Sunspot number

A numerical index of solar activity calculated from observed sunspot groups and individual sunspots. Its long record tracks the roughly 11-year solar cycle.

Wolf sunspot number: A sunspot index combining the number of groups with the number of individual spots, multiplied by a scale factor. This is the classic formula behind the traditional sunspot number.
Rudolf Wolf: A Swiss astronomer who established a systematic sunspot-counting program in the nineteenth century. Wolf devised the counting approach that gave the index its traditional name and formula.
Solar cycle: The recurring pattern of solar magnetic activity, averaging about eleven years between sunspot minima. The index’s rises and falls are the standard way to track the cycle.
10.7 cm solar radio flux: A measure of solar radio emission at a wavelength of 10.7 centimeters, widely used as an activity index. It responds to solar activity without relying on visible spot counts.
International Sunspot Number: A standardized sunspot index compiled from observations contributed by a worldwide network of observers. It applies coordinated counting and calibration to maintain a continuous global series.
Samuel Heinrich Schwabe: A German astronomer who discovered the approximately eleven-year variation in sunspot counts. His observations supplied the cyclic pattern the index was built to track.
Solar maximum: The phase of a solar cycle when solar activity and sunspot numbers are relatively high. Peak index values help identify and compare cycle maxima.
Solar irradiance: The solar power received per unit area at a specified distance, often measured at Earth's orbit. It measures emitted energy rather than the number of visible magnetic features.
Sunspot group: A cluster of sunspots on the solar surface, often associated with a single magnetic region. Each group contributes to the index separately from its individual spots.
Waldmeier discontinuity: A systematic change in the sunspot-number series associated with a change in counting practice around 1945. It created a historical break that later reconstructions sought to correct.

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