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Register a new userA New Calibrated Sunspot Group Series Since 1749: Statistics of Active Day Fractions
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Although the sunspot-number series have existed since the mid-19th century, they are still the subject of intense debate, with the largest uncertainty being related to the calibration of the visual acuity of individual observers in the past. Daisy-chain regression methods are applied to inter-calibrate the observers which may lead to significant bias and error accumulation. Here we present a novel method to calibrate the visual acuity of the key observers to the reference data set of Royal Greenwich Observatory sunspot groups for the period 1900-1976, using the statistics of the active-day fraction. For each observer we independently evaluate their observational thresholds [S_S] defined such that the observer is assumed to miss all of the groups with an area smaller than S_S and report all the groups larger than S_S. Next, using a Monte-Carlo method we construct, from the reference data set, a correction matrix for each observer. The correction matrices are significantly non-linear and cannot be approximated by a linear regression or proportionality. We emphasize that corrections based on a linear proportionality between annually averaged data lead to serious biases and distortions of the data. The correction matrices are applied to the original sunspot group records for each day, and finally the composite corrected series is produced for the period since 1748. The corrected series displays secular minima around 1800 (Dalton minimum) and 1900 (Gleissberg minimum), as well as the Modern grand maximum of activity in the second half of the 20th century. The uniqueness of the grand maximum is confirmed for the last 250 years. It is shown that the adoption of a linear relationship between the data of Wolf and Wolfer results in grossly inflated group numbers in the 18th and 19th centuries in some reconstructions.
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Long and consistent sunspot area records are important for understanding the 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 also systematic differences between them, so that these records need to be cross-calibrated before they can be reliably used for further studies. We produce a cross-calibrated and homogeneous record of total daily sunspot areas, both projected and corrected, covering the period between 1874 and 2019. A catalogue of calibrated individual group areas is also generated for the same period. We have 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. Mutual comparisons of the individual records have been employed 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. 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. We have also computed and provide the daily Photometric Sunspot Index (PSI) widely used, e.g., in empirical reconstructions of solar irradiance.
In this study, we used two methods to investigate the periodic behavior of sunspot counts in four categories for the time period January 1986-October 2013. These categories include the counts from simple (A and B), medium (C), large (D, E, and F), and final (H) sunspot groups. We used: i) the Multi-taper Method with red noise approximation, and ii) the Morlet wavelet transform for periodicity analysis. Our main findings are: (1) the solar rotation periodicity of about 25 to 37 days, which is of obvious significance, is found in all groups with at least a 95% significance level; (2) the periodic behavior of a cycle is strongly related to its amplitude and group distribution during the cycle; (3) the appearance of periods follow the amplitude of the investigated solar cycles, (4) meaningful periods do not appear during the minimum phases of the investigated cycles. We would like to underline that the cyclic behavior of all categories is not completely the same; there are some differences between these groups. This result can provide a clue for the better understanding of solar cycles.
Aims. Sunspot number is a benchmark series in many studies, but may still contain inhomogeneities and inconsistencies. In particular, an essential discrepancy exists between the two main sunspot number series, Wolf (WSN) and group (GSN) sunspot numbers, before 1848. The source of this discrepancy has so far remained unresolved. However, the recently digitized series of solar observations in 1825-1867 by Samuel Heinrich Schwabe, who was the primary observer of the WSN before 1848, makes such an assessment possible. Methods. We construct sunspot series, similar to WSN and GSN, but using only Schwabes data. These series, called WSN-S and GSN-S, respectively, were compared with the original WSN and GSN series for the period 1835-1867 to look for possible inhomogeneities. Results. We show that: (1) The GSN series is homogeneous and consistent with the Schwabe data throughout the entire studied period; (2) The WSN series decreases by roughly ~20% around 1848 caused by the change of the primary observer from Schwabe to Wolf and an inappropriate individual correction factor used for Schwabe in the WSN; (3) This implies a major inhomogeneity in the WSN, which needs to be corrected by reducing its values by 20% before 1848; (4) The corrected WSN series is in good agreement with the GSN series. This study supports the earlier conclusions that the GSN series is more consistent and homogeneous in the earlier part than the WSN series.
We use 5 test data series to quantify putative discontinuities around 1946 in 5 annual-mean sunspot number or group number sequences. The series tested are: the original and n
The Maunder minimum (MM) of greatly reduced solar activity took place in 1645-1715, but the exact level of sunspot activity is uncertain as based, to a large extent, on historical generic statements of the absence of spots on the Sun. Here we aim, using a conservative approach, to assess the level and length of solar cycle during the Maunder minimum, on the basis of direct historical records by astronomers of that time. A database of the active and inactive days (days with and without recorded sunspots on the solar disc respectively) is constructed for three models of different levels of conservatism (loose ML, optimum MO and strict MS models) regarding generic no-spot records. We have used the active day fraction to estimate the group sunspot number during the MM. A clear cyclic variability is found throughout the MM with peaks at around 1655--1657, 1675, 1684 and 1705, and possibly 1666, with the active day fraction not exceeding 0.2, 0.3 or 0.4 during the core MM, for the three models. Estimated sunspot numbers are found very low in accordance with a grand minimum of solar activity. We have found, for the core MM (1650-1700), that: (1) A large fraction of no-spot records, corresponding to the solar meridian observations, may be unreliable in the conventional database. (2) The active day fraction remained low (below 0.3-0.4) throughout the MM, indicating the low level of sunspot activity. (3) The solar cycle appears clearly during the core MM. (4) The length of the solar cycle during the core MM appears $9pm 1$ years, but there is an uncertainty in that. (5) The magnitude of the sunspot cycle during MM is assessed to be below 5-10 in sunspot numbers; A hypothesis of the high solar cycles during the MM is not confirmed.
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