then used to create a proxy of peak magnetic field strength based on sunspot areas from the RGO and the USAF/NOAA network for the period from 1874 to early 2012. Over this interval, the magnetic field proxy shows a clear solar cycle variation with an amplitude of 500-700 G and a weaker long-term trend. From 1874 to around 1920, the mean value of magnetic field proxy increases by about 300-350 G, and, following a broad maximum in 1920-1960, it decreases by about 300 G. Using the proxy for the magnetic field strength as the reference, we scale the MWO field measurements to the measurements of the magnetic fields in Pevtsov et al. (2011) to construct a combined data set of maximum sunspot field strengths extending from 1920 to early 2012. This combined data set shows strong solar cycle variations and no significant long-term trend (linear fit to the data yields a slope of $-0.2\pm$0.8 G year$^{-1}$). On the other hand, the peak sunspot field strengths observed at the minimum of the solar cycle show a gradual decline over the last three minima (corresponding to cycles 21-23) with a mean downward trend of $\approx$ 15 G year$^{-1}$.

When we examine the chirality or observed handedness of the chromospheric and coronal structures involved in the long-term build-up to eruptive events, we find that they evolve in very specific ways to form two and only two sets of large-scale chiral systems. Each system contains spatially separated components with both signs of chirality, the upper portion having negative (positive) chirality and the lower part possessing positive (negative) chirality. The components within a system are a filament channel (represented partially by sets of chromospheric fibrils), a filament (if present), a filament cavity, sometimes a sigmoid, and always an overlying arcade of coronal loops. When we view these components as parts of large-scale chiral systems, we more clearly see that it is not the individual components of chiral systems that erupt but rather it is the approximate upper parts of an entire evolving chiral system that erupts. We illustrate the typical pattern of build-up to eruptive solar events first without and then including the chirality in each stage of the build-up. We argue that a complete chiral system has one sign of handedness above the filament spine and the opposite handedness in the barbs and filament channel below the filament spine. If the spine has handedness, the observations favor its having the handedness of the filament cavity and coronal loops above. As the separate components of a chiral system form, we show that the system appears to maintain a balance of right-handed and left-handed features, thus preserving an initial near-zero net helicity. Each individual chiral system may produce many successive eruptive events above a single filament channel., Comment: 2nd ATST - EAST Workshop in Solar Physics: Magnetic Fields from the Photosphere to the Corona ASP Conference Series, Vol. 463

The measurement of solar magnetic fields using the Zeeman effect diagnostics has a fundamental 180 degree ambiguity in the determination of the azimuth angle of the transverse field component. There are several methods that are used in the community and each one has its merits and demerits. Here we present a disambiguation idea that is based on the assumption that most of the magnetic field on the sun is predominantly vertical. While the method is not applicable to penumbra or other features harboring predominantly horizontal fields like the sheared neutral lines, it is useful for regions where fields are predominantly vertical like network and plage areas. The method is tested with the full-disk solar vector magnetograms observed by the VSM/SOLIS instrument. We find that statistically about 60-85 % of the pixels in a typical full-disk magnetogram has field inclination in the range of 0-30 degrees with respect to the local solar normal, and thus can be successfully disambiguated by the proposed method. Due to its non-iterative nature, the present method is extremely fast and therefore can be used as a good initial guess for iterative schemes like nonpotential field computation (NPFC) method. Furthermore, the method is insensitive to noisy pixels as it does not depend upon the neighboring pixels or derivatives., Comment: 12 pages, 5 figures, in Solar Physics