Long-term studies of photospheric magnetic fields on the Sun

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.

Ant mazes and astronomy: Harlow Shapley's entomological experiments at Mount Wilson observatory and Pasadena, California

This article explores the entomological research of American astronomer Harlow Shapley (1885–1972). The focus is 1914–1921, the time that Shapley worked as a staff astronomer at Mount Wilson Observatory in the San Gabriel Mountains of California. The years 1919–1920 receive particular attention because they were the most active period for Shapley's ant experiments. During these years, Shapley undertook two primary programmes of study in ant physiology, each centred on a specific species native to Southern California—the velvety tree ant and the California harvester ant. Shapley's ant research was dependent upon the scientific environment of Mount Wilson Observatory because of several benefits and opportunities that the location offered him. The section ‘Observatory ants’ discusses the details of Shapley's research programme as well as the advantages given him by his location. The subsequent section, ‘Indoor ants’, discusses a change in environment for Shapley's ant experiments and how that change, from an outdoor to an indoor setting, changed the scope and nature of his research. This section also discusses the influences that Shapley's work came to have in the disciplines of entomology, ecology, general physiology, and behavioural science. The final section, ‘Living and non-living?’, discusses the impacts that Shapley's ant research came to have on his own later work concerning cosmic evolution and interdisciplinary co-operation among the sciences. In conclusion, we are left with evidence suggesting that Shapley's work in entomology was highly location-dependent while also possessing far-reaching consequences.

Reconstructing solar magnetic fields from historical observations

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.

Variations in ratio and correlation of solar magnetic fields in the Fe I 525.02 nm and Na I 589.59 nm lines according to Mount Wilson measurements during 2000–2012

Variations in the solar magnetic-field ratio over 13 years are analyzed, relying on the comparison of simultaneous measurements in two spectral lines at the Mount Wilson Observatory. The ratio and correlation coefficient are calculated over the general working range of measured magnetic-field values and in various ranges of field magnitudes. We study variations in both the parameters. We have found the following tenden-cies: i) the parameters show changes with solar cycle in the general case; ii) their dependence on magnetic-field magnitude is a nonlinear function of time, and this is especially pronounced in the ratio behavior; iii) several separate ranges of the field magnitudes can be distin-guished based on the behavioral patterns of variations in the ratio. We discuss correspondences between these ranges and the known structural objects of the solar atmosphere. This leads to a conclusion that the dependence of the parameters on magnetic-field magnitude and time is connected with the variety of magnetic structural components and their cyclic rearrangements. The reported results may be useful for solving interpretation problems of solar magnetic-field meas-urements and for the cross-calibration of applicable instruments. They can also be used for tasks related to the creation of a uniform long temporal series of solar magnetic-field data from various sources.

A Stellar Perspective on the Magnetic Future of the Sun

After decades of effort, the solar magnetic cycle is exceptionally well characterized, but it remains poorly understood. Pioneering work at the Mount Wilson Observatory demonstrated that other Sun-like stars also show regular activity cycles, and identified two distinct relationships between the rotation rate and the length of the cycle. The solar cycle appears to be an outlier, falling between the two stellar relationships, potentially threatening the very foundation of the solar-stellar connection. Recent discoveries emerging from NASA’s Kepler space telescope have started to shed light on this perplexing result, suggesting that the Sun’s rotation rate and magnetic field are currently in a transitional phase that occurs in all middle-aged stars. We have recently identified the manifestation of this magnetic transition in the best available data on stellar cycles. These observations suggest that the solar cycle is currently growing longer on stellar evolutionary timescales, and that the global dynamo may shut down entirely sometime in the next 0.8-2.4 Gyr. Future tests of this hypothesis will come from ground-based activity monitoring of Kepler targets that span the magnetic transition, and from asteroseismology with the TESS mission to determine precise masses and ages for bright stars with known cycles.

