sunspotnumber

The sunspot number is a quantitative index of solar activity based on observations of sunspots on the Sun’s visible surface. It serves as a simple proxy for the Sun’s magnetic activity and correlates with variations in solar irradiance and space weather conditions.

Traditionally, the International Standard Sunspot Number, also called the Wolf number, is computed from the observed count of sunspot groups and individual spots. A common form is R = k(10g + f), where g is the number of sunspot groups, f is the total number of individual sunspots, and k is a scaling factor that accounts for observer, instrument, and observing conditions. The index is compiled from worldwide observations to produce a continuous time series that spans several centuries.

Historical and contemporary effort has produced two main indices: the Wolf or International Sunspot Number (ISN) and the Group Sunspot Number (GSN). The ISN is maintained by the Solar Influences Data Analysis Center (SILSO) and has undergone recalibration, with Version 2.0 released in 2015 to address inhomogeneities in the historical record. The GSN, introduced by Hoyt and Schatten in 1998, uses group counts with a different calibration and serves as a complementary measure of solar activity.

Usage and significance of the sunspot number include studying the solar cycle, examining long-term trends in solar activity, and exploring connections to solar irradiance, geomagnetic activity, and climate-related research. Data series are publicly archived and widely used in solar physics, space weather forecasting, and related disciplines. Limitations arise from changes in instrumentation, observer practices, and data gaps, which modern recalibration efforts seek to mitigate.