V. M. S. Carrasco

A Revised Collection of Sunspot Group Numbers

We describe a revised collection of the number of sunspot groups from 1610 to the present. This new collection is based on the work of Hoyt and Schatten (Solar Phys. 179, 189, 1998). The main changes are the elimination of a considerable number of observations during the Maunder Minimum (hereafter, MM) and the inclusion of several long series of observations. Numerous minor changes are also described. Moreover, we have calculated the active-day percentage during the MM from this new collection as a reliable index of the solar activity. Thus, the level of solar activity obtained in this work is greater than the level obtained using the original Hoyt and Schatten data, although it remains compatible with a grand minimum of solar activity. The new collection is available in digital format.

Level and length of cyclic solar activity during the Maunder minimum as deduced from the active-day statistics

Aims. 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 gener ic statements of the absence of spots on the Sun. Here we aim, using a conservative approach, to assess the level and length of sol ar cycle during the Maunder minimum, on the basis of direct historical records by astronomers of that time. Methods. 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. Results. 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. Conclusions. 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 9± 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.

Sunspot Numbers and Areas from the Madrid Astronomical Observatory (1876 – 1986)

The solar program of the Astronomical Observatory of Madrid started in 1876. Observations were made in this institution to determine sunspot numbers and areas for ten solar cycles. The program was completed in 1986 and the resulting data have been published in various Spanish scientific publications. Four periods of this program (with different observers and instruments) were identified with the aid of the interesting metadata that has been made available. In the present work, the published data were retrieved and digitized. Their subsequent analysis showed that most of these data could be considered reliable given their very high correlation with reference indices (international sunspot number, group sunspot number, and sunspot area). An abrupt change emerged in the sunspots/groups ratio in 1946, which lasted until 1972.

Sunspots During the Maunder Minimum from Machina Coelestis by Hevelius

We revisited the sunspot observations published by Johannes Hevelius in his book Machina Coelestis (1679) corresponding to the period of 1653 – 1675 (just in the middle of the Maunder Minimum). We show detailed translations of the original Latin texts describing the sunspot records and provide the general context of these sunspot observations. From this source, we present an estimate of the annual values of the group sunspot number based only on the records that explicitly inform us of the presence or absence of sunspots. Although we obtain very low values of the group sunspot number, in accordance with a grand minimum of solar activity, these values are significantly higher in general than the values provided by Hoyt and Schatten (Solar Phys.179, 189, 1998) for the same period.

Forty two years counting spots: Solar observations by D.E. Hadden during 1890–1931 revisited

Abstract We have recovered the sunspot observations made by David E. Hadden during 1890–1931 from Alta, Iowa. We have digitized the available data published by Hadden in different astronomical journals. This data series have been analyzed and compared with the standard sunspot number series. Moreover, we provide additional information on two great sunspot groups, previously not described, that originated two important extreme episodes of space weather on February 1892 and September 1898.

Sunspot Observations During the Maunder Minimum from the Correspondence of John Flamsteed

We compile and analyze the sunspot observations made by John Flamsteed for the period 1672 – 1703, which corresponds to the second part of the Maunder Minimum. They appear in the correspondence of the famous astronomer. We include in an appendix the original texts of the sunspot records kept by Flamsteed. We compute an estimate of the level of solar activity using these records, and compare the results with the latest reconstructions of solar activity during the Maunder Minimum, obtaining values characteristic of a grand solar minimum. Finally, we discuss a phenomenon observed and described by Stephen Gray in 1705 that has been interpreted as a white-light flare.

Sunspot Catalogue of the Valencia Observatory (1920 – 1928)

A sunspot catalogue was maintained by the Astronomical Observatory of Valencia University (Spain) from 1920 to 1928. Here we present a machine-readable version of this catalogue (OV catalogue or OVc), including a quality-control analysis. Sunspot number (total and hemispheric) and sunspot area series are constructed using this catalogue. The OV catalogue data are compared with other available solar data, demonstrating that the present contribution provides the scientific community with a reliable catalogue of sunspot data.

Equivalence Relations Between the Cortie and Zürich Sunspot Group Morphological Classifications

Catalogues of sunspots have been available with useful information about sunspots or sunspot groups for approximately the last 150 years. However, the task of merging these catalogues is not simple. We suggest a method of converting the types of sunspot groups that was proposed by Cortie (Astrophys. J.13, 260, 1901) into the well-known Zürich types of sunspot groups. To achieve this, we used the sunspot catalogue of the Valencia University Observatory (from 1920 to 1928) in addition to the descriptions proposed by Cortie. To assess the quality of this conversion scheme, the Zürich type was computed from the Valencia catalogue, and the resulting contribution of each group type was compared to what can be found in other catalogues. The results show that the proposed scheme works well within the errors that are found in the different catalogues.

Monitoring the Solar Radius from the Royal Observatory of the Spanish Navy since 1773

The solar diameter has been monitored at the Royal Observatory of the Spanish Navy (today the Real Instituto y Observatorio de la Armada: ROA) almost continuously since its creation in 1753 (i.e. during the past 250 years). After a painstaking effort to collect data in the historical archive of this institution, we present here the data of the solar semidiameter from 1773 to 2006, making up an extensive new database for solar-radius measurements, which can be considered. We have calculated the solar semidiameter from the transit times registered by the observers (except for values of the solar radius from the modern Danjon astrolabe, which were published by ROA). These data were analysed to reveal any significant long-term trends, but no such trends were found. Therefore, the data sample confirms the constancy of the solar diameter during the past 250 years (approximately) within instrumental and methodological limits. Moreover, no relationship between solar radius and the new sunspot-number index has been found from measurements of the ROA. Finally, the mean value for the solar semidiameter (with one standard deviation) calculated from the observations made in the ROA (1773 – 2006), after applying corrections for refraction and diffraction, is equal to 958.87″±1.77″

A CRITICAL COMMENT ON THE CLAIMED RELATION BETWEEN THE SOLAR MAXIMUM AMPLITUDE AND MAX-MAX CYCLE LENGTH

958.87^{\prime\prime}\pm1.77^{\prime\prime}