Publications

This page will feature both publications arising from the Team meetings and short summaries of published articles on which the Team will build.

Andres Munoz-Jaramillo and Jose M. Vaquero

The sunspot series is one of the most used and famous data-set inside and outside of heliophysics.  Most users of the sunspot number series are unaware of how challenging it is to assemble a homogeneous series out of more than 700 observers, spanning the last 400 years, involving different equipment, observation techniques, magnification, and scientific understanding.   This is made particularly challenging by dry periods with few observations that can bridge historical gaps.   This Nature perspective proposes a new way of visualizing the sunspot series that accounts for its limitations and highlights periods of high uncertainty.  The goal is to raise awareness on these issues and inform better non-specialists that may be interested in long-term solar variability.

Sunspot Characteristics at the Onset of the Maunder Minimum Based on the Observations of Hevelius

V. M. S. Carrasco1J. M. VaqueroM. C. GallegoA. Muñoz-JaramilloG. de TomaP. GalavizR. ArltV. Senthamizh PavaiF. Sánchez-BajoJ. Villalba Álvarez, and J. M. Gómez

An analysis of the sunspot observations made by Hevelius during 1642–1645 is presented. These records are the only systematic sunspot observations just before the Maunder Minimum (MM). We have studied different phenomena meticulously recorded by Hevelius after translating the original Latin texts. We reevaluate the observations of sunspot groups by Hevelius during this period and obtain an average value 7% greater than that calculated from his observations given in the current group database. Furthermore, the average of the active day fraction obtained in this work from Hevelius’s records previous to the MM is significantly greater than the solar activity level obtained from Hevelius’s sunspot observations made during the MM (70% versus 30%). We also present the butterfly diagram obtained from the sunspot positions recorded by Hevelius for the period 1642–1645. It can be seen that no hemispheric asymmetry exists during this interval, in contrast with the MM. Hevelius noted a ~3-month period that appeared to lack sunspots in early 1645 that gave the first hint of the impending MM. Recent studies claim that the MM was not a grand minimum period, speculating that astronomers of that time, due to the Aristotelian ideas, did not record all sunspots that they observed, producing thus an underestimation of the solar activity level. However, we show that the good quality of the sunspot records made by Hevelius indicates that his reports of sunspots were true to the observations.

Sunspot Characteristics at the Onset of the Maunder Minimum Based on the Observations of Hevelius

Carrasco, Vaquero, Gallego, A. Muñoz-Jaramillo, G. de Toma, Galaviz, Arlt, Senthamizh Pavai, Sánchez-Bajo, Villalba Álvarez and Gómez

An analysis of the sunspot observations made by Hevelius during 1642–1645 is presented. These records are the only systematic sunspot observations just before the Maunder Minimum (MM). We have studied different phenomena meticulously recorded by Hevelius after translating the original Latin texts. We reevaluate the observations of sunspot groups by Hevelius during this period and obtain an average value 7% greater than that calculated from his observations given in the current group database. Furthermore, the average of the active day fraction obtained in this work from Hevelius’s records previous to the MM is significantly greater than the solar activity level obtained from Hevelius’s sunspot observations made during the MM (70% versus 30%). We also present the butterfly diagram obtained from the sunspot positions recorded by Hevelius for the period 1642–1645. It can be seen that no hemispheric asymmetry exists during this interval, in contrast with the MM. Hevelius noted a ~3-month period that appeared to lack sunspots in early 1645 that gave the first hint of the impending MM. Recent studies claim that the MM was not a grand minimum period, speculating that astronomers of that time, due to the Aristotelian ideas, did not record all sunspots that they observed, producing thus an underestimation of the solar activity level. However, we show that the good quality of the sunspot records made by Hevelius indicates that his reports of sunspots were true to the observations.

Thaddäus Derfflinger’s Sunspot Observations during 1802–1824: A Primary Reference to Understand the Dalton Minimum

Hisashi Hayakawa, Bruno P. Besser, Tomoya Iju, Rainer Arlt, Shoma Uneme, Shinsuke Imada, Philippe-A. Bourdin, and Amand Kraml

