solar cycle
The solar cycle is the Sun's roughly 11-year rhythm of waxing and waning activity. Over this period the number of sunspots, the frequency of flares, and the overall stormy temperament of the Sun rise to a busy peak (solar maximum) and then fall to a quiet lull (solar minimum), again and again.
The cycle is driven by the Sun's magnetic field slowly winding itself up and then unravelling. Because the Sun is a ball of gas, its equator spins faster than its poles, and this uneven rotation stretches and tangles the internal magnetic field over years. As the field gets more twisted, more of it bursts through the surface, producing more sunspots and more storms; then the field reorganises, the polarity flips, and the count drops. The famous butterfly diagram captures this: if you plot where sunspots appear over time, they start each cycle at higher latitudes and drift toward the equator, tracing wing-like shapes. A full magnetic cycle, with the Sun's north and south poles swapping and swapping back, actually takes about 22 years.
The solar cycle matters because it governs space weather, which affects satellites, astronauts, power grids, and radio communication, all of which are riskier near solar maximum. Long records of the cycle also connect to subtle changes in the Sun's brightness, and a famous 70-year lull called the Maunder Minimum, when sunspots nearly vanished, coincided with unusually cold winters in Europe — though the size of any climate link is still debated.
The butterfly diagram: early in each cycle sunspots appear around 30 degrees latitude; by the cycle's end they cluster near the equator, the pattern repeating like pairs of wings.
Sunspot latitudes drifting toward the equator over each 11-year cycle trace the famous butterfly wings.
The '11 years' is only an average. Individual cycles have run anywhere from about 9 to 14 years, and their strength varies a lot from one to the next; the Sun is not a precise clock.