Sunspot magnetic fields in 24 cycle of solar activity

The data of visual measurements of the magnetic field in sunspots umbra, obtained in four observatories during 24 cycles of solar activity, are compared. Magnetic field strength modules with averaging over each observatory are analyzed. The measurements taken in the Astronomical Observatory of the University of Kyiv (KAO) were used as base, where 170 spots were measured on average 4 times each at intervals of one to two daysduring the group passing through the solar disk. Measurements of the Ural, Crimean and Mount Wilson Observatory, performed within 24 hours of observation in the KAO, are used in the study. It was found that the average value of the magnetic field per cycle, obtained from the data of three observatories UrAO, CrAO and KAO, was 26.0 ± 0.3, 25.0 ± 0.2 and 25.2 ± 0.2 cT, respectively. They differ by no more than 3 standard errors, while the Mount Wilson Observatory gives a significantly lower magnetic field – 23.8 ± 0.1 cT. This result is confirmed by the work of R. Rezaei et al., 2015, which compares the visual measurements in Mount Wilson with Tenerife Infrared Polarimeter (TIP) and Facility InfRared Spectropolarimeter (FIRS) for the 23rd and 24th cycle of solar activity. The average magnetic field value for observations of TIP and FIRS in 24 cycles is 26.0 cT, which is slightly higher than we obtained from the visual observations of three observatories, while MtW shows a few centitesla less. The mean square error of the average per cycle of the magnetic field, based on the data of TIP and FIRS is 0.2 cT, so the difference with the MtW is reliable. An understatement of the measured intensity of the magnetic field of the spots requires a revision of the measurement calibration in Mount Wilson. The data of the Crimean Astrophysical Observatory can be considered reference for subsequent comparisons with the results of observations of other observatories. For the calculation of the annual index Bsp of magnetic fields of sunspot size 22-44 Mm, the data of visual measurements of all 4 observatories are traditionally used. We have found that variations in the magnetic field of sunspots are the same as in previous cycles: the peak occurs in the second-third years after the maximum number of spots. The average for the 24 cycle Bsp index is 24.9 ± 0.3 сT, which allows us to consider this magnetic cycle as moderately strong. Reduction of the sunspots magnetic field in 2018 to 23 cT allows it to expect in 2019 the highest near-ground level of galactic cosmic rays in the last half century.

Taking the Measure of Massive Stars and their Environments with the CHARA Array Long-baseline Interferometer

Most massive stars are so distant that their angular diameters are too small for direct resolution. However, the observational situation is now much more favorable, thanks to new opportunities available with optical/IR long-baseline interferometry. The Georgia State University Center for High Angular Resolution Astronomy Array at Mount Wilson Observatory is a six-telescope instrument with a maximum baseline of 330 meters, which is capable of resolving stellar disks with diameters as small as 0.2 milliarcsec. The distant stars are no longer out of range, and many kinds of investigations are possible. Here we summarize a number of studies involving angular diameter measurements and effective temperature estimates for OB stars, binary and multiple stars (including the σ Orionis system), and outflows in Luminous Blue Variables. An enlarged visitors program will begin in 2017 that will open many opportunities for new programs in high angular resolution astronomy.

Allan Rex Sandage. 18 June 1926 — 13 November 2010

Allan Sandage was an observational astronomer who was happiest at a telescope. On the sudden death of Edwin Hubble, Sandage inherited the programmes using the world’s largest optical telescope at Palomar to determine the distances and number counts of galaxies. Over many years he greatly revised the distance scale and, on reworking Hubble’s analysis, discovered the error that had led Hubble to doubt the interpretation of the galaxies’ redshifts as an expansion of the Universe. Sandage showed that there was a consistent age of creation for the stars, the elements and the cosmos. Through work with Baade and Schwarzschild he discovered the key to the interpretation of the colour–magnitude diagrams of star clusters in terms of stellar evolution. With others he founded galactic archaeology, interpreting the motions and elemental abundances of the oldest stars in terms of a model for the Galaxy’s formation. He published several fine atlases and catalogues of galaxies and a definitive history of the Mount Wilson Observatory.