We solicit manuscripts on this general subject for inclusion in a Topical Issue (TI) of Solar Physics. The deadline for submission of statements of interest (SOI) with a tentative title, author list, and three suggestions for referees is 15 June 2015, and the deadline for manuscript submission is 31 October 2015.

This Topical Issue is an outgrowth of the Sunspot Number Workshop Series (http://ssnworkshop.wikia.com/wiki/Home). Workshops were held at the National Solar Observatory in Sunspot, New Mexico (September, 2011), the Royal Observatory of Belgium in Brussels (May, 2012), the National Solar Observatory in Tucson, Arizona (January, 2013), and Specola Solare Ticinese in Locarno, Switzerland (May 2014). This Topical Issue is not a conference proceedings, and it is not limited to research presented at the workshop. Nor is it limited to scientists who participated in any of the workshops. All submissions must be completed original papers that meet the regular quality of the Journal. The Topical Issue will begin with several invited reviews to summarize the subject and frame the work in the research papers which follow. Please consult Solar Physics 290, 2015 ( http://link.springer.com/journal/11207/290/1/page/1 )
for a recent Topical Issue. Topics to be considered for the TI include:

* Origin and history of the sunspot number
* Historical sunspot observations
* Inhomogeneities in the current Group and International Sunspot Numbers
* Reconstructions of the Group and International sunspot numbers
* Uncertainties in the Group and International sunspot number time series
* Sunspot number catalogs/data bases/archives
* Sunspot number correlates and proxies (e.g. sunspot area, regular
geomagnetic variation, F10.7, 10Be)
* The sunspot number during the Maunder Minimum
* The sunspot number during the recent solar minimum and Cycle 24
* Applications and implications of the revised sunspot number series
* Possible future revision of the sunspot number

In order to respect the other contributors, we will be strict with deadlines
for submission, refereeing, and proofing. Articles that are late, either
because of delays in submission or protracted refereeing, will appear
individually in later issues of the Journal.

For further information, and submission of statements of intent, p e contact:
John Leibacher (john.leibacher@gmail.com) or Edward Cliver (ecliver@nso.edu)

John Leibacher (Solar Physics editor), Frederic Clette (guest editor), Edward
Cliver (guest editor), Laure Lefevre (guest editor), Leif Svalgaard (guest
editor), Jose Vaquero (guest editor)