Greenwich Observatory Research Articles

Jasper V Wall

Abstract Remembering Jasper V Wall, the final director of the Royal Greenwich Observatory

Two sunspot group populations and Gnevyshev-Waldmeier rule

Based on the materials of the Royal Greenwich Observatory catalog, the study of two different sunspot group populations continued: LLG – large long-lived, and SSG – small short-lived groups. The task of achieving a higher accuracy of the population separation parameter than before (Nagovitsyn & Pevtsov 2016, 2021) is being solved. A procedure for randomizing the lifetimes of sunspot groups observed once a day is proposed, which allows for statistical studies to achieve a higher time resolution. The form of the Gnevyshev-Waldmeier Rule is taken, which linearly connects the logarithm of the logS area and the lifetime of the sunspot group LT (over limited time intervals). It is shown that it has coefficients significantly different for SSG and LLG populations. The range of values of the group lifetime parameter separating the populations was found as LT∗=4.75±0.53 days, which is in agreement with the threshold values obtained earlier for the number of days of sunspot group observation: mSSG⩽5 days and mLLG>5 days. It is shown that the parameters of the bilognormal distribution of the sunspot group areas, obtained from their common grouped sample statistically by Levenberg–Marquardt method and with a preliminary division into lognormal distributions by lifetime, correspond to each other. It was clarified that, with an accuracy of up to a tenth of a day, the SSG population corresponds to the lifetimes of groups ⩽4.6 days, and the LLG population corresponds to times ⩾4.7 days. The results obtained make it possible to study various physical properties of SSG and LLG populations independently of each other in order to compare them and study their nature.

Neglect of Small Spots Remains an Explanation of the Difference Between SOON and RGO Spot Areas

I point out that the objection of Györi et al. to my finding that neglect of small spots could explain much of the puzzling difference between Royal Greenwich Observatory and Solar Optical Observation Network spot areas is not valid because their analysis was limited to only 1974–76, when activity was low. The 40%–50% difference occurs only at higher activity levels.

Revealing observatory networks through object stories: Object itineraries

In this paper, each contribution uses an object biography – or object itinerary – methodology, tracking their chosen objects from conception and purchase, across time and space, to today. The function of each object could change significantly over time, and these stories serve to remind us of the range of projects and activities that observatories might support or host, including overseas astronomical expeditions, a local time service, mathematical calculations, measurement of photographic plates, magnetic, meteorological and seismological observation, the hosting and dining of visitors, and the display and promotion of the observatory’s status. In each case, the object’s movements and location within particular spaces point the reader to consider a wide range of people, whether located within the observatory, beyond its walls, or within international scientific networks. The objects considered here are a London-made marine chronometer purchased by the Palermo Observatory, which was not used for navigation but was nevertheless well travelled; a large wooden table placed in the Octagon Room of the Royal Observatory Greenwich; and a British seismograph set up in at the Meteorological and Magnetic Observatory of the University of Coimbra. These itineraries also show how the value attached to objects changes over time: they shift from functional tools to obsolescence, then regain significance as their histories have been recovered and re-examined. This paper forms part of the collection ‘Revealing observatory networks through object stories’ where object stories are presented in three themed papers (with further papers being ‘Instrumental networks’ and ‘Observatory audiences’). The genesis of the collection is described in the ‘Introduction’.

Sunspot extraction and hemispheric statistics of YNAO sunspot drawings using deep learning

Sunspot drawings around the globe provide long historical records for understanding the long-term trends in the solar activity cycle. Yunnan Astronomical Observatory (YNAO) in China contributes to the relatively continuous sunspot drawings from 1957 to 2015. This paper proposes a new deep learning method named SPR-mask to extract pores, spots, umbrae and penumbrae in the YNAO sunspot drawings. SPR-mask consists of three parts: backbone, shared head and mask branch. It especially adopts a scale-aware attention network (SAAN) and a PointRend module in the mask branch to improve the accuracy of target edge segmentation. Besides that, each sunspot belonging to the northern or southern (N-S) hemisphere is determined by transforming its cartesian coordinates to spherical coordinates after extracting $P$ , $B_{0}$ and $L_{0}$ handwritten in sunspot drawings using a revised Lenet-5 deep learning method. The precision, recall and $AP$ of SPR-mask are 0.92, 0.93, and 0.92, respectively. The test results show the SPR-mask method has a good performance. The numbers and areas of pores, spots, umbrae and penumbrae for the N-S hemisphere are presented and analyzed separately. The YNAO data are also compared with Royal Greenwich Observatory (RGO), Kanzelhöhe Observatory (KSO) and Purple Mountain Astronomical Observatory (PMO) data. The results show similar trends, high correlations, and N-S asymmetries. All data of YNAO are publicly shared at https://github.com/yzs64/YNAO_sd/ , which are abundant and complementary to the other sunspot catalogs in the world.

