NASA Science
Glossary · The Sun & Space Weather
The solar cycle
An eleven-year rhythm in sunspots, flares and the Sun's entire magnetic field.
Sunspot numbers rise and fall on a roughly eleven-year cycle, and at each maximum the Sun's magnetic poles swap over, making the true cycle twenty-two years. Activity at maximum means more flares, more coronal mass ejections, more aurora and a measurably puffier upper atmosphere, which drags on satellites in low orbit and shortens their lives. The cycle is not clockwork — some maxima are feeble and one seventeenth-century stretch produced almost no spots at all — and predicting the next one remains genuinely difficult.
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Related terms
Coronal mass ejection
A billion tonnes of solar atmosphere thrown into space, and the cause of most serious aurorae.
Solar flare classes
A, B, C, M, X — a logarithmic scale for how hard the Sun just flashed in X-rays.
Kp index
A 0–9 scale of how disturbed Earth's magnetic field is right now.
Sunspot
A patch of the Sun where the magnetic field is strong enough to hold the heat back.
Questions
What is solar cycle?
An eleven-year rhythm in sunspots, flares and the Sun's entire magnetic field.
Why does it matter?
Activity at maximum means more flares, more coronal mass ejections, more aurora and a measurably puffier upper atmosphere, which drags on satellites in low orbit and shortens their lives. The cycle is not clockwork — some maxima are feeble and one seventeenth-century stretch produced almost no spots at all — and predicting the next one remains genuinely difficult.