A solar grand minimum is a prolonged interval, lasting several ordinary cycles, of strongly reduced sunspot activity. Historical records identify the Maunder minimum; cosmogenic carbon-14 and beryllium-10 provide longer indirect records because stronger heliospheric magnetic shielding generally reduces cosmic-ray production of these isotopes. Weak activity does not imply a zero solar magnetic field. Proposed causes include stochastic weakening of poloidal-field generation, nonlinear dynamo modulation, and intermittency near an excitation threshold.
The Maunder minimum is the interval approximately 1645–1715 when recorded sunspot activity was exceptionally low. It is the best-known historical solar grand minimum. Historical coverage and cosmogenic-isotope records require care in inferring its precise magnetic amplitude; evidence for weak modulation need not imply that the normal spot-producing dynamo disappeared completely.
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