Probability Distribution Functions of Sunspot Magnetic Flux

We investigated the probability distributions of sunspot area and magnetic flux by using data from the Royal Greenwich Observatory and USAF/NOAA. We constructed a sample of 2995 regions with maximum-development areas ≥500 MSH (millionths of solar hemisphere), covering 146.7 yr (1874–2020). The data were fitted by a power-law distribution and four two-parameter distributions (tapered-power-law, gamma, lognormal, and Weibull distributions). The power-law model was unfavorable compared to the four models in terms of AIC, and was not acceptable according to the classical Kolmogorov–Smirnov test. The lognormal and Weibull distributions were excluded because their behavior extended to smaller regions (S ≪ 500 MSH) do not connect to previously published results. Therefore, our choices were tapered-power-law and gamma distributions. The power-law portion of the tapered-power-law and gamma distributions was found to have a power exponent of 1.35–1.9. Due to the exponential falloff of these distributions, the expected frequencies of large sunspots are low. The largest sunspot group observed had an area of 6132 MSH, and the frequency of sunspots larger than 104 MSH was estimated to be every 3–8 × 104 yr. We also estimated the distributions of the Sun-as-a-star total sunspot areas. The largest total area covered by sunspots on record was 1.67% of the visible disk, and can be up to 2.7% by artificially increasing the lifetimes of large sunspots in an area evolution model. These values are still smaller than those found on active Sun-like stars.

Extending the sunspot area series from Kodaikanal Solar Observatory

Kodaikanal Solar Observatory (KoSO) possesses one of world’s longest and homogeneous records of sunspot observations that span more than a century (1904–2017). Interestingly, these observations (originally recorded in photographic plates/films) were taken with the same setup over this entire time period which makes this data unique and best suitable for long-term solar variability studies. A large part of this data, between 1921–2011, were digitized earlier and a catalog containing the detected sunspot parameters (e.g., area and location) was published in Mandal et al. (2017). In this article, we extend the earlier catalog by including new sets of data between 1904–1921 and 2011–2017. To this end, we digitize and calibrate these new datasets which include resolving the issue of random image orientation. We fix this by comparing the KoSO images with co-temporal data from Royal Greenwich Observatory. Following that, a semi-automated sunspot detection and automated umbra detection algorithm are implemented onto these calibrated images to detect sunspots and umbra. Additionally, during this catalog updation, we also filled data gaps in the existing KoSO sunspot catalog (1921–2011) by virtue of re-calibrating the “rouge” plates. This updated sunspot area series covering nearly 115 years (1904–2017) are being made available to the community and will be a unique source to study the long term variability of the Sun.

The Paris Observatory

The Paris Observatory is the oldest astronomical observatory that has operated continuously from its creation in 1667 to today. It was closely followed in 1675 by the Greenwich Observatory, which was closed in 1998 and turned into a museum. In 1926, Paris Observatory absorbed the Observatory of Physical Astronomy of Meudon, and created in 1953 a radio astronomy station in Nançay, so that it now operates on three different sites. We retrace its 350-years history here.

Reconstruction of the Solar Activity from the Catalogs of the Zurich Observatory

Catalogs of the Zurich Observatory contain positional information on sunspots, prominences, and faculae in the late 19th and early 20th centuries. This database is given in handwritten tabular form and was not systematically analyzed earlier. It is different from the sunspot-number time series made in Zurich and was obtained with a larger telescope. We trained a neural-network model for handwritten text recognition and present the database of reconstructed coordinates. The database obtained connects the earlier observations by Spörer with later programs of the 20th century and supplements the sunspot-group catalogs of the Royal Greenwich Observatory. We also expect that the presented machine-learning approach and its deep capabilities will motivate the processing of a wide bulk of astronomical data, which is still given in nondigitized form or as plain scanned images.