As we are heading toward the next solar cycle, presumably with a relatively small amplitude, it is of significant interest to reconstruct and describe the past secular minima on the basis of actual observations at the time. The Dalton Minimum is often considered one of the secular minima captured in the coverage of telescopic observations. Nevertheless, the reconstructions of the sunspot group number vary significantly, and the existing butterfly diagrams have a large data gap during the period. This is partially because most long-term observations at that time have remained unexplored in historical archives. Therefore, to improve our understanding on the Dalton Minimum, we have located two series of Thaddäus Derfflinger’s observational records spanning 1802–1824 (a summary manuscript and logbooks), as well as his Brander’s 5.5 feet azimuthal quadrant preserved in the Kremsmünster Observatory. We have revised the existing Derfflinger’s sunspot group number with Waldmeier classification, and eliminated all the existing “spotless days” to remove contaminations from solar elevation observations. We have reconstructed the butterfly diagram on the basis of his observations and illustrated sunspot distributions in both solar hemispheres. Our article aims to revise the trend of Derfflinger’s sunspot group number and to bridge a data gap of the existing butterfly diagrams around the Dalton Minimum. Our results confirm that the Dalton Minimum is significantly different from the Maunder Minimum, both in terms of cycle amplitudes and sunspot distributions. Therefore, the Dalton Minimum is more likely a secular minimum in the long-term solar activity, while further investigations for the observations at that time are required.

Some “essential reading” on the sunspot recalibration effort, courtesy of Laure Lefevre:

Here is a summary of proposed reading for the future ISSI workshops on the Recalibration of the Sunspot Number Series.

First, historical background about the Sunspot Number (Zurich method) can be found in Friedli (2016a) and Svalgaard, Cagnotti and Cortesi (2017).

Friedli (2016a): doi:10.1007/s11207-016-0907-0
Svalgaard, Cagnotti and Cortesi (2017): doi:10.1007/s11207-016-1024-9

Of course the historical reconstruction of the Group Number can be found in Hoyt & Schatten (1998a and b).

Hoyt & Schatten (1998a): doi:10.1023/A:1005007527816
Hoyt & Schatten (1998b): doi:10.1023/A:1005056326158

As the latest workshops were organized in triads the rest is organized in order of the program of the latest sworkshop held in Brussels June 12-15th 2017.

1. Clette & Lefèvre (2016): Clette et al. (2016a 2016b) and Clette et al. (2014) describe corrections applied directly to version one of the Sunspot Number SSN_V1 (published on the SILSO website: http://www.sidc.be/silso/versionarchive) to SSN_V2 (http://www.sidc.be/silso/datafiles). Contrary to the GN series described below, this is not a reconstruction based on the original sources, but rather a set of corrections applied after carefully establishing the reasons behind them, the periods of correction, and defining the level at which they should be applied. On this subject, the “Waldmeier Jump” due to a weighting of the spots in the counting of stations of the Zurich network is also discussed in the series of articles by Mike Lockwood (see point 2).

Clette et al. (2014): https://arxiv.org/abs/1407.3231, doi:10.1007/s11214-014-0074-2
Clette et al. (2016a):https://link.springer.com/article/10.1007%2Fs11207-016-1014-y
Clette et al. (2016b):https://arxiv.org/abs/1507.07803, doi:10.1007/s11207-016-0875-4

2. Lockwood et al. (2014, 2016): The output series described in Lockwood et al. (2014, 2016) concern the SSN and focus on the correction to be applied around 1945. The so-called “Waldmeier Jump” referred to in the last section. The Lockwood method basically compares different series supposed to have similar properties as the SSN (related to the SSN like geomagnetic data, or very close to the SSN_V1, such as the Number of groups and sunspot areas extracted from the Royal Greenwich Observatory data) around the jump assuming that the mean difference before and after the jump should be consistent with zero. They compare the series to SSN_V1 before and after the alleged jump with one side of the jump corrected for different supposed jump levels in SSN_V1.

Lockwood et al. (2014): http://onlinelibrary.wiley.com/doi/10.1002/2014JA019970/abstract;jsessionid=81F0BFB70D64EB87585BCC59E3248BD4.f03t01
Lockwood et al. (2016a): https://arxiv.org/abs/1601.06441,doi:10.1007/s11207-016-0855-8
Lockwood et al. (2016d): https://arxiv.org/abs/1605.05149, doi:10.1007/s11207-016-0967-1

3. Friedli (2016) proposes a Recalculation of the Wolf series from 1849 to 1893. However, as most of the methods used by the authors are very disputable, it has still not been accepted for publication. Here is a summary of what should appear in the paper if it is published.

