WDC-SILSO: a full modernization of the multi-century sunspot recordFrédéric Clette, Laure Lefèvre

World Data Center SILSO, Sunspot Index and Long-term Solar Observations, Royal Observatory of Belgium, Brussels

Abstract:We present here the World Data Center SILSO that preserves the Sunspot Number time series, ourlongest record of solar activity. We highlight the main achievements of the last 5 years, during which theWDC SILSO was deeply modernized and the base data series was entirely recalibrated for the first timesince its creation by R. Wolf in 1849. We describe the new corrections applied to the standard SunspotNumber series. We also present the new array of WDC-SILSO data products.

Looking towards the future, we outline the new coordinated process now being implemented by WDC-SILSO under supervision of the International Astronomical Union to implement future upgrades of theSunspot Number and their official approval. Indeed, this largely static heritage has now become again amodern living data set open to new progresses in solar research, and destined to evolve in parallel withmore detailed but only-recent solar measurements.

References:• Fröhlich, C., Lean, J. 2004: Astron. & Astroph. Rev., Vol 12/4, 273-320, DOI 10.1007/s00159-004-0024-1• Hoyt, D. V. and Schatten, K. H., 1998: Solar Physics, Vol 179, p.189, and Vol 181, p.491., DOI 10.1023/A:1005007527816

• Svalgaard, L., Schatten, K.H. 2016: Solar Phys, Online First, DOI 10.1007/s11207-015-0815-8 (arXiv:1506.00755)• Usoskin, I.G., Solanki, S.K., Kovaltsov, G.A. 2007: Astron. & Astroph., Vol471/1, 301-309, DOI 10.1051/0004-6361:20077704

A complete revision of the Sunspot Number series

In 2011, a 4-year community effort was initiated to carry out a full critical revision of the historicalSunspot Number time series. It was structured around four international Sunspot Number Workshopsand was completed with the official release of the first-ever revised version of this reference series onJuly 1st , 2015, presented at the General Assembly of the Astronomical Union (Honolulu, Aug. 2016).

This re-calibration revealed several large inhomogeneities (up to 40%), both in the SN series and itsmore recent counterpart, the Group Number series (GN, Hoyt & Schatten 1998, Svalgaard & Schatten 2016). Thecorrections are shown in Figures 2 and 3 and are explained in detail in a comprehensive review paper:

Clette, F., Svalgaard, L., Vaquero, J.M., and Cliver, E.W.: 2014, “Revisiting the Sunspot Number. A 400-Year Perspective on the Solar Cycle”, Space Science Reviews 186, pp. 35-103.

More recently, a special topical issue of Solar Physics, the main journal devoted to solar research, wasdedicated to the new Sunspot Number. It will be published by the end of 2016 and will included morethan 40 papers (> 800 pages).

Figure 2: Comparison (top) and ratio (bottom) betweenthe original and recalibrated Sunspot Number SN.

Figure 3: Comparison and ratio between the originaland new reconstructed Group Number GN.

GN

Figure 4: Overall secular trends in peak cycle amplitudes over the past 300 years (dashed lines) for theoriginal Sunspot Number (green), the original Group Number (red) and the new Sunspot Number andGroup Number series (blue and purple respectively) , the latter two now match closely.

Introducing the WDC - SILSO

The World Data Center SILSO (Sunspot Index and Long-term Solar Observations, formerly known as SIDC) ispreserving the longest record of solar activity, which spans the last four centuries. This unique SunspotNumber (SN) time series documents the variations of the last 35 11-year solar cycles. The WDC alsocoordinates a worldwide network of more than 85 stations to continuously extend this series and providemid-term forecasts of the solar cycle on a monthly basis.

The Sunspot Number is derived from the total number of sunspots on the Sun (Ns) and the number ofsunspot groups (Ng), by the formula 10 Ng + Ns. It is based on all visual observations since the invention ofthe telescope in 1610 and was introduced by Rudolf Wolf in 1849. Originally at the Zurich Observatory(Switzerland), its production was moved to Brussels in 1981. The statistical calculation of the SunspotNumber, based on multiple stations, was then fully computerized. Since 2011, a full modernization of theWDC products and archives was undertaken. It involved all aspects: end-to-end revision of reference dataseries, modernization of heritage software and data archives, and a new Web data portal.