A Sunspot Catalog by Rafael Carrasco at the Madrid Astronomical Observatory for the Period 1931 – 1933

A sunspot catalog was published by the Madrid Astronomical Observatory from sunspot observations made by Rafael Carrasco and his assistants for the period November 1931 – December 1933. We have digitized this catalog and carried out a quality control to detect inconsistencies. We present a machine-readable version of this sunspot catalog together with an erratum list. Moreover, we compared the Madrid sunspot number and area with other reference series. We found that sunspots in the northern hemisphere were predominant during this period, confirming previous studies. We have also analyzed the group morphological typologies registered in the catalog and show a butterfly diagram drawn with this new information. A comparison with the sunspot catalog by the Royal Greenwich Observatory shows that almost all the groups included in the Carrasco’s catalog are also present in the aforementioned catalog.

Obstacles encountered by four major European astronomical observatories belonging to academies in the 18th century

It is known that, in the first half of the 18th century, the conditions for astronomy at the Imperial Observatory of St-Petersburg, directed by Joseph-Nicolas Delisle, were comparable to those enjoyed by astronomers at the royal observatories of Paris and Greenwich created in the previous century. But what about the public observatories created in the first half of the 18th century in Berlin, Uppsala and Bologna? The rich correspondence maintained by Joseph-Nicolas Delisle with the astronomers working in these observatories provides elements of an answer to this question. It also provides more precise information on Delisle’s working conditions at the St-Petersburg Observatory. In this article, we present a comparative analysis of the obstacles encountered by astronomers at these different observatories, and the particular contexts in which they operated, including a breakdown by observatory of salaries and expenditure on astronomy equipment.

Sunspot Catalog (1921–1935) and Area Series (1886–1940) from the Stonyhurst College Observatory

Abstract A sunspot observation program was started at the end of the 19th century at the Stonyhurst College Observatory (hereafter SCO) by Father Perry, director of the observatory at that time. A digitization of the daily sunspot area series recorded in this observatory from 1886 to 1940 (with a gap between 1889 and 1897) is provided in this work. This depicts one of the oldest sunspot area series available. A comparison of this series with contemporary area series made in other observatories shows that SCO generally recorded larger areas than those in some of the observatories of that time such as, for example, the Royal Greenwich Observatory (RGO). Furthermore, SCO published a sunspot group catalog for the period 1921–1935. We provide a machine-readable version of this catalog. We compared the SCO group number series with other sunspot data obtained from other observatories. In this case, for example, the RGO systematically recorded more groups than the SCO. We compared SCO and RGO area distribution functions obtaining the calibration constant between both data sets. We also obtained the butterfly diagram from the group latitudes recorded by SCO and compared the percentages of group types computed from the SCO catalog with those from Valencia Observatory (following the Cortie morphological classification of sunspot groups), identifying their similarities and differences.

Eleanor Margaret Burbidge. 12 August 1919—5 April 2020

Margaret Burbidge was one of the great observational astronomers of the twentieth century. She had a natural aptitude for instrumentation, observation and the interpretation of spectroscopic data, coupled with an instinct for making optimum use of the observing facilities to which she gained access. Following her rigorous training in observational astrophysics and managing the University of London Observatory, she and Geoff Burbidge (FRS 1968), whom she married in 1948, led a peripatetic life for the following decade or more. During this time they made their most important pioneering contribution to astrophysics through their collaboration with Willy Fowler and Fred Hoyle (FRS 1957) on the origin of the chemical elements. Their famous and comprehensive B 2 FH paper of 1957 described the numerous nuclear processes that led to the synthesis of the heavy elements in stars, with element abundance values supported by decisive observational evidence. They went on to the University of Chicago/Yerkes, making pioneering observations of the kinematics of galaxies. In 1962, they both became tenured professors at the University of California at San Diego, their base for the rest of their careers. There, they were at the heart of the exciting early years of unravelling the properties of quasars, writing the first monograph on these objects in 1967. After an unhappy brief episode as the first woman director of the Royal Greenwich Observatory in the UK, Margaret returned to San Diego, where she played a major role in the construction and exploitation of the faint object spectrograph of the Hubble Space Telescope, specializing in the study of the absorption line spectra of quasars. Most of her early papers were co-authored with Geoff, who supported her in every way throughout their happy marriage. In her understated way, she became a role model for women astronomers, breaking down the conscious and unconscious bias against women she encountered throughout her career. Margaret led by example and made every effort to support future generations of women astronomers.