Homogeneity is considered as the most important property of the Wolf series of sunspot relative numbers since without a stable scale no valid conclusions about variations in the long-term course of the solar activity can be drawn. However, the homogeneity testing of the Wolf series is a difficult task, since the raw data and the data reduction methods are widely unknown. Based on hitherto unpublished original sources we reconstruct the calibration algorithms and data reduction methods and discuss their impact on the homogeneity of the Wolf series. Furthermore, we provide evidence that the practice of weighting the individual spots according to their extent started already with the assistants of Wolfer and was passed to their successors, including Brunner and Waldmeier. Thus, no additional long-term correction has to be applied to keep the homogeneity of the Wolf series. Based on Alfred Wolfer as standard observer, we recalculate the course of the Wolf series from 1849 to 1893, correcting for the widely disregarded diminishing of Wolf’s eyesight during the late 1870s and the 1880s and for the hitherto unrecognized scale jump from the 83/1320~mm Fraunhofer refractor to the 40/700~mm Parisian instrument. Before 1849 the level of the Wolf series has to be lowered further, since Wolf overestimated the k- factor of Heinrich Schwabe.

4. Svalgaard & Schatten (2016): This article describes a reconstruction of a complete Group Number series using a backbone method. Contrary to the excessive use of “Daisy chaining” used in previous reconstructions, the backbone enables to actually minimize the number of links in the “chain”. Lockwood et al. (2016b and c) discuss the use of Daisy chaining and the propagation of possible errors made on the linear fit between two observers.

Lockwood et al. (2016b): https://arxiv.org/abs/1605.01948, doi:10.1007/s11207-016-0913-2
Lockwood et al. (2016c): https://arxiv.org/abs/1510.07809, doi:10.1007/s11207-015-0829-2

5. Usoskin et al. (2016a): the main paper describing the Active Day Fraction (ADF) method is Usoskin et al. (2016a). A slight modification of the method with new observers added is developped in Willamo et al. (2017) and the computation of monthly and yearly values is described in Usoskin, Mursula and Kovaltsov (2003). Usoskin et al. (2016b) analyses in more details the possibility of assuming the difference of reported groups is stronlgy (i.e. strongly enough that other effects are negligible) related to the acuity of the observer, i.e. his capacity at seing/reporting spots of a size above a certain threshold.

Usoskin, Mursula and Kovaltsov (2003):https://link.springer.com/article/10.1023%2FB%3ASOLA.0000013029.99907.97
Usoskin et al. (2016a):https://arxiv.org/abs/1512.06421, doi:10.1007/s11207-015-0838-1
Usoskin et al. (2016b):https://arxiv.org/pdf/1609.00569.pdf, doi:10.1007/s11207-016-0993-z
Willamo et al. (2017): https://arxiv.org/pdf/1705.05109.pdf, https://doi.org/10.1051/0004-6361/201629839

Note that, apart from the ADF method, the Usoskin et al. (2016) paper also contains a new method to scale on observer to the other based on “correspondance matrices”. This part of the work is used especially in Chatzistergos et al. (2017).

6. Chatzistergos et al. (2017): This article describes a “backbone” reconstruction of group numbers that is based on the “correspondance matrices” described in Usoskin et al. (2017) instead of using a “simple” linear dependency between observers as in Svalgaard and Schatten (2016). Also note that the data from Svalgaard and Schatten (2016) is based on the scaling of yearly data, while the series from Chatzistergos et al. (2017) is based on daily data that is later interpolated to yearly.

Chatzistergos et al. (2017): https://arxiv.org/abs/1702.06183, https://doi.org/10.1051/0004-6361/201630045

In that vein, Andres-Munoz Jaramillo discussed a way of doing the “backbone” without any human bias at the Sunspot Workshop in Reading in Feb. 2017, by constructing the backbone with unbiased criteria based on the quality of the relations between observers and choosing who to link to who with an automatic method. You can ask him about his ideas.

Two articles from Ed Cliver are also really interesting to read. Cliver & Ling (2016) investigates how the original Hoyt & Schatten (1998) series was built, including some inconsistencies and anomalies and Cliver (2017) confronts the recent reconstructions with the ground truth that observations cannot degrade with time between the 19th and 20th century.

Cliver & Ling (2016): doi:10.1007/s11207-015-0841-6
Cliver (2017): doi:10.1007/s11207-016-0929-7

Aside from these references to specific time series, the articles from Thierry Dudok de Wit on how to fill gaps and the study of uncertainties in the Sunspot Number might be of interest.

Dudok de Wit (2011):https://arxiv.org/pdf/1107.4253.pdf, https://doi.org/10.1051/0004-6361/201117024
Dudok de Wit et al. (2016):https://arxiv.org/pdf/1608.05261.pdf, doi:10.1007/s11207-016-0970-6