Multiple applications of the sunspot record

• Solar physics: solar cycle, physical models of the solarmagnetic activity

• Earth climatology: solar irradiance and solar windreconstructions

• Paleo-climatology: cosmogenic isotope record calibration

• Operational civilian and military uses:• Telecommunication: long-distance propagation, blackouts

• Navigation: GNSS disturbances and errors

• Spaceflight: atmospheric drag on LEO spacecrafts, solar flare risk

• Energy: electrical power grids, pipeline corrosion

The re-calibrated SN and GN series are now largely reconciled and show much more uniform cycleamplitudes over past centuries (Figure 4). The revised secular trend in peak cycle amplitudes is lessthan 5%/century, compared to 15%/century for the original Sunspot Number and 40%/century for theoriginal Group Number. This thus questions the notion of a Grand Maximum with unprecedented cycleamplitudes occurring in the mid-20th century (e.g. Usoskin et al. 2007 ).

Figure 1: Cosmogenic isotope record(10Be, 14C) calibrated on the SunspotNumber (dark blue) over the last 4centuries. (Fröhlich & Lean 2004 )

Medieval maximum Spörer Maunder Dalton

A new expanded SILSO Web site

Together with the revision of the Sunspot Number itself, the heritage Fortran software was entirely revisedand rewritten (Python) and the data storage and format was upgraded (MySQL database). For thedistribution of our new data sets and of the associated information (metadata), we also created an entirelynew Web site (on line since June 2015) with new improved graphical displays of the various data sets.

In view of future upgrades of the Sunspot Number series, we also implemented a version numberingscheme. All versions, past and current (now V2.0), are accessible together with a full documentation.

Wide range of

SILSO data products

Sunspot numbers (prime reference):Daily, monthly, yearly, 13-month smoothed series

USET solar images and drawings(Uccle station)

12-month mid-term solar activity forecasts (2 methods)

Real-time sunspot numbers for Space weather applications

Hemispheric sunspot numbers

Conclusion 1: Entirely new perspectives for the future

This epochal revision of the reference sunspot record proved to be a great success, as beyond the upgradeof the series itself, it led to a full revival of solar-cycle research, with more than 50 papers published on thesubject in just the last 18 months (more than in the last 50 years). It also leads to a deep transition in therole of the World Data Center, which is now strongly expanded, as outlined in the diagram below.

Given this steep growth in published results, a new global coordination of future research and results isnow being implemented under the supervision of the International Astronomical Union (Inter-DivisionWorking Group on Solar Synoptic Observations) for the official validation of future modifications of thesunspot reference series. This will be led by the WDC-SILSO, which will centralize the input data and codes,and subsequently archive and distribute the resulting validated output.

http://sidc.be/silso

Therefore, we started issuing alert messages,addressing in particular the WDS and IAUcommunities and we want to prompt urgentrescuing actions. As other data centers alsoface precarious working conditions, we hopeto share experiences, which might helpidentifying solutions. However, our coreproducts hold a very specific position, quiteunlike most other WDCs, as outlined in thesketch at right, namely: small data volumes,global scope without territorial base andabsence of a direct link with our vastcommunity of actual end-users.

Static unquestioned archive Modern evolving data set

Formerly (Zurich, SIDC): Repository + mechanical extension of the heritage series

Now (WDC –SILSO): Clearinghouse for all new published results (whole series)• Central hub for input sunspot data (database of all raw observations) • Coordinated evaluation of methods and validation of future versions (IAU supervision)• Upgrade of the production method to follow scientific progresses

INPUTSolar activity

WDC - SILSO

IMPACTEnd-Users

Geographical baseRESPONSEAtmosphereIonosphere

MagnetosphereEnvironment

INFLUENCEglobal

Grand MinimaDalton

Conclusion 2: Threatened future for the WDC – SILSO and solar synoptic observations at the ROB

The WDC-SILSO operations rest almost entirely on the structural resources provided by the host institute,(Royal Observatory of Belgium ROB). Those resources, primarily manpower, were always limited butdeclined over recent years (now, only three part-time collaborators, of which only one permanent scientist).

Given the growing needs of the scientific community and the recent expansion of the WDC-SILSO missions(see conclusion 1 above) , our means now clearly fell below the sustainability threshold. The long-termcontinuation and quality standards are not guaranteed anymore in the ROB context.

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