Hemispheric sunspot numbers 1874–2020

Context. Previous studies show significant north–south asymmetries for various features and indicators of solar activity. These findings suggest some decoupling between the two hemispheres over the solar cycle evolution, which is in agreement with dynamo theories. For the most important solar activity index, the sunspot numbers, so far only limited data are available for the two hemispheres independently. Aims. The aim of this study is to create a continuous series of daily and monthly hemispheric sunspot numbers (HSNs) from 1874 to 2020, which will be continuously expanded in the future with the HSNs provided by SILSO. Methods. Based on the available daily measurements of hemispheric sunspot areas from 1874 to 2016 from Greenwich Royal Observatory and National Oceanic and Atmospheric Administration, we derive the relative fractions of the northern and southern activity. These fractions are applied to the international sunspot number (ISN) to derive the HSNs. This method and obtained data are validated against published HSNs for the period 1945–2004 and those provided by SILSO for 1992 to 2016. Results. We provide a continuous data series and catalogue of daily, monthly mean, and 13-month smoothed monthly mean HSNs for the time range 1874–2020 –fully covering solar cycles 12 to 24– that are consistent with the newly calibrated ISN (Clette et al., 2014). Validation of the reconstructed HSNs against the direct data available since 1945 reveals a high level of consistency, with Pearson correlation coefficients of r = 0.94 (0.97) for the daily (monthly mean) data. The cumulative hemispheric asymmetries for cycles 12–24 give a mean value of 16%, with no obvious pattern in north–south predominance over the cycle evolution. The strongest asymmetry occurs for cycle no. 19, in which the northern hemisphere shows a cumulated predominance of 42%. The phase shift between the peaks of solar activity in the two hemispheres may be up to 28 months, with a mean absolute value over cycles 12–24 of 16.8 months. The phase shifts reveal an overall asymmetry of the northern hemisphere reaching its cycle maximum earlier (in 10 out of 13 cases), with a mean signed phase shift of −7.6 months. Relating the ISN and HSN peak growth rates during the cycle rise phase with the cycle amplitude reveals higher correlations when considering the two hemispheres individually, with r ≈ 0.9. Conclusions. Our findings provide further evidence that to some degree the solar cycle evolves independently in the two hemispheres, and demonstrate that empirical solar cycle prediction methods can be improved by investigating the solar cycle dynamics in terms of the HSN evolution.

Average annual total sunspot area in the last 410 yr: the most probable values and limits of their uncertainties

ABSTRACT The aim of this work is to create a long (410-yr) series of average annual total sunspot areas AR – a physically based index of sunspot activity. We use telescopic observations of the AR index in 1832–1868 and 1875–2020, as well as the relationship between AR and long series of sunspot indices SN (international sunspot numbers, version 2.0) and sunspot groups GN (Svalgaard and Schatten’s version of group sunspot numbers). The Royal Greenwich Observatory series after 1976 is extended by the Kislovodsk Mountain Astronomical Station data. When reconstructing AR from SN, it is taken into account that the function AR = f(SN) has a non-linear systematic character and uncertainty associated with the heterogeneity of these indices. Therefore, in addition to modelling the most probable AR values, predictive limits of reconstruction uncertainty are determined. In the interval 1610–1699 we carried out the reconstruction on the basis of the GN series using the previously proposed decomposition in pseudo-phase space (DPS) method. The resulting series NO21y is freely available online. We show that for this series the empirical Gnevyshev–Ohl rule and Waldmeier effect are fulfilled. Wavelet analysis reveals periodicities of 8.4–13.8 yr for the main cycle (with a sharp decrease of the period before the global Maunder and Dalton minima) and a two-component Gleissberg cycle with typical periods of 50–60 and 90–110 yr.

“Generative” Indices of Sunspot Solar Activity: 145-Year Composite Series

Traditional indices, which we call apparent indices, describe the current state of solar activity and are very important for understanding the influence of the Sun on Earth. To study the productivity of the solar dynamo, however, we need a different index that would describe each magnetic region once – at the moment of its maximum area. We refer to such indices as generative ones. The aim of this work is to produce such a generative series improving the procedure by Nagovitsyn et al. (Geomagn. Aeron. 58, 1170, 2018). We use sunspot group data from two archives – Royal Greenwich Observatory (1875 – 1976) and Kislovodsk Mountain Astronomical Station (1955 – 2019). We produce generative indices with annual values of sunspot group areas $A$ , and counts of sunspot groups $C$ . Unified 145-year series of these indices were compiled. We study the characteristics of the indices $A$ and $C$ , and indices derived from them for two sunspot group populations – large long-living groups and small short-living groups according to Nagovitsyn and Pevtsov (Astrophys. J. 833, 94, 2016) – for the northern and southern hemispheres separately. We find a 50-year variation in several indices on the 145-year interval, and detect a similar phase variation in the north-south asymmetry taken with the opposite sign. Perhaps this indicates the existence of another secular cycle along with the Gleissberg cycle on the Sun. The obtained series of indices $A$ and $C$ can be useful for studying the long-term properties of the solar dynamo.

