J. M. Vaquero

Revisiting the Sunspot Number

Our knowledge of the long-term evolution of solar activity and of its primary modulation, the 11-year cycle, largely depends on a single direct observational record: the visual sunspot counts that retrace the last 4 centuries, since the invention of the astronomical telescope. Currently, this activity index is available in two main forms: the International Sunspot Number initiated by R. Wolf in 1849 and the Group Number constructed more recently by Hoyt and Schatten (Sol. Phys. 179:189–219, 1998a, 181:491–512, 1998b). Unfortunately, those two series do not match by various aspects, inducing confusions and contradictions when used in crucial contemporary studies of the solar dynamo or of the solar forcing on the Earth climate. Recently, new efforts have been undertaken to diagnose and correct flaws and biases affecting both sunspot series, in the framework of a series of dedicated Sunspot Number Workshops. Here, we present a global overview of our current understanding of the sunspot number calibration.After retracing the construction of those two composite series, we present the new concepts and methods used to self-consistently re-calibrate the original sunspot series. While the early part of the sunspot record before 1800 is still characterized by large uncertainties due to poorly observed periods, the more recent sunspot numbers are mainly affected by three main inhomogeneities: in 1880–1915 for the Group Number and in 1947 and 1980–2014 for the Sunspot Number.After establishing those new corrections, we then consider the implications on our knowledge of solar activity over the last 400 years. The newly corrected series clearly indicates a progressive decline of solar activity before the onset of the Maunder Minimum, while the slowly rising trend of the activity after the Maunder Minimum is strongly reduced, suggesting that by the mid 18th century, solar activity had already returned to levels equivalent to those observed in recent solar cycles in the 20th century. We finally conclude with future prospects opened by this epochal revision of the Sunspot Number, the first one since Wolf himself, and its reconciliation with the Group Number, a long-awaited modernization that will feed solar cycle research into the 21st century.

Historical sunspot observations: A review

Abstract Early observations of sunspots were made by the naked eye. We discuss here the possibilities to use these records to study the long-term change in the Sun. Other historical sunspot observations with camera obscura are also discussed. The best record of the behaviour of the Sun is available for the last four centuries thanks to the observations of sunspots with telescope. These observations allow us to know the number, position, and area of sunspots as well as some specific episodes like the Maunder Minimum, optical flares, etc. Rudolf Wolf developed the first reconstruction of solar activity in the 19th century. Another reconstruction was made by Hoyt and Schatten in 1998 which improves the database and uses a new methodological approach. Here we also discuss some mistakes, pending tasks and minor improvements related to sunspot observations.

Trends in Block-Seasonal Extreme Rainfall over the Iberian Peninsula in the Second Half of the Twentieth Century

Abstract In this study trends in extreme rainfall over the Iberian Peninsula at a daily scale in the second half of the twentieth century have been detected and analyzed. For this goal 35 stations evenly distributed over the region of study covering the period 1958–97 have been studied. Two different approaches have been used. The first one consists of the nonparametric Mann–Kendall test and the Sen method. The second approach is based on the statistical theory of extreme values, involving time-dependent parameters in order to be able to reflect possible temporal changes in the frequency distribution. Results from both methods agree, confirming the reliability of the analysis. Negative trends are found for the west and southwest of the Iberian Peninsula in spring and winter. In autumn a spatial dipolar pattern appears, but trends are not so evident.

The Maunder minimum (1645–1715) was indeed a grand minimum: A reassessment of multiple datasets

Aims. Although the time of the Maunder minimum (1645–1715) is widely known as a period of extremely low solar activity, it is still being debated whether solar activity during that period might have been moderate or even higher than the current solar cycle #24. We have revisited all existing evidence and datasets, both direct and indirect, to assess the level of solar activity during the Maunder minimum. Methods. We discuss the East Asian naked-eye sunspot observations, the telescopic solar observations, the fraction of sunspot active days, the latitudinal extent of sunspot positions, auroral sightings at high latitudes, cosmogenic radionuclide data as well as solar eclipse observations for that period. We also consider peculiar features of the Sun (very strong hemispheric asymmetry of the sunspot location, unusual differential rotation and the lack of the K-corona) that imply a special mode of solar activity during the Maunder minimum. Results. The level of solar activity during the Maunder minimum is reassessed on the basis of all available datasets. Conclusions. We conclude that solar activity was indeed at an exceptionally low level during the Maunder minimum. Although the exact level is still unclear, it was definitely lower than during the Dalton minimum of around 1800 and significantly below that of the current solar cycle #24. Claims of a moderate-to-high level of solar activity during the Maunder minimum are rejected with a high confidence level.