Bi-lognormal Distribution of Sunspot Group Areas

Abstract We use daily observations from the Royal Greenwich Observatory and Kislovodsk Mountain Astronomical Station of Pulkovo Observatory to study the distribution properties of sunspot areas. To mitigate the poor statistics in the distribution of small areas, we introduce a “precision randomization” approach based on the assumption that all measured areas have a random component within the measurement uncertainty of 1 millionth of the solar hemisphere (M.S.H.). We confirm the presence of two distinct components in the distribution of sunspots previously reported by several authors, and show that the area distribution is described by the sum of two lognormal distributions responsible for small and large groups (sunspots), respectively. We also demonstrate that the area of the main spots in the groups correlates well with the total area of the group, and, thus, the findings derived for the total group areas are equally applicable to the main spots of groups.

The cause of the appearance or disappearance of the Rieger-type periodicity in the northern and southern hemispheres of the sun

Abstract We use a continuous wavelet transform to analyse the daily hemispheric sunspot area data from the Greenwich Royal Observatory during cycles 12–24 and then study the cause of the appearance or disappearance of the Rieger-type periodicity in the northern and southern hemispheres during a certain cycle. The Rieger-type periodicity in the northern and southern hemispheres should be developed independently in the two hemispheres. This periodicity in the northern hemisphere is generally anti-correlated with the long-term variations in the mean solar cycle strength of hemispheric activity, but the correlation of the two parameters in the southern hemisphere shows a weak correlation. The appearance or disappearance of Rieger-type periodicity in the northern and southern hemispheres during a certain solar cycle is not directly correlated with their corresponding hemispheric mean activity strength but should be related to the strength of the hemispheric activity during sunspot maximum times, which hints the Rieger-type periodicity is more related to temporal evolution of toroidal magnetic field. The Rieger-type periodicity in the two hemispheres disappears in those solar cycles with relatively weak hemispheric activity during sunspot maximum times. The reason for the disappearance of this periodicity may be due to the combined influence of relatively weak toroidal magnetic fields and torsional oscillations, the differential rotation parameters vary through the solar cycle and may not remain more or less unchanged during some time, which does not permit the strong growth of magnetic Rossby waves.

Eleanor Margaret Burbidge

Eleanor Margaret Burbidge

Sunspot area catalog revisited: Daily cross-calibrated areas since 1874

Context. Long and consistent sunspot area records are important for understanding long-term solar activity and variability. Multiple observatories around the globe have regularly recorded sunspot areas, but such individual records only cover restricted periods of time. Furthermore, there are systematic differences between these records and require cross-calibration before they can reliably be used for further studies. Aims. We produce a cross-calibrated and homogeneous record of total daily sunspot areas, both projected and corrected, covering the period between 1874 and 2019. In addition, we generated a catalog of calibrated individual group areas for the same period. Methods. We compared the data from nine archives: Royal Greenwich Observatory (RGO), Kislovodsk, Pulkovo, Debrecen, Kodaikanal, Solar Optical Observing Network (SOON), Rome, Catania, and Yunnan Observatories, covering the period between 1874 and 2019. Cross-comparisons of the individual records were done to produce homogeneous and inter-calibrated records of daily projected and corrected areas. As in earlier studies, the basis of the composite is formed by the data from RGO. After 1976, the only datasets used are those from Kislovodsk, Pulkovo, and Debrecen observatories. This choice was made based on the temporal coverage and the quality of the data. While there are still 776 days missing in the final composite, these remaining gaps could not be filled with data from the other archives as the missing days lie either before 1922 or after 2016 and none of the additional archives cover these periods. Results. In contrast to the SOON data used in previous area composites for the post-RGO period, the properties of the data from Kislovodsk and Pulkovo are very similar to those from the RGO series. They also directly overlap the RGO data in time, which makes their cross-calibration with RGO much more reliable. Indeed, comparing our area catalog with previous such composites, we find improvements both in data quality and coverage. We also computed the daily Photometric Sunspot Index, which is widely used, for example, in empirical reconstructions of solar irradiance.