Revisited Sunspot Data: A New Scenario for the Onset of the Maunder Minimum

The Maunder minimum forms an archetype for the Grand minima, and detailed knowledge of its temporal development has important consequences for the solar dynamo theory dealing with long-term solar activity evolution. Here, we reconsider the current paradigm of the Grand minimum general scenario by using newly recovered sunspot observations by G. Marcgraf and revising some earlier uncertain data for the period 1636-1642, i.e., one solar cycle before the beginning of the Maunder minimum. The new and revised data dramatically change the magnitude of the sunspot cycle just before the Maunder minimum, from 60-70 down to about 20, implying a possibly gradual onset of the minimum with reduced activity started two cycles before it. This revised scenario of the Maunder minimum changes, through the paradigm for Grand solar/stellar activity minima, the observational constraint on the solar/stellar dynamo theories focused on long-term studies and occurrence of Grand minima.

The sun recorded through history

Preface.- The Sun.- The Solar Structure.- The Photosphere.- Observing the Solar Surface.- The Chromosphere.- The Corona.- The Solar Wind.- 3-D Topology of the Magnetic Field.- Observing the Outer Layers.- Time Scales of Solar Variability.- Solar Terrestrial Relations.- Naked-eye Sunspots.- The Human Eye as a Detector of Light.- Visibility Criteria.- Naked-eye Sunspot Observations.- Naked-eye Sunspots and Temporal Evolution of Solar Activity.- Solar Drawings.- Pretelescopic Instruments.- The Invention of the Telescope.- First Telescopic Observations of Sunspots.- The Maunder Minimum.- The Rise of Solar Activity and the Dalton Minimum: 18th and 19th Centuries.- Sunspots Drawings in the Photography Era.- The First Granulation Drawings.- Sunspot Fine Structures.- Faculae.- White-light Flares.- The Outer Layers of the Sun.- The Influence of the Eye in Solar Drawings.- Physics from Drawings.- Modern Solar Drawings.- Solar Eclipses.- The Basics of Solar Eclipses.- Historical Solar Eclipse Observations.- Science Using Early Reports of Solar Eclipses.- The Solar Diameter and the Astronomical Unit.- The Earths Orbit.- Measuring the Known World.- Observing Methods of Solar Diameter.- Theoretical Background.- Long-term Variations.- Planetary Transits.- Terrestrial Aurorae and Solar-Terrestrial Relations.- Auroral Physics in Brief.- Folklore, Omen and Myths.- Reports During the Last Two Millennia.- The Search for the Cause.- Catalogues of Aurorae Observations.- Aurorae and Secular Solar Activity.- Aurora and Great Space Weather Events.- Reconstruction of Solar Activity During the Telescopic Era.- Wolfs Reconstruction.- The Reconstruction by Hoyt and Schatten.- Improving and Finding Lost Observations.- Final Comments.- Index.

A categorization method applied to the study of urban road traffic noise

The present work summarizes a study of the hypothesis that urban noise can be stratified by measuring street noise according to a prior classification of a towns streets according to their use in communicating the different zones of the town. The method was applied to five medium-sized Spanish towns (Vitoria-Gasteiz, Salamanca, Badajoz, Cáceres, and Mérida) with populations ranging from 218 000 down to 50000 and with different socio-economic characteristics, climate, etc. As the initial hypothesis of the work was that traffic is the main source of urban noise and is also the principal cause of the variability of the sound levels measured in urban settings, the study focused only on the five nonpedestrian categories of streets. The continuous equivalent sound level (Leq) was employed in the statistical analysis as it is commonly used as a general noise index, and other noise indicators such as L(DN) or L(DEN) are calculated from it. It was found that, although differences between the medians were not statistically significant in some of the towns for certain pairs of adjacent categories, the differences between pairs of nonadjacent categories were always significant, indicative of the stratification of noise in these five towns. Further studies on other medium-sized towns and on large towns and small villages would be needed to test whether the present definition of street categories is extensible elsewhere without modification.

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.

A HISTORICAL ANALOG OF 2005 HURRICANE VINCE

This paper documents a rare spell of severe weather in Spain that took place during the mid-nineteenth century when a tropical storm struck the southwest of the country on 29 October 1842. The use of a variety of independent documentary sources has provided unprecedented scope for the analysis of this event, allowing it to be set within its wider context, and for a judgement to be made on its tropical origin. The evidence suggests that this was similar, though stronger, to the more recent Hurricane Vince, which made landfall in Spain on 10 October 2005. This case study not only places Hurricane Vince, suggested at the time to have been unique, in its more proper long-term context, but it also demonstrates how documentary sources can improve our wider understanding of climate dynamics during historical times in the Atlantic basin.